Ion and Laser Induced Plasmas – High Energy Density in Matter

8/3/2019 Ion and Laser Induced Plasmas High Energy Density in Matter

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4 Ion and Laser Induced Plasmas High EnergyDensity in Matter

OverviewHeavy ion beams are a new tool to investigate dense plasma phenomena associatedwith high energy density matter. A number of methods are available to generate highenergy density matter. These are intense laser beams, exploding wires, gas guns, andfast z-pinches. The advantage of heavy ion beams is that only here the amount of energy deposited in matter is known with high precision, as well as the temporal andspatial distribution of the energy in the initial state. GSI-Darmstadt is the first andonly accelerator laboratory worldwide where a powerful and intense heavy ion beam ahigh-energy laser beam are available. This unique combination facilitates novel andpioneering beam-plasma interaction experiments to study the structure and theproperties of matter under extreme conditions of high energy density [1]. Already atthe currently existing power level for the heavy ion beam (10 GW/g) and the laserbeam (51011 W/cm2) unexpected results have been found.

Beam-plasma interaction experiments demonstrated a stopping power of fullyionized matter for heavy ions which is up to forty times higher than thestopping power of cold non-ionized matter. It could be shown that the chargestate distribution of heavy ions traversing a plasma is significantly differentfrom the distribution obtained from interaction processes with a cold gas [2].

In a laser irradiation experiment a directed beam of heavy ions emerging fromthe target was detected, where the total energy of the observed ions wasapproximately two orders of magnitude higher than it was predicted fromstandard scaling relations [3].

The new facility SIS 100 with a heavy ion synchrotron for high intensity heavy ionbeams will extend the available beam deposition power by more than two orders of magnitude beyond the currently available power. Therefore it will be possible toperform laboratory experiments under controlled and reproducible conditions to studyproperties of matter under conditions similar to those in the interior of stars or largeplanets.

Plasma phenomena do occur in quite different areas of physics. In the early phase of the universe matter consisted of quarks and gluons forming a state that is calledquark-gluon plasma and the investigation of the quark-gluon plasma properties is oneof the research topics GSI scientists are engaged in. The properties of compressednuclear matter is yet another field of intense research in nuclear physics. Theexperiments are searching for phase transitions and aim to measure the properties of nuclear matter produced in the extreme conditions of heavy ion collisions. Though theexperimental techniques are different from those applied in plasma physics, the basicunderlying physical phenomena show similarities to the effects that are investigated

to probe atomic and molecular matter under the conditions of high energy density.


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Interaction processes of ion beams with bulk matter have a wide range of applicationto modify matter properties of the irradiated samples. If a multi-kilojoule beam isdeposited in matter during a time span, which is short compared to the hydrodynamicresponse time of the material, a state of high energy density is induced. This resultsin dramatic changes of characteristic target properties. Phase transitions are inducedand shock waves may be generated which lead to a high compression state of thetarget material, and to the metallization of condensed hydrogen [4,5] and othermolecular crystals. Thus, yet widely unexplored regions of the metallic-phasediagrams and critical points of the gas-fluid and plasma phase transitions in stronglycompressed matter become readily accessible under reproducible conditions in thelaboratory (Figure 4.1).

Figure 4.1: Exploring matter under extreme conditions of high energy density. Instantaneousenergy deposition in a sample leads to a heated volume at solid state density o and extreme

pressure. Subsequent expansion of the material along the expansion isentropes allows to studythe phase diagram of matter under high pressure and less than solid state density. In thisregime the critical points of metals and plasma phase transitions are predicted. The regimeabove the solid state density can only be accessed in compression experiments using single andmultiple shocks. A single planar shock wave can drive matter only up to fourfold density.Therefore only multiple shock waves and the near isentropic compression allow to reach theultrahigh density regime


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4.1 Science Case

4.1.1 Introduction

Basic research of plasma physics with intense heavy ion and laser beams is motivatedby the fact that heavy ion beams are excellent tools to generate dense plasma. Theenergy deposition of ion beams into a non-ionized target is a reasonably well-understood process and direct heating of a well-defined extended volume is achievedby a rather homogeneous energy deposition. High precision experiments to study theproperties of bulk matter require a detailed knowledge about the exact amount of energy deposited into the sample as well as the spatial and time distribution of theenergy inside the target volume. This demand is intrinsically fulfilled by the verynature of the interaction processes of heavy ions with matter themselves. Theyprovide a detailed picture of the energy deposition inside the target volume as shownin Figure 4.2,where a heavy ion beam is injected into a solid crystal made from argongas at cryogenic temperature The energy deposition profile is revealed throughinteraction processes emitting light in the visible regime. The highest energydeposition occurs at the end of the range. This regime is commonly called the Bragg

peak and its position is precisely determined by the total ion energy. A higher beamenergy would cause the Bragg peak to shift out of the target. In this case the target isheated in a very homogeneous manner. If the ion beam is intense enough, the beamheated target volume is transformed into a dense plasma. Thus intense ion beamsopen new opportunities to investigate the interaction phenomena of heavy ion beamswith dense plasma and they allow to study the hydrodynamic and radiative propertiesof beam heated matter with high precision experiments, and improved orcomplementary techniques. In order to achieve this ambitious goal it is also necessaryto include the development of new diagnostic techniques and the design of appropriateheavy ion targets.

Figure 4.2: Neon beam of 300 MeV/u penetrating an Ar crystal. The beam deposition time is1 s. The false colors represent the spatial energy deposition. This framing picture demonstratesthat heavy ion beams offer the opportunity to measure the total amount of energy deposited intothe material sample as well as the spatial and temporal distribution of the energy. While thetarget material during the first part of the range is heated in a rather homogeneous manner an

increased energy deposition occurs at the Bragg peakend of the range.


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The investigation of interaction processes of high-energy heavy ion beams with matteris a research topic with a long tradition at accelerator laboratories. At least for simplecollision systems a basic understanding of the dominant atomic and nuclearinteraction mechanisms has been achieved. However, energy loss and chargeexchange reactions with dense, ionized matter have added new and interestingaspects to this field [6-11]. Since most visible matter, that constitutes our universe isin a state of high energy density, dense, strongly coupled plasmas are an interestingresearch object. The properties of matter under conditions of high density andpressure are often summarized by an equation that relates the pressure, or energydensity to the matter density of the sample. Such an equation is called theequation of state (EOS) of the material. The determination of the proper equation of state, theconversion of kinetic energy of the ion beam into radiation, and the hydrodynamicbehavior of dense plasmas are key issues to a basic understanding of dense plasmaphenomena.

Figure 4.3: Parameter range of plasmas produced by direct impact of intense heavy ion andlaser beams on solid targets compared to different stellar plasmas and the ignition parameters

for magnetic confinement (MCF) and inertial confinement (ICF) plasmas.

A plasma map covering many orders of magnitude in temperature and density isshown inFigure 4.3.The dominating physical properties of plasma vary considerablybetween the extremes of low density very high temperature plasma in the top leftcorner of the figure, where the magnetic confinement fusion plasma is situated, downto the low temperature high density regime of dense plasma. A dividing line ismarked where the plasma parameter is equal to unity. This parameter is a measure


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of the relative strength of Coulomb interaction phenomena compared to the thermaleffects and is given by the ratio of the potential energy (e2/40d) to the thermalenergy (kT) of the plasma constituents. In the high temperature low-density regimethe plasma parameter is smaller than unity and the plasmas occurring in thisparameter regime are called ideal plasmas. Below the dividing line interactionprocesses determine the properties of plasma and therefore the term strongly coupledplasma is used. Quantum effects are expected, where the inter particle distance d iscomparable to the de Broglie wavelength b of the particles. The line of b=d is alsomarked in Figure 4.3.

Plasma targets used for experiments at GSI are shown in comparison with typicalplasmas of astrophysical interest, as well as fusion plasmas. Interaction processesbetween intense ion beams and plasmas are an ideal tool to probe high energy densityplasmas and to investigate their properties. The kinetic energy of the heavy ions can

exactly be tailored to the experimental conditions. Ions penetrate deep into thevolume of the plasma target. Energy loss and the final charge state distribution of theions are the typical signals which characterize the beam plasma interaction processes,and which allow to draw conclusions about the plasma target properties [13,14]. Thegeneral interest in these measurements is the high precision determination of thedeposition power, an analysis of recombination and charge exchange processes, andthe hydrodynamic response of matter irradiated with high intensity heavy ion beams.

