The ELTEST Experiment - Infn

The ELTEST ExperimentR. Pozzoli, M. Cavenago, F. De Luca, M. De Poli,V. Petrillo, M. Romè, L. Serafini, G. Bettega, and A. IlliberiINFN Sezione di Milano and Dipartimento di Fisica Università di MilanoINFN Laboratori Nazionali di Legnaro19 September 2005INFN Gr V, Napoli 19/09/05

ELTEST 1AbstractA proposal for the investigation of the occurrence and evolution of structures due tospace charge processes in a low-energy pulsed by means of Thomson scattering oflaser light is presented.INFN Gr V, Napoli 19/09/05

ELTEST 2Introduction• A key question in electron beam physics is the convective evolution (alongthe 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 theINFN Gr V, Napoli 19/09/05

ELTEST 3relativistic 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)]) :ω 2 pγω 2 c∝IβγB 2 = const, (1)r2 where ω p is the plasma frequency, ω c is the electron cyclotron frequency, Ithe beam current, B the magnetic induction, r the radius of the beam, andβ, γ refer to beam electrons.INFN Gr V, Napoli 19/09/05

ELTEST 4The ELETRASP Experiment

ELTEST 4The ELETRASP ExperimentFigure 1: Eltrap viewINFN Gr V, Napoli 19/09/05

ELTEST 5The 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)]).INFN Gr V, Napoli 19/09/05

ELTEST 6Figure 2: Eltrap schematics with the pulsed sourceINFN Gr V, Napoli 19/09/05

ELTEST 7Figure 3: Evolution of a merger process observed in Eltrap (The enlargementsof the three pictures are different)INFN Gr V, Napoli 19/09/05

ELTEST 8Figure 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)]).INFN Gr V, Napoli 19/09/05

ELTEST 9Figure 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.INFN Gr V, Napoli 19/09/05

ELTEST 10Figure 6: Phase space (z vz) during the evolution of a space charge dominated beam.INFN Gr V, Napoli 19/09/05

ELTEST 11Figure 7: Phase space (z vz) during the evolution of a beam with a barrier.INFN Gr V, Napoli 19/09/05

ELTEST 12A 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.INFN Gr V, Napoli 19/09/05

ELTEST 13Figure 8: Planar sourceINFN Gr V, Napoli 19/09/05

ELTEST 14The 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 10 9 cm −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)]).INFN Gr V, Napoli 19/09/05

ELTEST 15The frequency of the scattered radiation is mω s with1 + βω s (θ) ≈ ω i1 − β cos θ(2)where ω i is the the frequency of the incident wave and the scattering angleis θ s = 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 parametera = eE 0mω 0 c ≈ 0.22 λ P(r s 1GW )1 2 (3)where E 0 is the electric field amplitude, λ the wavelength of laser radiation,INFN Gr V, Napoli 19/09/05

ELTEST 16r s 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

ELTEST 17Numerical simulation of beam dynamics and scattered field give in fact wellmeasurable numbers of electrons, as shown in fig 9INFN Gr V, Napoli 19/09/05

ELTEST 18Figure 9: Numerical simulation of backscattered power (V. Petrillo)The conceptual design of the experiment is shown in fig. 10INFN Gr V, Napoli 19/09/05

ELTEST 19ELTESTThomson backscattering from electron beam in ELTRAPlaserλ= 1064 nm0.30 – 0.40 J per pulse(e.g. Brilliant - Quantel)1 degreePMT300-880 nm(e.g. H8711 seriesHamamatsu)1-10 degreesdegreesdegreesbeam dumpphosphor screenCCDConceptual 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 densitydistribution.Figure 10: Conceptual schematics of ELTEST experimentINFN Gr V, Napoli 19/09/05

ELTEST 20Preventivi di spesaLaser 1064 nm, 360 mJ, 5ns 35 (con modulo a 532) + ivaSistema fotomoltiplicatori o fotodiodi 15 + ivaOttica per laser 5 + ivaSistema di oscuramento 4Settore flangiato 15 + ivaGeneratore di ritardo per trigger 4 + ivaFaraday cup e dumping del fascio 5 + ivaForno per sorgente planare 12 + ivaINFN Gr V, Napoli 19/09/05

ELTEST 21Piano delle ricerche2006 disegno costruttivo + ordine apparati2007 acquisto laser, assemblaggio e prime misure2008 misure e sviluppo codiciINFN Gr V, Napoli 19/09/05

ELTEST 22References[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 HighEnergy 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., andRomé, M., Rev. Scient. Instr., 74, 3991 (2003).[Bettega et al.(2004)] Bettega, G., Cavaliere, F., Illiberi, A., Pozzoli, R., Romé, M., Cavenago, M., and Tsidulko,Y., Appl. Phys. Lett., 84, 3807 (2004).[Tsidulko et al.(2005)] Tsidulko, Y., Pozzoli, R., and Romé, M., J. Comp. Phys., 209, 406 (2005).[Cavenago et al.(2004)] Cavenago, M., Bettega, G., Cavaliere, F., Illiberi, A., Pozzoli, R., Rome’, M., andSerafini, 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).INFN Gr V, Napoli 19/09/05