n.a. ebrahim et al- first observations of acceleration of injected electrons in a laser plasma...

8/3/2019 N.A. Ebrahim et al- First Observations of Acceleration of Injected Electrons in a Laser Plasma Beatwave Experiment

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FIRST OBSERVATIONS OF ACCELERATION OF INJECTED ELECTRONS IN A LASER PLASMA BEATWAVE EXPERIMENTTH3-44

N.A. Ebrahim*, F. Martin, P. Bordeur, E.A. Heighway**, J.P. Matte, H. Pepin and P. LavigneINRS-Energie, Varennes, Quebec, Canada JOL 2P0

Abstract

We report the first experimental observati ons ofaccelerati on of injected electrons -in a laser driven

plasma beatwave. The plasma waves were excited in anionized gas jet, using a short pulse high intensityCO: laser with two collinearly propagating beams (at

A = 9.6 em and 10.6 um) to exci te a fast wave

(VP = c). The source of electrons was a laserplasma produced on an aluminum slab target by a third,

synchronized CO2 laser beam. A double-focusing dipole

magnet was used to energy select and inject electronsinto the beatwave, and a second magnetic spectrographwas used to analyze the acceler ated electrons.Electron acceleration was only observed when the

appropriate resonant plasma density was produced(= ID" cme3), the two laser lines were incident onthe plasma, and electrons were injected into this

plasma from an external source.

Introduction

The laser plasma beatwave accelerator was firstdiscussed by Tajima and Dawson, in thy context of an

ultra-high energy particle accelerator . The physical

mechanism underlying this scheme is ths optical mixingof laser light in an underdense plasma , which exc itesan electron plasma wave as a result of the beatpondermotive force. The potential troughs associ atedwith the longitudinal plasma wave can trap sufficient-

ly energetic electrons injected from an external

source and accelerate them to relativistic energies.Theoreticall y at least, accelerati ng gradients of

order Jne V/cm seem possible, where ne is theplasma density in units of cmm3.

A study of the plasma beatwave concept involves astudy of three basi c problems: that of producing a

source of underdense plasma of an appropriate size anddensity; that of generating a plasma wave with laser

beams and understanding the mechanisms of amplitudegrowth, saturation and decay of this wave, as well asits effect in modifying the plasma density; andfinally that of injecting electrons into such asystem, trapping the electrons and accelerati ng them

to relativistic energy. The first of these problemshas been investigated extensivel y over the past twodecades in the field of plasma physics. Plasmasources suitable for a study of the beatwave concept

are arc discharge plasmas, exploding submicron foils,pinch discharges (both 'Z' and '0' pinches) and gas

jet plasmas. Th second problem has been studied both

;;ar",:;al;:,'di',':l gnd in computer simulati ons in* . There have also been recent

experiment s to identify and establish the existence of

large amplitude plasma wav%s as a result of the opti-

cal mixing of laser light . The problem of particletrapping and accelerati on has also bfec studied quite

extensively in computer simulations * over the pasttwo decades, since wave-partic le interaction is aproblem of general interest in plasma physics. How-ever there have not been direct experimental studiesto verify important aspects of the theoretical pre-dictions.

*

**

Atomic Energy of Canada Limited

Chalk River Nuclear Laboratories

Chalk River, Ontario, Canada KDJ 1JD

Now at Los Alamo s National Laboratory

Los Alamos, New Mexico, USA 87545

This paper .discusses a novel experimental

approach to test the laser plasma beatwave acceleratorconcept. In this approach, the three main elements of

the beatwave concept discussed above (i.e., the plasmasource, electron plasma wave and the injection

electron source) are all driven by a single, shortpulse Co p laser operating at two frequencies. This

greatly reduces the complexities of such an experimentby reducing the generally difficult problem of

synchroni zation of a short laser pulse, a rapidlyvarying plasma density and a short burst of injection

electrons, to a simple problem of setting up appropri-ate optical delays bet ween the various laser beams

emanating from a single laser. Nevertheless, this

approach gives us a great deal of flexibility toconduct a wide range of experiments necessary to study

the generation and dynamic s of the beat plasma wave aswell as the important problem of wave-particl e inter-action in a high phase-veloci ty electron plasma wave.