Concerning the aspect of basic research, physics of dense plasmas is a well-establishedfield of research, a domain of pulsed power generators like shock tubes, light gasguns, exploding wires as well as powerful laser and light ion beam facilities. Duringthe last decades they have produced a wealth of results, motivated predominantly bythe application to inertial confinement. With heavy ion beams, research in this fieldhas been carried out in recent years [6-12] using the available low intensity beams atthe GSI accelerator facilities, where the heavy-ion beam energy ranged from a fewkeV/u up to several hundred MeV/u. Gas-discharge, z-pinch, and later also laserplasmas from the nhelix laser system (n anosecond h igh e nergy laser for ionex periments) were used initially for beam-plasma interaction experiments asindicated in Figure 4.3. The intense beam from a high currentr adio-f requency-q uadrupole RFQ-accelerator was used to study first beam induced plasma from an ionbeam heated gas target. With higher beam intensities available now, solid-statetargets are the subjects of investigation. After completion and the achievement of thefull performance of the new high current injector, target temperatures of up to severaleV will be obtained. With the new synchrotron facility the intensity for beams of veryheavy ions will be increased by another factor of 100 which allow the investigation of fast hydrodynamic phenomena of dense plasmas, the physics of hot compressedmatter, its equation of state, predicted phase transitions and material properties in anew regime of pressure and temperature. Sophisticated target designs like amagnetically insulated cylindrical target will eventually open the temperature regimearound 100 eV for ion beam driven plasmas, whereradiation physics is starting to bethe new dominant feature. This regime is obviously relevant to the stellar interior and

stellar atmospheres, and other basic phenomena of astrophysics.


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Basic research into the properties of dense plasmas has also been motivated by theaspect to explore the physics basis for an alternative route to fusion energy.InertialConfinement Fusion (ICF) driven by heavy ion beams exhibits a promisingperspective. Compared to laser and light ion drivers which are single-shot facilities,the heavy ion accelerator is an excellent high repetition rate machine with highefficiency and is therefore a natural candidate for a reactor driver. On the other hand,the European Fusion Program is almost exclusively devoted to magnetic confinementfusion (MCF). This decision was made more than forty years ago, when the EuropeanFusion Program was launched. At that time the restriction to MCF had some logicbecause the route to fusion energy seemed to be paved already, taking into accountthe enormous progress and success introduced by Tokamak machines. Meanwhile thesituation has changed dramatically. Laser fusion experiments have experimentallydemonstrated similar temperature and confinement parameters. Moreover, in 1993the U.S. Department of Energy has declassified nearly all target issues relevant to

fusion energy. Different from Japan, the U.S., and the Russian Federation, however,where traditionally strong civilian inertial fusion energy activities exist, Europeanscience policy is still ignoring this promising opportunity.

A figure of merit for the progress of fusion experiments is often given in terms of theconfinement parameter nT, which is the product of density n, the confinement time,and the plasma temperature T. Comparing the two confinement options in terms of the achieved confinement parameter withexisting facilities, inertial confinement is asadvanced as magnetic confinement: With the next generation of facilities, ITER forMCF and two new high-power Nd-glass laser facilities NIF and Megajoule for ICF,now under construction in Livermore and in Bordeaux,ignition is expected to beachieved for both fusion concepts. As a consequence,the investigation of the key issues

for ICF drivers and target physics is mandatory and ICF with heavy ion beams has anexcellent basis with the existing expertise of a large and experienced Europeanaccelerator community. Therefore, a new facility at GSI can contribute within a basicscience research concept to a great number of problems related to inertial fusionenerg y and will allow - as the most advanced accelerator facility with high-powerheavy ion beams - to carry out an excellent research and development program for thespecific needs of adriver accelerator as well as for the physics of dense plasmas whichis attractive for a worldwide community.

Intense ion beams are necessary to provide a new and powerful high precision tool tocreate and investigate extreme states of matter in the laboratory under reproducibleconditions. A fast growing community of university groups and of internationalresearch teams is getting interested in the physics of dense, strongly correlatedplasmas and should therefore be provided with resources to investigate this researchtopic. Based on these considerations the following major topics will be studied with anew accelerator facility at GSI:


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interaction processes of ions with plasma in a large regime of density andtemperature

elementary processes in dense plasma.

These include such items as high-pressure metal physics, phase transitions, shockcompressed non-ideal plasmas, hydrodynamics of beam driven plasmas, radiationtransport and opacity measurements in dense plasmas, as well as measurements of phase diagrams and the equation of state. The new facility will provide the capabilityto address relevant problems of high power ion accelerators for a variety of applications in the field of

accelerator physics ,

physics of intense beams , their transport and focusing,

target physics .

With these features a highly innovative research potential will become availablewhich already at the present level is attracting young scientists from universitygroups and research laboratories worldwide.


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4.1.2 Matter under Extreme Conditions

Since most of the matter in the universe is in the plasma state and is not accessible todirect observation, high-energy density plasmas are research objects of fundamental

interest. The dense plasma in the interior of the sun is at the origin of energygeneration for the energy needs of our planet. Stellar atmospheres, stellar dynamics,and the conditions in the interior of large planets and brown dwarfs are other objectsof astrophysical research (seeFigure 4.3)with relevance to dense plasma phenomena.The most prominent processes in these dense plasmas involve opacities, radiativetransfer, electronic transport and nuclear reactions. Plasmas in the laboratory cansimulate some of these conditions. Such plasmas are created by high currentdischarges, explosively driven shock waves, and by irradiation of samples withintense laser beams. High current ion beams are now a new tool in this field of research. The characteristic features that distinguish intense ion beams from other

tools and methods to create and analyze extreme matter properties, are due toproperties inherent to the ions and to the accelerators producing them, such as:

the energy deposition properties of ions, constituting a source of heating densematter in a volume,

the interaction of ions with the heated material, which provides new diagnosticmethods, like energy loss, charge state distribution, emission of characteristicx-rays from the target and the projectile, nuclear reactions along theinteraction region, and the emission of secondary particles and -rays,

the repetition rate capability of accelerators.Therefore, ion beams can produce interesting plasma states where, due to the ionrange in matter, macroscopic volumes of dense matter with comparatively smallgradients of temperature and density prevail, and conditions near equilibrium can beprepared for experimental investigation. These issues are therefore of interest to alarge scientific community and establish interdisciplinary links between variousbranches of physics, like atomic and molecular physics, plasma physics and nuclearphysics, condensed matter physics, statistical and many particle physics.

4.1.2.1 Interaction Processes of Ions with Plasma

Beam plasma interaction physics is a research field that derives its motivationalready from the interest in the multitude of basic phenomena in atomic, plasma, andnuclear physics that govern energy loss processes of charged particles in matter. Sincemany years, interaction processes of ions with matter have constituted a classicalresearch topic of nuclear physics, leading to a wealth of applications, particularly inmedical and material sciences. Interaction experiments were, however, restricted totargets of cold matter. Only since a few years, interaction phenomena with ionizedgases and hot plasmas have been studied. The charge state evolution and the energyloss of heavy ions passing through plasma show pronounced differences when

compared to the passage through cold gases and solid-state matter.


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First experiments to study beam plasma interaction phenomena made use of externally created plasmas, which had to be integrated into the accelerator beamline.While the route will be continued to use external plasmas as in the early experiments,the proposed accelerator schemes allow to place the emphasis on dense, stronglycoupled plasmas in the temperature range of up to several 10 eV, which are createddirectly by the heavy ion beam. The properties of matter in this temperature anddensity regime are characterized by the fact that the potential energy of the particlesdue to their mutual Coulomb interaction (e2/4od) is equal to, or even exceeds thethermal energy (kT). In order to access this area of strongly-coupled, or non-idealplasmas, and to study the effects associated with intense beams, like collectivephenomena, instabilities, and hydrodynamic target response, it is necessary toperform irradiation experiments at the highest available intensity level.

Highly charged ion species prevail in high temperature plasmas. Such conditions with

plasma temperatures of up to 300 eV and higher, depending on the actual plasmasize, can be explored in beam plasma interaction experiments with laser drivenplasma targets. With a combination of laser- and ion-beam plasmas the interestingtransition regime from non-ideal to ideal plasmas will be accessible. For this purposea kilojoule-class high-power laser is necessary. Other experimental conditions call fora laser system of the same specifications for short-time diagnostics and x-raybacklighting to study the hydrodynamics and the opacities of beam driven plasmas.These demands will be met in the near future by the Petawatt High-Energy Laser forIon EXperiments (PHELIX), which is currently under construction at the acceleratorfacilities of GSI.

