High-Intensity Ion Beam Neutralization and Drift ......Applications and techniques of ion beam...

Applications and techniques of ion beam focusing NTX and NDCX-1 experiments Simultaneous beam compression Summary

High-Intensity Ion Beam Neutralization andDrift Compression Experiments

Prabir K. Roy1

1Lawrence Berkeley National Laboratory (LBNL)Berkeley, CA 94720

March 08, 2010

(with S. S. Yu, P.A. Seidl, and B.G. Logan et al., HIFS-VNL Collaboration)

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Outline

1 Applications and techniques of ion beam focusingIon beam applicationsTechniques of ion beam focusingBeam space charge neutralization for radial compressionDrift compression of an ion beam

2 NTX and NDCX-1 experimentsBeam neutralization resultsDrift compression results

3 Simultaneous beam compressionPlasma channel with a final focus solenoidAxial plasma densityFabrication of a new plasma sourceDesign and fabrication of a radial plasma probeOutcome of initial beam compression experiments

4 Summary2 / 37


Ion beam applications

Intense ion beams of low kinetic energy offer an attractiveapproach to heat dense matter uniformly to extreme conditions

1 Ion beam energy deposition(dE

dx

)is nearly classical and

shock-free.2 Ion beams have the ability to heat target materials

conductorsinsulatorsfoams, and powder

with high repetition rates.

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Ion beam applications

High energy density physics and heavy-ion-driven inertial fusionrequire the simultaneous transverse and longitudinal beamcompression of an ion beam to achieve high intensities

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Techniques of ion beam focusing

Small beam spot sizes and short pulse lengths are achievable withbeam neutralization and longitudinal compression.

The beam travels along the z axis with velocity vz . Both typesof compression are critical for achieving high intensities ontarget, since power per unit area is ∼ 1

r2b

and ∼ 1tp

.

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Beam space charge neutralization for radial compression

In neutralization experiment, defocusing nature of the electricself-fields of space-charge-dominated ion beams is reduced byspace-charge neutralizing using plasma

Electrons from a plasma areentrained by the beam andneutralize the space chargesuch that the pulse focuses onthe target in a nearly ballisticmanner to a small spot, limitedonly by longitudinal andtransverse emittance. Typically,

np

Znb≥ 1

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Beam space charge neutralization for radial compression

Neutralization and beam transport in plasma

1 Key scaling parameter for beam transport is the perveance(ratio of the beam space charge to kinetic energy).

k =2Ib

IAB2i

, IA =βiγimic3

eZ,

where, IA: Alfven current and IB: Beam current2 Provided kmi

kme≥ 1, electrons from this plasma can

accelerate in the beam space-charge potential to the beamvelocity. This condition limits the minimum residual spacecharge potential to 1

2mev2i

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Drift compression of an ion beam

Concept of longitudinal beam compression

The axial compression is achieved with an inductionbunching module (IBM)

Compressed beam bunchhas higher space chargedensity than uncompressedbeam bunch section. Thishigher space charge cancontribute to beam blow-upbefore reaching the target ordiagnostic location.Therefore, the compressedbeam must be neutralizedwith an appropriate plasmadensity.

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Drift compression of an ion beam

IBM produces a head-to-tail velocity ramp that longitudinallycompress the beam for a short pulse length.

If the compressed pulse length is dominated by the longitudinalbeam temperature TL, the compressed pulse length is

tf =dL

v2z

√2kTL

mi

where, vz , dL, mi and k are the mean longitudinal beam velocity,drift length, ion mass, and Boltzmann constant, respectively. TL

is an effective temperature including the effects of errors in thetilt waveform. 9 / 37


Beam neutralization results

Sketch of Neutralized Transport Experiment (NTX) to demonstrateradial compression of a beam

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Neutralized transport experiment (NTX)

NTX used a higher current (∼25 mA) and higher energy(250-350 keV) K+ beam. The beam perveance (∼10−3) waseffectively neutralized with RF and cathodic-arc plasma sources

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Typical NTX Ion Beam Characterization

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Beam current density increased > 100x for radial beamcompression (NTX alone)

P.K.Roy et al., POP, 11, No 5, (2004), 2890.13 / 37



Sensitivity Study of Neutralization

P. K. Roy et al.,NIM A, 544 (2005) 225.

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Drift compression results

Neutralized drift compression experiment (NDCX-1)

NDCX uses many components of the former NTX. The axialcompression is achieved with an induction-bunching module(IBM) inserted after the matching section.

