The ELTEST Experiment ELTEST Experiment R. Pozzoli, M. Cavenago, F. De Luca, M. De Poli, V....

The ELTEST Experiment

R. Pozzoli, M. Cavenago, F. De Luca, M. De Poli,V. Petrillo, M. Rome, L. Serafini, G. Bettega, and A. Illiberi

INFN Sezione di Milano and Dipartimento di Fisica Universita di MilanoINFN Laboratori Nazionali di Legnaro

19 September 2005

INFN Gr V, Napoli 19/09/05

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AbstractA proposal for the investigation of the occurrence and evolution of structures due to

space charge processes in a low-energy pulsed by means of Thomson scattering oflaser light is presented.


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Introduction• A key question in electron beam physics is the convective evolution (along

the beam) of small non uniformities (due, e.g. to the emitting cathode or tothe presence of grids or small errors in the alignment of the system) inspace charge regime and their effects on the beam emittance (e.g.wavebreaking due to transverse plasma oscillations, longitudinal expansionleading to debunching and reduction of the peak current). Thesephenomena are important up to energies of about 150 MeV in photoinjectorproduced beams with pulse duration of few ps and current of somehundreds A [Serafini and Rosenzweig(1997)], and can limit the achievablebrightness in photoinjectors [Fraser et al.(1986)], and the performance ofinteresting applications as X ray FEL’s and plasma based accelerators (seee.g. [Ferrario et al.(1999)]). Direct experimental investigation of thecharacteristics of such beams is very difficult due to the small beam width(some hundreds microns) and limited flexibility of the overall apparatus.

It is possible to investigate the same space charge phenomena in beamswith lower energy (few keV) which can exhibit a similar behavior as the


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relativistic beam produced by the photo injector if current, energy and beamradius are scaled maintaining the following parameter constant (see e.g.[Davidson and Qin(2001)]) :

ω2p

γω2c

∝ I

βγB2r2= const, (1)

where ωp is the plasma frequency, ωc is the electron cyclotron frequency, Ithe beam current, B the magnetic induction, r the radius of the beam, andβ, γ refer to beam electrons.


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The ELETRASP Experiment

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The ELETRASP Experiment

Figure 1: Eltrap view


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The mentioned processes arise from the collective (collisionless)interactions occurring in single species plasmas and have been thoroughlyinvestigated in electron or ion plasmas confined in Malmberg-Penningdevices (see e.g. [Anderegg et al.(2002)] and references therein), where avariety of nonlinear dynamical regimes have been found. The ELTRAPdevice in Milan is devoted to such type of investigation. In the ELETRASPexperiment the transverse distribution of the electron density of alow-energy laminar beam has been investigated in ELTRAP([Amoretti et al.(2003)]), using a phosphor screen and high contrast CCDcamera. This has allowed the detection of the formation of coherentstructure arising from the transverse instability due to space charge and theestimate of their transport along a stationary beam; this being obtained byvarying the magnetic field strength, since the inverse time scale of theprocess is proportional to n

B ([Bettega et al.(2004)]).


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Figure 2: Eltrap schematics with the pulsed source


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Figure 3: Evolution of a merger process observed in Eltrap (The enlargementsof the three pictures are different)


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Figure 4: Evolution of an electron beam in ELTRAP in the space charge regime. The imageshave been taken using different values of the confining magnetic field (decreasing from left tothe right picture).

The obtained results have been interpreted by means of numericalsimulation codes developed in Milano (e.g. [Tsidulko et al.(2005)]).


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Figure 5: Computation of the beam evolution in ELTRAP using the PIC code MEP. Left:macroparticle distribution in a thin longitudinal layer containing the axis, computed at a timeclose to a stationary state. Right: macroparticle distribution in a thin layer on the phosphorscreen, at the same time.


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Figure 6: Phase space (z vz) during the evolution of a space charge dominated beam.


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Figure 7: Phase space (z vz) during the evolution of a beam with a barrier.


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A more detailed investigation of the stationary beam will be performedusing a planar source developed in Legnaro [Cavenago et al.(2004)]. Thisallows to use the whole cross section of ELTRAP to achieve a largerplasma and produce a higher current. This research will continue under theELTEST experiment to achieve a better uniformity in the emission andreduce the thermal load with the appropriate use of different materials. Inaddition to the stationary source a pulsed electron gun with a 4-5 ns pulseof 10 keV electrons is being completed and will be installed in ELTRAP nextOctober. This will allow the investigation of a pulsed beam with length ofabout 40 cm.


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Figure 8: Planar source


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The ELTEST experimentThe results obtained in ELTRAP have shown that the development ofstructures is quite fast; the use of a pulsed beam will increase the electrondensity n up to about 109cm−3 leading to an even faster process. In theELTEST experiment (acronym in Italian of ”testing transient laminarelectron beam by means of thomson scattering” ) the density variation inthe pulsed or even the stationary beam will be investigated by means of thedetection of back scattered radiation from a laser beam.

In the literature the interest towards Thomson backscattering techniques onrelativistic beams is high due to their use in x ray production and electronacceleration processes in plasmas. Incoherent as well as coherentThomson scattering techniques are commonly used as diagnostic methodin fusion research. Thomson backscattering as diagnostic tool forrelativistic beam has been used in the past (e.g. by Chen and Marshall in1984 [Chen and Marshall(1984)]).