4.1.2.2 Hydrodynamic Response of Beam Driven Plasmas

Energy deposition processes of ion beams in matter determine locally the properties of the heated material with respect to temperature, density and pressure [15]. Thehydrodynamic response of the target is time-dependent and is also a limiting factorfor the temperature, which can be achieved in the heating process. Investigations of the hydrodynamic target response involve time and space dependent measurements of density , temperature T, and the pressure p. The measured results are thencompared to simulations and thus test the assumptions about the equation of stateand transport properties.

Presently, no models of material behavior for pressures between 10 and 100 Mbarhave been experimentally validated. At low pressures, below a few Mbar, impact andstatic experiments provide much data showing the validity of statistical mechanicalmodels. At higher pressures few experiments have been performed in Russia and inthe U.S. with lasers, but with insufficient accuracy due to high gradients andinhomogeneities of small-scale laser plasmas. For real advances in this field asignificant progress concerning the uniformity of the irradiating driver beam and animprovement of the involved time and length scales are indispensable. Thesedemands can, however, be met by heavy ion beam heated dense matter. This effortwill substantially increase our understanding of dense fluids, especially the predictedeffects of pressure ionization and phase boundaries. In the past, experiments had


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concentrated just on the simplest and most fundamental measurements. There hardlyexist any experimental data on transport properties such as thermal and electricalconductivity at these conditions. The main effects will be based on turbulenthydrodynamics, caused by instabilities, radiative transfer and electronic heatconduction. If the latter mechanism is dominant, magnetically insulated targets couldachieve even very high temperatures.

With the unique combination of the intense heavy ion beam and a powerful sub-picosecond laser, experiments in the totally unexplored field of matter underconditions similar to those deep inside stellar objects with keV temperatures or morethan 100 times solid density seem to be feasible. Presently, with the SIS-18 beamfocused by the plasma lens to diameters smaller than 600m, specific powerdepositions up to several GW/g are obtained, and induced pressures of up to 10 kbarare accessible. With the high intensity fine focused SIS 100 beam, shockwaves in the

Mbar regime will be reached in solid targets at low temperatures of about 1 eV. Atthese parameters hydrogen, iodine, xenon and other cryogenic gas crystals areexpected to perform a phase transition towards a metallic state. Unexplored regions of the equation of state (EOS) at extremely high densities in matter will be revealed,which will give new insight into processes involved in the evolution of stars, and thecomposition of stars and large planets like Jupiter and others. The SIS 100 beam, if moderately focused will enable the generation and precise characterization of planarshocks in the range of several ten kbar. The results from experiments using this beamproperty will be used for direct measurements of the Hugoniot curve and will lead toconsiderable improvements of the EOS models and simulations.

4.1.2.3 Phase Transitions

A prominent feature of Equation-of-State physics is the occurrence of phasetransitions in cold compressed material, e.g. the insulator to metal transition of diamond at 10 Mbar, the insulator to metal transition of solid hydrogen at about 5Mbar, or the plasma phase transitions at temperatures of about 1 eV. As most of theprevious experiments were based on shock wave techniques on the principalHugoniot, these phase transitions were not accessible because they require nearlyisentropic compression. Heavy ion heated systems with their intrinsic large time andlength scales offer a promising alternative to explore these phase transitions in

precision experiments. With heavy ion beams the very interesting area of non-ideal,strongly coupled plasmas is accessible to experimental investigation. To illustrate thiscapability we show numerical simulations in this regime on the cylindricalcompression of hydrogen, which were carried out with respect to hydrogenmetallization envisaged by imploding multi-layered cylindrical targets for the SIS-100.

Wigner and Huntigton first suggested metallization of hydrogen in 1935 [16]. Theypredicted that normal molecular hydrogen, which is an insulator, would transforminto a mono-atomic metallic system when subjected to pressures of the order of 0.25Mbar. This problem has attracted much attention of a significant number of physicistsover the past decades because of the later predictions that metallized hydrogen may


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possess some very special and exotic properties. For example, Ashcroft predicted thatit could be a room temperature super-conductor [17]. If this were the case, then it willhave an immediate and revolutionary impact on our daily life. At present, the besthigh-temperature superconductors work at about 150 K ( -123 degrees Celsius) andmust be cooled by liquid nitrogen which makes them impractical for daily use.Because of its high density, metallic hydrogen could store a huge amount of energythat will be released when the solid is transformed to gas. It could thus be used as aclean and highly efficient propellant for space travel. This however requires that thesample can be quenched from a state of high pressure to an ambient state, whichmeans that the atoms of the sample will not fly apart once the imposed high pressureis released. It has been predicted by Brovman et al. [18] that once produced at highpressures, the metallized hydrogen may remain in a metastable phase even after thepressure is released. In addition to the above potential industrial applications, thesuccess in creation of metallized hydrogen would provide new insight into the

structure of the giant planets, Jupiter and Saturn.Several techniques have been used to compress samples of hydrogen by applyingstatic as well as transient pressures. The most popular method of generating staticpressure is that of a diamond anvil cell (DAC) [19]. Transient pressures have beencreated using gas guns and high intensity lasers [20]. Pressures in excess of 2 Mbarhave been generated but the final goal of hydrogen metallization has not yet beenachieved, although significant progress has been made towards the understanding of the equation-of-state of highly compressed hydrogen. This shows that the criticalvalue of 0.25 Mbar pressure predicted by Wigner and Huntigton is incorrect. Modernestimates suggest that one would require pressures in the range of 3-5 Mbar, adensity of the order of 1 g/cm3 and a temperature not exceeding a few eV in order tobring about such a transition.

We believe that an intense beam of energetic heavy ions could be a very effective anda much superior tool to create metallized hydrogen by imploding an appropriatelyshaped multi-layered cylindrical target. This is because the slow compression drivenby an ion beam will lead to a low entropy implosion that is necessary to achieve theabove physical conditions. In addition to that, the heavy ion imploded samples havedimensions of the order of a few mm, which is very large compared to those created byother methods like lasers and the diamond anvil cell. The lifetime of ion-beam driventargets is of the order of a few hundred ns, which is long enough for experimentalinvestigations but still short enough to avoid any significant diffusion of hydrogenfrom the sample. Moreover, the physical conditions of the material are quite uniformthroughout the sample. This fact eases the experimental investigations and theresults will become more reliable.

In order to design such experiments at the SIS-100 facility, we have carried outextensive numerical simulations [5,21,22]. These simulations were performed withthe two-dimensional computer code BIG-2 [23].


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Beam Parameters and Simulation Results

The target design proposed for this experiment is shown inFigure 4.4and it consistsof a cylinder of frozen hydrogen. A thick cylindrical shell made of solid lead encloses

the hydrogen region. The right face of this target is irradiated by the ion beam withan annular (ring shaped) focal spot. It is arranged in such a way that the hydrogenregion is not directly irradiated by the projectile particles. The length L of the cylinderis chosen to be less than the range (penetration depth) of the projectile ions so thatthey loose only a part of their energy in the lead shell and emerge from the oppositeface of the cylinder with a much reduced energy. Since the Bragg peak does not lieinside the target, the energy deposition is approximately uniform along the particletrajectory. Moreover, the outer radius Ro of the ring shaped focal spot is considered tobe smaller than the outer target radius Rt.

Figure 4.4: Beam-target configuration: The cylinder length L is assumed to be 5.0 mm, theradius R h of the hydrogen layer is 0.4 mm while the outer target radius R t is considered to be2.5 mm. The beam has a ring shaped focal spot with an inner radius, R i = 0.6 mm and an outerradius, R o = 1.6 mm. The beam deposition profile along the radial direction is parabolic andthe pulse duration is assumed to be 50 ns. The temporal profile of the beam power is alsoconsidered to be parabolic. It is to be noted that the range of the 1 GeV/u uranium ions in solidcold lead is about 1.5 cm while the target length is 5.0 mm which means that the target is a"sub range" target [24] and the energy deposition is uniform along the particle trajectory.

As a result of the energy deposition inside the target, a high temperature and highpressure zone is created in the lead shell surrounding the hydrogen region. A shell of solid cold lead follows the high-pressure region. The high pressure that is created inthe middle of the target launches shock waves along the radial direction, inward aswell as outward. The inward moving shock wave travels towards the axis where it isreflected and a return shock is created that moves outwards. The return shock isagain reflected from the hydrogen-lead boundary, which is moving inwards, therebycompressing the hydrogen slowly. As a result of multiple shock reflection between thetarget axis and the hydrogen-lead boundary, and due to a slow target implosion, oneachieves a very low entropy compression of the hydrogen layer that leads to the

theoretically predicted physical conditions for hydrogen metallization, namely, a


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density of 1 g/cm3, a pressure of 5 Mbar and a temperature of less than 1 eV. Thetarget is irradiated with an intense uranium beam which has a particle energy of 1GeV/u and the total number of particles in the beam pulse is 5 1011. The situationdescribed inFigure 4.4has been investigated using a sophisticated two-dimensionalhydrodynamic computer code BIG-2 [23].