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Three Fast (ns) diagnostic systems deployed for NDCX

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Beam bunching observed as induction bunching module (IBM)turned on.

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First time ≥50 times longitudinal beam compression measured

IBM operated at ±80kV, a ±12.5% velocity ramp is imparted to150–200 ns subset of the several µs beam pulse.

P. K. Roy. etal., PRL, 95,(2005) 234801.18 / 37


Initial NTX and NDCX-1 experiments demonstrated that sufficientplasma is essential for radial and longitudinal compression.

1 The beam intensity, immediately after the longitudinalcompression, is significantly higher than theuncompressed beam, illustrating that a higher plasmadensity is required to maintain a small beam spot size.

2 Also to focus the intense beam to a target,a final focussolenoid is suggested.

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Plasma channel with a final focus solenoid

A space charge neutralizing plasma channel for simultaneousbeam drift compression

1 FEPS injects plasma in the radial direction.2 CAPS provides in axis plasma.3 A FFS provides beam focusing field near the target.

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Plasma channel with a final focus solenoid

A strong final focus magnet increases beam density near thetarget plane

8T FFSmagnet with837-µspulses and apeak currentof 21.7 kA

1.5 kJ/pulse,whichnecessitatesactivecooling.

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Axial plasma density

Measurement of axial plasma density using single Langmuir probe

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Axial plasma density

The two-FCAPS provides a plasma density of ≤ 1012cm−3 whenthe FFS was powered

Maximum plasma density≤ 2.8x1012

cm−3 between the gap ofthe FFS and the targetplane, where the plasmaelectron density was lowerdue to the FFS field andFilter field.

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Fabrication of a new plasma source

A higher plasma density may further decrease the beam spot size

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FCAPS system with bent filter replaced by a CAPS system withstraight filter for axial higher density plasma

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The new four– CAP provides ∼ 30 times greater plasma than thetwo-FCAPS with bent filters

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Design and fabrication of a radial plasma probe

An array of 37 collectors to measure radial and axial plasmadistribution

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The axial drive unit of the probe array

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Plasma forms a thin column of diameter ∼5 mm along the 8Tsolenoid axis

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Transverse and axial plasma distribution (surface plot) using the37 collectors

P. K. Roy et al., NIM A 606 (2009) 22-30.30 / 37



3-D electrostatic PIC simulation shows qualitative agreement withexperiment

1 With velocity of 3 cm/µs,temperature of 7 eV.

2 The density rapidly fallsoff, off-axis inside thesolenoid.

3 The peak density is∼10x smaller thanexperiment is due to asmaller injected plasmadensity out of theFCAPS.

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The four-CAPS provides axial plasma of ≥ 1013 /cm3 when the FFSwas powered

The plasma (electrons and ions) drift into the bore usingthe diffusive B magnetic field map and the CAPS B field.

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Integration of the plasma channel with beam drift compressionexperiments

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A profile of ≤ 2.5 mm (at FWHM) was measured as operated forsimultaneous radial and longitudinal compression.

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Outcome of initial beam compression experiments

Physics results presented in this talk have established a basicbackground for an on going 11M$ NDCX II ARRA project for targetheating experiment at LBNL.

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Outcome of initial beam compression experiments

An artistic view of target heating

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Summary

1 Successfully accomplished > 100x radial compression.2 Successfully accomplished >50x longitudinal compression.3 Developed a radial plasma diagnostic in parallel of three

other diagnostics.4 Simultaneous radial and longitudinal compression system

is being exploring the warm dense matter phenomenon atpresent.

5 The presented experimental results established confidenceon many high intense beam experiments as NDCX-IIARRA project.

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High-Intensity Ion Beam Neutralization and Drift ......Applications and techniques of ion beam...

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