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The frequency of the scattered radiation is mωs with

ωs(θ) ≈ ωi1 + β

1− β cos θ(2)

where ωi is the the frequency of the incident wave and the scattering angleis θs = π − θ.

The evaluation of scattered power for incoherent scattering can be donestarting from the motion of single electron in the laser field (see e.g. Landauand Lifshitz and Jackson) and computing the relevant radiation field. Thedeviation of the motion induced by the laser field from the simple oscillationdriven by the electric field of the laser pulse depends on the parameter

a =eE0

mω0c≈ 0.22

λ

rs(

P

1GW)12 (3)

where E0 is the electric field amplitude, λ the wavelength of laser radiation,


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rs the laser spot size and the evaluation is made for a linear polarized fieldand a Gaussian radiation beam [He et al.(2003)].

In the case under consideration a << 1; the scattered power is at thefrequency ωs only, and the scattered power per solid angle is

p[eV/s] ≈ 4.75 · 1010 ε[J ]∆t[ns]

1r2sγ

2

(cos θ − β)2

(1− β cos θ)6(4)

where ε is the energy of the laser pulse and ∆t its duration.

The number of scattered photons per solid angle by a laser pulse is thengiven by

Ns ≈ 1.2 · 10−6λ0[µm]ε[J ]n[cm−3]L[cm] (5)

times a angular factor.


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Numerical simulation of beam dynamics and scattered field give in fact wellmeasurable numbers of electrons, as shown in fig 9


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Figure 9: Numerical simulation of backscattered power (V. Petrillo)

The conceptual design of the experiment is shown in fig. 10


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beam dump

laser λ= 1064 nm 0.30 – 0.40 J per pulse (e.g. Brilliant - Quantel)

PMT 300-880 nm (e.g. H8711 series Hamamatsu)

1 degree

1-10 degrees degrees degrees

ELTEST Thomson backscattering from electron beam in ELTRAP

Conceptual design of ELTEST experiment. The optical system is not shown. As additional diagnostic, the beam can be dumped on a phosphor screen, and a CCD camera can analyze the electron density distribution.

CCD phosphor screen

Figure 10: Conceptual schematics of ELTEST experiment


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Preventivi di spesa

Laser 1064 nm, 360 mJ, 5ns 35 (con modulo a 532) + iva

Sistema fotomoltiplicatori o fotodiodi 15 + iva

Ottica per laser 5 + iva

Sistema di oscuramento 4

Settore flangiato 15 + iva

Generatore di ritardo per trigger 4 + iva

Faraday cup e dumping del fascio 5 + iva

Forno per sorgente planare 12 + iva


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Piano delle ricerche

2006 disegno costruttivo + ordine apparati

2007 acquisto laser, assemblaggio e prime misure

2008 misure e sviluppo codici


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References[Serafini and Rosenzweig(1997)] Serafini, L., and Rosenzweig, J. B., Phys. Rev. E, 55, 7565 (1997).[Fraser et al.(1986)] Fraser, J. S., Sheffield, R. L., and Gray, E. R., Nucl. Instr. Meth., A250, 71 (1986).[Ferrario et al.(1999)] Ferrario, M., Clendenin, J. E., Palmer, D. T., Rosenzweig, J. B., and Serafini, L. (1999),

Proceedings of the 2nd ICFA Adv. Acc. Workshop on The Physics of High Brightness Beams, UCLA.[Davidson and Qin(2001)] Davidson, R. C., and Qin, H., Physics of Intense Charged Particle Beams in High

Energy Accelerators, Imperial College Press and World Scientific, London, UK, 2001.[Anderegg et al.(2002)] Anderegg, F., Schweikhard, L., and eds, C. D., Non-neutral Plasma Physics IV, AIP,

Melville, NY, USA, 2002.[Amoretti et al.(2003)] Amoretti, M., Bettega, G., Cavaliere, F., Cavenago, M., De Luca, F., Pozzoli, R., and

Rome, M., Rev. Scient. Instr., 74, 3991 (2003).[Bettega et al.(2004)] Bettega, G., Cavaliere, F., Illiberi, A., Pozzoli, R., Rome, M., Cavenago, M., and Tsidulko,

Y., Appl. Phys. Lett., 84, 3807 (2004).[Tsidulko et al.(2005)] Tsidulko, Y., Pozzoli, R., and Rome, M., J. Comp. Phys., 209, 406 (2005).[Cavenago et al.(2004)] Cavenago, M., Bettega, G., Cavaliere, F., Illiberi, A., Pozzoli, R., Rome’, M., and

Serafini, L. (2004), Proceedings of EPAC 2004.[Chen and Marshall(1984)] Chen, S., and Marshall, T., Phys. Rev. Lett., 52, 425 (1984).[He et al.(2003)] He, F., Lau, Y., and Kowalczyk, R., Phys. Rev. Lett., 90, 055002–1 (2003).


The ELTEST Experiment ELTEST Experiment R. Pozzoli, M. Cavenago, F. De Luca, M. De Poli, V....

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