Figure 4.5: Target density on a radius-length plane at t = 150 ns. The axis of the cylindricaltarget is at R=0. The beam heated material starts to compress the inner hydrogen layer

Figure 4.5 shows as result of the simulation the target density on a radius-lengthplane at t = 150 ns. It is seen that a shock wave is moving outward along the target

radius into the lead region. The lead-hydrogen boundary has moved inwards as theradius of the hydrogen-lead boundary has been reduced to 0.35 mm compared to theinitial value of 0.4 mm. This results in a compression of the hydrogen layer. Ahead of the hydrogen-lead boundary, a shock is propagating along the radial direction and isreflected at the cylinder axis. A return shock is thus generated that propagatesoutwards, along the cylinder radius. The return shock is again reflected at thehydrogen-lead boundary and multiple shock reflection takes place while the hydrogen-lead boundary continues to move inwards, slowly compressing the hydrogen layer.This compression scheme leads to the theoretically predicted metallization conditionsin the hydrogen sample, as shown inFigure 4.6and Figure 4.7.


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Figure 4.6: Density, temperature and pressure profile along the target axis at t=220 ns assimulated by the BIG-2 code [23] for the target and beam parameters as given in Figure 4.4 andan intensity of 510 11 ions.

Figure 4.7: Same as Figure 4.6 but as function of the target radius in the middle (L = 2.5 mm)of the target.


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In Figure 4.6we plot density, temperature and pressure along the target axis (at r =0.0 mm) and at t = 220 ns. It is seen that the material density is of the order of 1g/cm3, the pressure is about 5 Mbar and the temperature is of the order of 0.2 eV.Figure 4.7 shows the same variables of density, temperature and pressure as inFigure 4.6,but along the radius, at L = 2.5 mm (middle of the cylinder) and at t = 220ns. It is seen that the density is of the order of 1.3 g/cm3; the pressure is above 5 Mbarwhile the average temperature is of the order of 0.2 eV, which is quite low.

These physical conditions that are the theoretically predicted conditions for hydrogenmetallization exist inside the sample for about 50 ns, which is long enough to carryout experimental investigations. On the other hand, this time is short enough to avoidany significant loss of hydrogen from the sample due to diffusion. It is also seen thatthe compressed sample is about 4 mm long and has a 100-micron radius. Thesedimensions are very large compared to other techniques including lasers, diamond

anvil cell and gas guns. Another important advantage of using ion beam inducedcompression is that the physical conditions in the compressed sample are veryuniform which is very difficult to achieve with laser beams.


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4.1.3 Equation of State Physics

The regime of density and temperature, which is accessible through intense heavy ionbeam irradiation of matter, is shown inFigure 4.3.As long as the temperature stays

below approximately 10 eV and the density is within one order of magnitude of solid-state density, the properties of matter are dominated by strong correlation effects dueto the fact that the thermal energy is small compared to the potential energy of charged particles at small inter-particle distances. The internal energy Ei(T,) as wellas the pressure p(T, ) the so-called equation of state (EOS), which generally arefunctions of temperature T and density, are known in this regime from theoreticalmodels mainly. Only few reliable measurements exist. Even for hydrogen, the mostabundant element in the universe, recent experiments have shown large deviationsfrom widely used theoretical predictions [25]. Benchmark experiments for theequation of state therefore are an absolute necessity to gain a deeper inside into the

properties and the behavior of high energy density matter. EOS data are of greatimportance to understand e.g. the core of giant gas planets, to describe the inner coreof the earth, as well as to estimate the hydro-behaviour of matter used in inertialfusion energy scenarios. Promising results with very high pressures were achieved,when spatially uniform and steadily moving shock waves without preheat of thematerial ahead of the shock became possible by powerful laser pulses [25-28]. In theseexperiments a strong single shock was produced which drives the matter to a point onthe Hugoniot curve. Solid targets were heated up to several eV by shocks in the10Mbar regime. EOS-related studies have also been carried out with the pulsedproton beam of the Karlsruhe Light Ion Facility KALIF [30] during recent years.

4.1.3.1 Equation of state studies with heavy ion beams

The new SIS-100 is expected to deliver beam energies on the order of 100 kJ, resultingin power densities of several TW/g when focused to a moderate beam-spot of 2 mmdiameter. A rather homogeneously illuminated beam spot is essentially to produceplanar shockwaves in forward direction, with shock velocities exceeding 10 km/s. Theprogress in this field is intimately connected to the continuous development, thereliability and the achieved high standard of accelerator physics and technology [29].Intense heavy ion beams can be applied to any material of interest, either a metal or adielectric, unlike, for example, electrical wire explosion method, and only ion beams

are in general capable of depositing high energy density within a relatively largetarget volume.

To keep the entropy low during compression and allow at the same time highcompression ratios of several times the initial solid density a multiple-shocktechnique is desirable [4,5,31]. As result of the multiple shock waves, the compressionprocess consists of a series of reverberating shock waves running back and forthinside the investigated piece of matter.


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Figure 4.8: Flyer scheme and hydrogen density ratio vs. mass cell number and time.

This is illustrated by Figure 4.8, were a target layer will be impacted by a planarpiston flying with a velocity of several km/s in the range of the target sound velocity.The perspective density plot for a frozen hydrogen layer shows multiple density jumpscaused by the reverberating shocks. Since low entropy compression of a 100m thicklayer takes tens of nanoseconds, heavy ion beams from the proposed new acceleratorSIS-100 with a pulse duration of 50 100 ns seem to be well-matched drivers. Additionally, the use of these particle beams does not involve complications due totarget preheat by fast electrons or hard x-rays generated in laser-driven shockexperiments.

Figure 4.9: a) Flow diagram, b) Trajectory of a mass cell (circles) and the hydrogen isentrope forinitially 10 K.

Figure 4.9 shows a one-dimensional hydrosimulation in planar geometry with a

specific ion beam deposition power of 0.5 TW/g [32]. The flow diagram exhibits


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reverberations of the shock inside the hydrogen layer as well as the rarefaction wavefollowed by a second shock in the cold part of the gold pusher. The compression isalmost isentropic, as it can be concluded from the coincidence of the mass celltrajectories with the subsequent isentrope in the pressure-density space. SESAMEequation-of-state tables were used in this numerical experiment. A schematic view on a relevant planar slow compression experiment with the futureGSI heavy ion beam is plotted inFigure 4.10.The investigated target and part of thehigh-Z ion beam absorber (here frozen hydrogen and gold respectively) are screenedby a beam stopper. The other heavy ions deposit part of their energy in the absorberplates. An amount of 3% of an overall beam energy not higher than 1.5 kJ would bealready sufficient to get a specific deposition power of 1 TW/g in both flyer plates of this scheme.

Figure 4.10: Proposed planar compression experiment.

Two counterpropagating shock waves will compress the sample. This process may beobserved here in a direction perpendicular to the shock motion. The cold flyer platesshould also be good protectors of the compressed target layer against instabilitiesfrom the energy deposition region. Two-dimensional hydrosimulations of the schemehave shown, that rarefaction waves running inwards from the sample boundariesperpendicular to the shock motion will remarkably influence the homogeneity of the

compression process only after several tens of nanoseconds. This hydrodynamic effectcan be avoided or at least minimized only by use of sufficiently short heavy ion pulses. A careful design of this experiment would allow measuring characteristic parametersof the compression process in a near-planar geometry. Only under these conditionsthe determination of a shock velocity seems to be possible. Since a large part of the ionbeam must be screened out to meet the homogeneity demands of the proposed scheme,a powerful heavy ion pulse will be needed. The new accelerator facility at GSI is wellsuited to perform such new type of high-precision measurements of EOS data at highcompression ratios not attainable by other techniques.

The hydro processes in cylindrical targets irradiated by the ion beams with theparameters expected for SIS-100 accelerator facility were considered in detail recently


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[33,34], and it was demonstrated that with the envisioned conditions again highcompression rates of matter inside such targets can be achieved. This scheme is welladapted to the geometry of the heavy ion beam, the shock front in this geometry ishowever non-planar.

As far as the transverse intensity profile of the heavy ion beam is sufficiently smootha further experimental scenario is possible, where a nearly planar shock front isinduced in heavy ion beam excited matter around the Bragg-peak region (Figure4.11). This shock front situation will be used to determine the equation of state (EOS)parameters of the material under consideration. Beam target interaction simulationsshow that e.g. in gold plasmas induced from an intense uranium ion beam,temperatures of up to 50 eV are to be expected at the Bragg peak region. This willrelease a shockwave in forward direction, traveling with a speed of approximately10 km/s. At the same time it will induce a temperature wave with rather constant

amplitude of up to 10 eV. The density in the shock front will rise to more than twicethe solid-state density. In this process hot, dense plasma is created. The emittedradiation spectrum is almost entirely characterized by the properties of blackbodyradiation. The peak wavelength emitted by a blackbody radiator at a temperaturecorresponding to 10 eV is centered around 20 nm, a wavelength region that is opaquein almost all materials [35]. Direct access to the light emitting volume of the purematerial thus is not possible in most cases, and comparative measurements arehindered by different opacities of the materials involved. On the other hand, for mostother beam target combinations, shock velocities, as well as temperatures will bealmost one order of magnitude less, strongly reducing the amount of emitted light,

and the access to the light-emitting region will be easier.

Figure 4.11: Schematic target configuration for shock velocity measurements. The ion beam B isstopped in a heavy pusher material P, releasing a shock wave towards the target material T.The step in the target will have a height (depending of the calculated shock velocity) on theorder of 100 m, resulting in a time difference of shock arrival on the order of 10ns, easily to bemeasured by optical means.

To deduce equation of state data, usually shock velocity, matter velocity, andtemperature are the measured observables [36,37]. Shock velocity is generallyobtained from a planar shockwave, which is moving towards a step, perpendicular tothe shock direction, as indicated inFigure 4.11.The time difference of the arrival of

the shock front at the outer surface of the two steps can easily be converted to the


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shock velocity. However, the hydro dynamical properties of the target have to becalculated beforehand, in order to design the step height and step extension properlyaccording to the shock velocity and shock shape conditions. In general, a typical stepheight will be on the order of 100m, resulting in a time difference of shock arrival onthe order of 10 ns with a shock velocity of 10 km/s. Using a streak camera with 10 psresolution, shock velocities thus can be derived with an accuracy of a few percent.Care has to be taken to investigate only the planar region of the released shock, butwith proper optics a spot size of about 200m dimension will be sufficient.

When the shock reaches the surface, the compressed and heated matter willevaporate into the surrounding vacuum of the target chamber. Matter velocities aresignificantly lower than shock wave velocities, reaching from 0.5 to 5 km/s. Severaltechniques can be used to measure these velocities, with time resolved shadowgraphyor schlieren measurements being the most widely used ones. Careful analysis of the

beam shape is inevitable in any case, since an Abel-inversion of the optical density inthe shadow has to be performed. Also, time resolved Doppler broadening and Dopplershift of an impinging and reflected laser beam can be used to study velocitydistribution of evaporated particles. Since time scales are on the order of severalhundred ns up to microseconds, particle velocities can be measured with the same oreven better accuracy than shock velocities.

Figure 4.12: Schematic target configuration for temperature measurements of target T and pusher material P. The ion beam B releases a shock wave inside the pusher, which compressesand heats up the target. The thermal radiation can be observed through a cladding C, which

prevents the target material from immediate evaporation. Thermal radiation of the pusher andtarget material will be focused by mirrors M to the analyzing spectrometers S.

The third entity to be measured is target temperature. Since a direct temperaturemeasurement by blackbody radiation analysis inside the beam-target interactionvolume is not possible in the range from 1 to 30 eV, an indirect method of measuringthe temperature on the target surface, induced by shock wave heating, can be applied.Hydro-calculation, including beam-stopping behaviour in a heated and expandingtarget has then to be compared with the results, measured. At a plasma temperaturerange, not exceeding 20 eV, blackbody radiation can be expected. For each projectile-


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target combination, thus the appropriate wavelength regime to be observed has to becalculated and measurements shall be performed in a way that during the time of measurement, the peak of the Planck-curve shifts through the observed wavelengthinterval. Since almost any solid material absorbs radiation in the wavelength regimeof interest, a suitable transparent surface layer of well-defined thickness has to beapplied to the target. This cladding is going to prevent evaporation of target materialduring the observation time, at the same time allowing blackbody radiation to beemitted from inside the compressed matter. A schematic sketch of the target is shownin Figure 4.12.

For any expected target surface temperature up to 5 eV, resulting in a Planck peakwavelength of about 50 nm, a several hundredm thick layer of neon is proposed, orargon, in case the blackbody peak emission happens to coincide with excitonicemissions of crystalline neon. Neon crystals are completely transparent essentially

down to the neon resonance lines at 74 nm, with correction to be applied atwavelengths lower than 85 nm. Depending on the actual strength of the compressionwave, direct access to the pure target surface is given, while at a temperature of morethan 2eV the compressed and heated neon itself is radiating as a blackbody beingexcited to an estimated electron density on the order of 1020cm-3 according to the Sahaequation at solid-state density. For compressions, resulting in temperatures in excessof 5eV, the regular VUV spectroscopy is no longer perfectly suited, since the peak of the Planck curve shifts towards 10 nm at a temperature of 20 eV. EUV/x-rayspectroscopy, combined with VUV spectroscopy thus is proposed to be applied. Thecladding's best choice in the short wavelength range therefore is lithium, with a

20 nm thick protective layer of LiF. This combination can be used down to the cutoff wavelength at about 23 nm. At even shorter wavelengths beryllium might be theappropriate choice. However, since the cladding is evaporated during the experiment,safety aspects associated with the use of beryllium have to be considered, and boronmight serve the purpose equally well. In any case, the optical depth of the surfacelayer is on the order of one micrometer, which results in an observation time of 0.1 to1 ns. A relative calibration of the monochromators is mandatory, since thetemperature has to be determined by the shape of the recorded spectrum incomparison to blackbody radiation.

4.1.3.2 Two-sided Irradiation of Cylindrical Targets

The key part of the planned new accelerator facility is a synchrotron complexconsisting of two separate synchrotron accelerator rings, in addition to the alreadyexisting synchrotron SIS-18. This opens the possibility to irradiate the target withdifferent ion species of the same magnetic rigidity at the same time. This feature isespecially interesting for diagnostic purposes of an ion beam heated target.Furthermore the synchrotron complex may also be used to irradiate a target fromopposite sides simultaneously. This feature is not implemented in the present layoutfor the new facility and is regarded as a promising option of the new acceleratorfacility. Phase-locking the RF of both machines would allow to vary the arrival time


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on the target and allow to study collisions of shock waves, an important aspect inunderstanding the dynamics of Supernovae.

Figure 4.13: Irradiation geometry for the following simulations. The shad ing illustrates theintensity of the local deposition, and the arrows the shock wave propagation directions.

As a start we have studied the collision of two opposite beams of about equalproperties. The first series of simulations were devoted to the possibilities of generating shock waves in quite diverse configurations using two beams.Figure 4.13illustrates the generic possibilities: overlapping the deposition regions yields alocalized region of higher temperatures, while separating them leads to the collision of shock waves with possibly higher densities achievable. These cases were compared tothat of a one-sided single-beam illumination. The simulations were carried out usingthe codeCaveat [38]. The target was a solid gold cylinder of 3 mm radius and with alength adjusted to produce the various situations depicted above (6 mm for the uppertwo cases, 8 mm for the colliding-beam case). For the beams a flat temporal profile of 19 kJ in 50 ns was assumed and the deposition was calculated based on a SRIM [39]simulation assuming an ion energy of 500 MeV per nucleon, which yields a total rangeof about 3.5 mm.

This ion energy was selected because the range is comparable to the radius, yieldingan attractive geometry: for different ion energies effects will be analogous. The totalenergy corresponds to 1012 ions in the pulse. The radial profile of the beam was takento be one of two characteristic cases: constant up to the maximum radius of 2 mm orGaussian with =1 mm. As is well known [40], the two types of radial profile showquite different behavior, but here we concentrate only on the features due to the newirradiation geometry. The time dependence of the maximum density in the target isshown inFigure 4.14.


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Figure 4.14: Time dependence of the highest compression in the target for the different scenariosstudied.

It is clearly seen how the higher deposition leads to enhanced compression with astrong increase when the two shock waves collide. The additional strong jump forflat deposition in the colliding shocks case caused by the inward convergence of the

material due to the higher deposition in the outer radial parts of the target. In thetemperatures, the overlapping case with the addition of two deposition regions showsthe highest temperatures. The maximum values are also summarized in the followingtable:

Beam profile Case max [g cm-3] Tmax [eV]

Flat single pulse 34.4 14.6

overlapping 32.3 13.0

colliding 50.4 13.1

Gaussian single pulse 34.3 14.6

overlapping 25.5 18.6

colliding 43.5 14.0


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While the overlapping of the deposition regions seems to offer only the advantage of higher temperatures in the zone of enhanced deposition, the increased densityachieved in the colliding shock wave situation is clearly pronounced and it has theadditional advantage of being at rest after the shock collision.

Figure 4.15: Energy deposition in a shorter target for one- and two-sided irradiation.

For the flat radial profile the high-density region is a thin sheet at rest in the centerof the target but expanding and is at relatively low temperatures of less than 2 eV.For the Gaussian beam profile the situation is more complicated and less useful.Because of the higher speed of the shock waves near the axis of the cylinder, thecollision happens first on the axis and then the high-density zone expands as a ring tothe sides as the collision point moves outward. An additional possible use of overlapping deposition regions could be to heat the target in a more homogeneousway. Although the most prominent feature of the deposition curve is the Bragg peak,it should be noted that the energy loss always has a noticeable slope. To study thiseffect we compare the energy distribution in a target irradiated from both sides, butonly 1.25 mm long in order to avoid the Bragg peaks, with that irradiated by a beamof double the intensity from one side only (seeFigure 4.15).

While the deposition from both sides does not provide a perfectly homogeneousheating, the advantages are clearly visible: instead of the strongly increasingdeposition for a single beam, we get a much more, though not completely, constantbehavior for the two-sided deposition. To investigate the practical consequences, westudied a case of a hollow Lead cylinder irradiated by a hollow beam in aconfiguration very similar to that investigated by Tahir et al. [41]. The simulationswere done using Caveat [38]. A hollow lead cylinder of 0.5 mm inner, 2.5 mm outer

radius and 1.25 mm length is irradiated by a beam confined to the radial range 0.5 up


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to 1.5 mm. The beam consists of Uranium ions with an ion energy of 200 MeV pernucleon delivered over 50 ns. The total beam energy is 1.525 kJ, the temporaldeposition profile is parabolic, while the radial one corresponds to an invertedparabola between the inner and outer radii of the beam. The deposition is againcomputed using SRIM [39].

Figure 4.16: The inner part of a hollow cylindrical target as discussed at a time close to thecollision on the axis in each case. The colors indicate density as shown by the scale on the left.The upper plot is for single-sided and the lower one for double-sided illumination by the heavy-ion beams.

The hydrodynamic flow caused by the beam converges to the cylinder axis and thedensity distribution near the time of hitting the axis is shown inFigure 4.16. Asexpected, the absence of the Bragg peak because of the shorter length of the targetmakes the inward motion already a bit more uniform. In the case of one-sidedillumination, the axis will clearly be hit in a point which moves to the right with time,while for the two-sided illumination there is a much more uniform region of highdensities created, which also stays stationary for some period.


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4.1.3.3 Magnetized implosions driven by intense ion beams and radiationphysics

Once intense beams of heavy ions become available through the proposed accelerator

scenario they will immediately open new ways to generate high energy density statesin matter. Due to special energy deposition features of the energy depositionmechanisms by fast heavy ions, such experiments will make a contribution of theirown to fundamental research into the structure and properties of matter underextreme conditions.

Self Sustained Magnetized Implosions

Since the beam deposition characteristics of high-energy heavy ion beams in densematter favor a cylindrical geometry the most simple experimental scenario will be aquasi-cylindrical plasma volume created by focusing an ion beam onto a uniformsample. The peak pressure and temperature values that can be attained in this wayhave been analyzed recently at GSI [43] and it was demonstrated that intense heavyion beams and the cylindrical target geometry are well suited for implosionexperiments. The aim of such experiments is to obtain states of matter, which arecharacterized by a maximum concentration of energy. Hydrodynamic expansion is inthis case considered to be an effect, which is adverse but unavoidable. At the sametime, hydrodynamic flow in a converging geometry can well be employed to enhancethe initial energy concentration created by the energy deposition of the ion beam. Aspecial beam focus geometry, e.g. an annular beam is necessary for this purpose. Alsorecently an annular beam focus has been experimentally demonstrated with the

plasma lens [42]. In such implosions the initial pressure, which is generated by thedirect heating of target material by deposition of heavy ion beam energy inside thetarget volume, can be enhanced by more than a factor of 10. Moreover, the cylindricaltarget geometry offers simple methods for magnetization. If an external magneticfield is introduced, the effect of magneto-thermal insulation may allow to reachextremely high temperatures accompanied by significant thermonuclear neutronyields even with beam intensities that result in a specific deposition power of someTW/g [43,44].

Hydrodynamic consistency between the total amount of energy deposited into thetarget ( 100 200 kJ/g) and the focal spot radius (1mm) sets a limit on the pulseduration tp for the ion pulse below 100 ns, which is in agreement with the designparameters considered here. The imploding configuration consists of a hollow metallic(Au) cylinder, filled with low-density deuterium gas. The heavy ion beam with anannular focus impinges onto the target parallel to the cylinder axis and heats thetarget material uniformly along the cylinder axis inside the volume determined by theouter and inner focal radii. In this case the cavity is imploded by the cold innerportion of the liner, which is accelerated inward by the amount of heated mattersurrounding it. The peak pressure values obtained in 1-D simulations range up toalmost 500 Mbar. In reality, drive asymmetries, target imperfections and instabilitiesnear the stagnation state will distort the 1-D results towards more moderate plasmaparameters.


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Figure 4.17: The principle of magnetized targets for heavy ion beam experiments. The focus of the ion beam is annular. Highest plasma parameters are obtained where the annular beam

focus heats the liner material only in the annular focus region and the remaining cold portionof the liner is accelerated inward by the beam heated material. An initial magnetic field of approximately 30 T is required to reach the self-sustained magnetic implosion regime.

The situation may considerably improve with an additionally applied magnetic field,which is introduced in axial direction into the target plasmas shown inFigure 4.17.Aseries of detailed magneto-hydrodynamic (MHD) simulations, taking into accountparameters, which are within reach of the SIS-100 facility, have been performed byBasko [43]. The results indicate that it is possible to obtain a situation, where a self-sustained magnetic implosion (SSMI) is achieved. In this situation the plasmatemperature of the low-density deuterium gas exceeds the temperature of the non-magnetized situation considerably and is expected to reach a value as high as 1 3keV. However, the threshold for the self-sustained magnetic insulation (SSMI) regimecan only be met with an initial magnetic field of B0 = 30 T, which certainly is achallenge to the experimental conditions.

Radiation Physics

When the plasma temperature approaches 100 eV radiation becomes more and moreimportant and finally dominates the energy transport and the energy partition in theplasma. This regime has been addressed by high power lasers like NOVA at LLNLwhere Planckian temperatures above 200 eV have been measured, and by light ions atSandia National Laboratories where a hohlraum temperature of 160 eV has beenachieved with multi-Mega-Ampere beams of Li at an energy of 9 MeV. It is highlydesirable that a heavy ion facility can participate in this region of radiation-dominated physics to complement the other high energy density facilities with itsunique features. Except for special cases of low density plasmas as discussed formagnetically insulated targets it will be the low temperature regime that will beaccessible with the planned facility. However, the prospects for future improvements


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by a high current low charge state injector e.g. with U4+ ions and improved bunchingcapabilities are tempting. Therefore the radiation physics issue is discussed herebriefly.

The radiative opacity of the material plays a key role in any object where radiationdominates the energy transport mechanisms like in stellar objects and radiativelyheated hohlraums. Opacity controls the flow of energy inside stars and in particularthe instability of stars is especially sensitive to nuances of the opacity. Thereforeprecision experiments are necessary to optimize the models of stellar structure. As theradiative opacity is a harmonic mean of the spectral-dependent opacity which dependson the atomic structure of the involved matter, experiments in this field couldstimulate the advance of the theoretical description of highly charged ions in plasma,like plasma ionization balance, the rate processes, spectral line shapes and transitionenergies. Opacity simulation codes having recently been reviewed at an international

workshop in Garching have made large progress. However, those simulation codesavailable have to be benchmarked with experimental data.

The conversion efficiency of a converter in an inertial fusion scenario also dependsstrongly on the radiative opacity of the involved material. High-Z materials haveshown good conversion efficiencies, because of their large radiative opacities. Butactual converter targets will probably consist of a delicate mixture of high and low-Zmaterial in order to have an optimum of both properties, radiation conversion andradiation transport. Such issues have also to be addressed in experiments with heavyion beams.

The conversion of kinetic energy of the ion beam into radiation is a fundamentalphysics question. There exists no substitute experimental technique to address thisproblem other than to use the most intense ion beams to irradiate a target andmeasure the emitted radiation spectrum. One important goal for the new facilityshould therefore be to obtain radiation temperatures as high as possible, even insmall samples of matter. An essential issue in this context is that of preheat by lightfragments and other side products of ion beam interaction with matter, such aselectrons, -rays and nuclear reaction products. The influence of these products on thehydrodynamic flow can be studied with irradiated planar targets and x-raybacklighting techniques. A related problem that can be studied is that of radiative

smoothing of surface inhomogeneities causing hydrodynamic instabilities.Currently, the SIS-18 beam creates samples of high-energy density in matter withtemperatures of the order of 1 eV. Numerical simulations predict [33] that after thefinal completion of the intensity upgrade and the bunch compressor, several eV can beachieved in a solid lead target. The SIS-100 facility will deliver a much more intensebeam compared to the SIS-18. It is expected that the SIS-100 beam will deliver 1012 particles of uranium in a pulse length of about 50 ns long. Simulations show that asuitable energy for plasma physics experiments is 400 MeV/u [34]. The range of theseparticles in cold lead is about 4.25 mm. In our simulations a cylindrical lead target

with 2 mm length is considered. The target is thus sub range as the particles deposit


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only a fraction of their energy in the target. Since however the Bragg peak does not lieinside the target, the energy deposition is uniform along the particle trajectory. Thisgives rise to a uniform heating. The simulation shows that with the specific powerdeposition of about 4 TW/g a temperature of the order of 20 eV is created. Thecorresponding pressure of this target is about 6 Mbar.


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4.1.4 Intense ion and laser beams synergetics with the PHELIX laser

The new prospects of the SIS-100 accelerator to generate dense plasmas forfundamental research will be further complemented and enhanced by the Petawatt

High Energy Laser for Heavy Ion Experiments (PHELIX) laser system, which iscurrently under construction [37]. This project was developed at GSI realizing theunique synergetic possibilities offered by the combination of a high power laser andan intense heavy ion beam in various fields of fundamental research, such as plasmaphysics, atomic physics, nuclear physics, laboratory astrophysics and materialresearch and is highly regarded within these communities world-wide. The new areasof research accessible with the new SIS-100 will further improve the outstandingposition of GSI in these fields of basic research. The combination of intense heavy ionbeams and high power laser in the kJ regime with the additional option of shortpulses to achieve a laser power in excess of 1015 W will be a unique research tool for

many years. Leading laser laboratories worldwide have immediately supported thisproject and have offered their collaboration. GSI is therefore able to attract a newcommunity and will be able to provide unparalleled research conditions to scientistsfrom university groups and research teams on an international scale.

Especially for the exciting and widely unexplored field of dense plasma research it isof great interest to access the matter properties, for example present in the interior of the earth, giant planets and stellar atmospheres, in the laboratory to preciselydetermine the characteristics of this matter under such extreme conditions. Thereforeit is mandatory to transform a macroscopic sample of matter into a rather cold, buthighly compressed state and furthermore to determine its characteristics with thehighest amount of accuracy.

Moreover, the combination of a high power laser with the intense, short ion bunchesdelivered by the SIS-100 allows not only the measurement of the matter properties,but it will also be possible to manipulate these exotic states of matter in a controlledand reproducible way in the laboratory for the first time.

4.1.4.1 PHELIX as a unique diagnostic tool

The investigation of dense plasmas is a challenging task, because most of thestandard diagnostic means, which are well known from atomic and plasma physics,fail due to the high density and exotic electronic behavior of the sample to be probed[45]. Spectroscopy in the visible and near UV-range is usually not applicable due tothe huge opacities of the sample. Similar restrictions appear to interferometricmeasurements and optical imaging. Moreover, detectors, which are mechanicallyattached to the sample, are either subject to destruction, or may alter the conditionsof the sample in a way, which decreases the accuracy of the measurement.

Furthermore, these exotic states of matter are available in the laboratory only in ahighly transient state. These kind of experiments therefore require a high temporalresolution of the diagnostics which has to be at least of order of nanoseconds or evenbetter. However, well known techniques to investigate plasma states at and above the


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solid state density have been developed and tested at many laboratories worldwideand are employed and developed by the GSI plasma physics group for several yearsalready [46-50].For the diagnostic techniques presented in this section, we generallythink of the diagnostic radiation as an illumination produced behind the targetmaterials and detected after passing through the target. Hence, the light source iscommonly called a Backlighter. The propagation of electro-magnetic waves in anyplasma is restricted to densities, where the density dependent frequency of plasmawaves is lower than the incident frequency of the light wave. The density dependentplasma frequency is given by

p = (nee2/0me)1/2

where ne, e, and me are the electron density, charge and mass and0 is the electricfield constant.

Therefore, visible and ultra-violet light is no longer propagating in dense plasmas. Toinvestigate the rapidly changing properties of dense plasmas at or well above the solidstate, short x-ray bursts of high brightness are required. High-Z, laser generatedplasmas are known to be suitable sources of such radiation and are therefore used asdiagnostic tools in many laboratories [51-54]. The efficiency to convert laser light intohigh energy x-rays of 10 keV or more depends on the irradiance provided by the laserbeam. In order to provide a sufficient x-ray intensity the irradiance of the laser beamshould be at least 1015 W/cm2 or even higher. The spatial resolution of theexperiments requires a spot size diameter below 50 m.

The GSI- PHELIX system is designed to serve as a versatile driver for x-raybacklighting of heavy ion driven targets. The laser is equipped with a front-end,which is able to generate multiple laser pulses of sub-ns pulse duration, with eachpulse to be timed independently. Therefore transient phenomena, like the dense,strongly coupled plasmas generated by the SIS-100 can be investigated with hightemporal resolution.

Backlighter scenarios to investigate plasmas generated by the SIS-100:

The point projection requirement applies for imaging techniques in the photon energy

range between 4-12 keV, and a 25m spatial resolution. For x-rays in this energyrange a laser intensity of 1-30 PW/cm2 is required The resolution requirement sets thespot size to about 50 m for a standard magnification m=2. To avoid motionalblurring, the maximum acceptable pulse length is between 0.5 and 1 ns. To providemultiple snapshots on a single heavy ion experiment, multiple pulses are required.For experiments to investigate the temporal evolution of sample features that areeither 1D, repeating, or random, one can simply gate different segments of a singlepoint-projected x-ray image at different times. This allows for a single laser focus tobe sufficient (Figure 4.18). If it is necessary to view the temporal evolution of samplesthat are 2D or non-repetitive, one needs, a whole series of short pulses, and spatiallydistinct point backlighters (i.e., laser foci) for each frame. The laser foci are typicallyseparated by a few millimeters (Figure 4.19).


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Figure 4.18: Examples for dense plasma diagnostics. Single point projection geometry. Thewhole temporal evolution of a 1-D sample can be studied in a single shot experiment.

Figure 4.19: Multiple point projection. The laser spots are separated by a few mm due to a shiftof the laser wavelength for each pulse. This allows for the analysis of 2-D samples (tomography)with high temporal resolution.

The area backlighting requirements apply for 1-3 keV photon energies overapproximately a 1 mm spot. A uniform spot of ~1 mm can be created by inserting anappropriate random phase plate. An irradiance (power) of 0.1-0.3 PW/cm2 (1-2 TWtotal power) is required to create sources with x-ray energies of 1 to a few keV over


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this spot size. The area backlighting option allows for absorption spectroscopy, whichprovides opacity information of the sample (Figure 4.20).

Figure 4.20: Example for large-area, low-energy (1-2 keV) backlighting. To provide ahomogenous focal spot, the laser beam can be smoothes by SSD (smoothing by spectraldispersion).

The measurement using x-rays can be extended and complemented by the use of short, intense pulses of energetic protons. These intense, low-emittance ion beamshave recently been discovered at experiments with short pulse lasers and are subjectof great interest since [55]. Petawatt class lasers, like PHELIX, have demonstratedtheir capability of generating intense (several hundred kiloamperes), short (severalpicoseconds), energetic (up to 60 MeV), beams of protons with an excellent beamquality (normalized beam emittance ~ 0.2 pi mm mrad), which are suitable forradiographic measurements. Due to the different interaction mechanism, these beamsmay provide complementary information about the properties of matter underextreme conditions. One unique aspect of laser accelerated protons as compared toconventional RF accelerated beams is that the instantaneous beam current can bequite large, that is, a huge number of protons can be accelerated in a single, shortpulse, having a duration of order of picoseconds, comparable to the laser pulseduration. Therefore transient phenomena like dense plasmas can be studied withhighest temporal resolution [56].

The SIS-100 will be a unique tool to adiabatically compress matter to densities wellabove the solid-state density while maintaining a relatively low temperature. Theresulting strongly coupled plasma is enclosed by the compressing pusher material.The pusher that is heated by the intense heavy ion beam is made of high-Z materials

in order to effectively convert the heavy ion beam energy into kinetic energy of the


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pusher. Therefore, x-ray radiography does not work well for the imploding sample,because of the low x-ray attenuation (especially in case of hydrogen samples) of thecompressed plasma, and because the surrounding pusher will attenuate the x-raysmuch more than the compressed matter to be investigated. Here, proton energy lossimaging can be used to directly image the sample throughout the entire course of theimplosion.

Figure 4.21: First experimental demonstration of short pulse proton radiography by laseraccelerated protons. The target consisted of four layers of plastic (Kapton) placed behind a high-Z shielding (see left inset). The proton beam was accelerated by a 100 TW laser at a few cmdistance from the sample. The result demonstrates the capability of laser-accelerated protons toimage the light element with excellent resolution. The total time of exposure was just a couple of

picoseconds.

Figure 4.21shows the first experimental proof of this technique performed within aninternational collaboration at a 100 TW laser (LULI, Ecole Polytechnique). Several

layers of low-Z plastic were mounted behind a high-Z (Tantalum) shielding. The resultdemonstrates the unique capability of laser accelerated proton beams to image a low-Z sample. In combination with x-ray absorption measurements, proton beam imagingwill allow a direct measurement of the matter opacities, which then provides a wholeset of information about the atomic structure of matter at high pressures.GSI is theonly facility in the world with a powerful Petawatt laser system in combination to anintense heavy ion driver. Therefore it will be an outstanding place to investigatematter properties for example present in giant planets or the interior of our earth.


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4.1.4.2 The combined capabilities of PHELIX and the SIS-100

The SIS-100 with its intense, short heavy ion pulses can generate dense, stronglycoupled plasmas superior to other methods. Thus it will become possible to explore

the regime of this state of matter with high accuracy for the first time. Augmented bythe unique capabilities of the PHELIX laser the combination of these two facilities canextend the range of measurements in new areas.

Laser light propagation in dense plasmas:

Since ultra-intense laser pulses are now available for experimental research it hasbecome of great interest to study propagation properties of such laser pulses inplasmas. This issue is also of importance for a variety of applications like short pulselaser cutting of sensitive material or the so-called fast ignitor approach to inertialconfinement fusion [57]. Irradiation at a level of 1019 W/cm2 and higher results in anew regime of plasma physics, the relativistic plasmas, where the mean oscillatoryvelocity of the electrons becomes close to the speed of light [58]. Therefore the electronmass changes significantly, the frequency of electron plasma waves changes and thelaser light propagation is therefore modified with respect to the regular laser-targetinteraction. Moreover, the light pressure reaches magnitudes in the Gigabar rangeand is therefore similar or even exceeding the pressure in the center of the sun.Nonlinear effects lead to self-focusing and channeling. For experiments investigatingthese phenomena clean initial conditions are the key to precision measurements. Theion beam delivered by the SIS-100 is able to create volumetric plasmas of high densitywithout the presence of large gradients that reduces the accuracy of the experiment.

Hence, the combination of a powerful heavy ion accelerator to prepare the samplewith a short pulse laser to interact with opens the possibility to precision experimentson light propagation in overdense plasmas.

Equation of state and the characterization of dense plasmas:

As presented in this proposal, the SIS-100 will be able to generate highly compressedstates of matter with clean, well-known initial conditions. In addition, once thesample is prepared by the ion beam the physical conditions can be altered by thePHELIX laser within a time span much shorter than the hydrodynamic response timeof the matter. For example, a sample of metallic hydrogen can be heated by the shortpulse laser either on one side by direct irradiation or heated throughout the volumeinstantaneously by laser generated ion beam impact. Whereas for example thepropagation of shock waves and heat conduction can be studied in the first case, theresponse of the target properties to sudden temperature variations and the successivephase transition could be explored in the second case. To summarize, due to thecombined capabilities of laser and ion beam drivers, the door to a whole new set of experiments in dense plasma physics may be opened.


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Additional synergetic effects:

The use of the GSI plasma lens as a superior final focusing device will be explained indetail in the technical part of the proposal. This instrument has been in use at GSI for

several years and has also been proven to be an important device for high energydensity in matter research [59]. Using a powerful laser to drive a micro-plasma lens,as proposed in several international conferences may further reduce the final spot sizeand hence increase the specific energy deposition of the intense heavy ion beam fromthe SIS-100. The laser beam, irradiating a solid target expels a huge number of electrons from it, thereby charging the target to high voltages. If a secondary target isplaced in close vicinity to the irradiated one and bathed into these electron cloud, oneeffectively created a fast charged capacitor. If this voltage is released in a connectingcircuit, designed like a small plasma lens, huge currents may be generated that willfocus a passing ion beam due to the induced magnetic fields. This may drive heavy ion

beam irradiated targets, mounted closely behind the lens to high temperatures andfurther extend the range of plasmas accessible with the new facility.

Figure 4.22: Remnants of the supernova 1987a. The ring structures are caused by theinteraction of the propagating shock waves with the interstellar background medium.


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Supernova shock wave experiments:

The propagation of shock waves of a supernova explosion into the interstellar mediumis of general astrophysical importance because it releases the heavy elements created

in the final phase of the supernova into the universe and mixes them with thebackground medium (Figure 4.22). At a certain point of the shock wave propagation itis pumped with energy dumped by the huge amount of neutrinos (~ 1046 J) created inthe supernova explosion. The response of a propagating shock wave on a suddenenergy deposition is therefore an interesting topic of astrophysics research. With thecombination of the SIS-100 and the PHELIX laser, this question can be studied in thelaboratory. A shock wave, launched by a tailored laser pulse from the PHELIX laserpropagates through a sample that is then volumetrically heated by an intense heavyion pulse, delivered by the SIS-100. The response of the shock wave, its velocity, oreven a breakup, predicted by numerical calculations, can be studied under well-

defined experimental conditions repeatable in the laboratory.Plasma spectroscopy in high-energy astrophysics:

The observation of violent events, like quasars, exploding galaxies or pulsars, refers tothe detection of radiation in the x-ray andray regime extending to optical and UV-light to reveal the nature of the emitting objects. The astronomers problem is toobtain a spectrum showing as much details as possible. However, a second problem isto have the knowledge base on non-equilibrium spectra that allows inference to bedrawn from the data. One poorly understood area is the low-temperature radiationdominated plasma. These plasmas are irradiated by dilute sources and very far from

local thermal equilibrium (LTE). One of the uncertainties is the plasma energetics.Perhaps a lot of energy is tied up in states that are not evident in the spectrum. Oneexample is the Bowen mechanism. In 1924 Ira Bowen found, that in some nebulae theemission lines of OIII arising from the upper state 2p3d3P2 were greatly enhancedrelative to other members of the same multiplet. She discovered that there was acoincidence between He Ly and the OIII line producing fluorescence. The Bowenmechanism was analyzed theoretically in the 60s and 70s but still does not have asecure theoretical footing and needs experimental verification.

An experimental exploration can be done creating a large volume of plasma with a

temperature of 2-3 eV in which oxygen and nitrogen are mostly doubly ionized. Asecond region of hotter plasma that serves as a light source for the first may thenilluminate this volume under investigation. The column density of the large plasma,perhaps 1020 cm-2, could be ample to provide the necessary optical depth in He Ly andperhaps also in OIII 374,436 as well. Whereas the new SIS-100 will be an ideal toolto homogeneously drive the large plasma avoiding the large amount of backgroundradiation present in laser driven plasmas, PHELIX will be able to serve as a driver forthe second, hotter plasma. This approach shows the unique experimental possibilitiesavailable with such a combination for a whole variety of experiments in laboratory-based astrophysics.


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4.1.5 Spectroscopy experiments with intense ion and laser beams toinvestigate dense plasma properties

4.1.5.1 Introduction

The planned new accelerator facility combined with a high-power laser opensoutstanding future prospects for novel spectroscopy experiments to study theproperties of hot, dense matter. The parameter regime of future GSI experiments isexceptional, and standard methods of spectroscopy almost entirely fail and can hardlybe modified to serve for the future needs.

Figure 4.23: Space resolved x-ray image of magnesium plasma produced with the nhelix laser.He-like resonance and intercombination lines show strong emission up to cm-distances from thetarget surface (Z' = 0.00 cm). Li-like satellite emission is much more restricted in space

Since 1999 "Spectroscopy" has been included into the plasma physics activities at GSI[35,36]

Ion and Laser Induced Plasmas – High Energy Density in Matter

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Transcript of Ion and Laser Induced Plasmas – High Energy Density in Matter