Indeed as we point out later, a simple extension of

our present approach may well have important impli-

cations for a configuration which would be appropriate

for the test of a rmltis tage beatwave concept as well.

Experiments and Results--

Laser Facility

The experiments were performed with a high intensity

CO2 laser system shown schematically in Fig. 1. This

system can be operated in a single frequency

[lo P(2O)l or dual frequency [9 P(20), 10 P(I6)'Jmode. In the single frequency operation, the lase{chain amplifies the 10 P(20) line at 944.2 cm'

(A = 10.6 ~a) to produce a 1.0 ns FWHM pulse con-

taining 75 Joules maximum. For the dual wavelength

experiments, the laser chail was used to amplify the9 P(20) line at 1046.8 cm' (X = 9.55 un) and the10 P(16) line at 947.74 cm-' (X = 10.55 in) to yield

30 Joules maxi mum in each of the two wavelengths.Since these lines have similar small signal gains, a

good control of the relative energy at each frequency

could be maintained. The use of reflective focusing

optics and parallel windows all along the laser chain

ensures good beam collinearity for the two wave-

lengths. The P- polarized light beam (main beam in

Fig. 1) was focused at near normal incidence by an,

f/6, 60 cm focal length parabola. At best focus, 50%

of the energy was contained within a 120 un diameter

focal spot. Only those shots for which the synchroni-

zation between the pulses at each frequency was betterthan 100 ps as measured on a 1 GHz oscilloscope, were

retained for analysis.

Plasma Source

When two travelling electromagneti c waves, (wg,k,) at 9.6 um and (~1, kl) at 10.6 wn , are injectedinto an underdense plasma, the beat of the two wavc;s

gives rise to a non-linear pondermotive force V CE >which excites plasma oscillations at the frequency

and?equency

wavenumber kp which satisfy the usual

and wavenumber matching conditions

= %-WI and kp = k,-kl. For the two wave-

l?&gths used in these experiments this [ysonance con-

dition occurs at a plasma density of 10 cmw3 giving

553

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pinch discharge, we have used the much simpler tech-nique of laser breakdbwn of a low pressure gas, as

this technique has several desirable features. Sincebreakdown of the aas is limited to a high laser

intensity region in-the focal volume, where the beatplasma wave is excited as well, it is not necessary totransport the laser beam through a significant region

of the plasma. The initial gas filling pressur e and

i the laser intensity are adjusted so that the resonantdensity is reached near the peak of the laser pulse,

in order to obtain the maxi mum saturated amplitude of

the plasma wav e in a time scale short enough to avoidthe deleterious effects due to the-ion motion. .

The plasma density and the density profile weremeasured with a Mach'Zehnder interferometer using part

of the main CO? laser beam (- 4%) as a probe beam,

aft:;, the necessar y spatial filtering (see Fig. 1,. The olasma was imaoed with an afocal tele-

scope, with a' magnification-of two and recorded by

burning a Polaroid (TM) film. Since the probe pulsehas a duration of 1.0 ns we cannot obtain any temporalresolution of the ionization phase where the electron

density increases very rapidly. Consequently we haveobtained interferoarams at 2.0 ns and 5.0 ns after the-_- -~ ~~plasma producing Ctz laser pulse for static pressuresof 0.3 Torr < p < 2.0 Torr of dry air. Since recombi-

nation and particle diffusion are negligible on thistime scale,. interferometry gives the peak electrondensitv. The Abel inverted electron density profile

(Fig. 2) shows the plasma to be cylindrical -with adiameter of 2.0 mn FWHM. The laser beam has a focal

diameter of 0.12 mn and a depth of focus of I.5 mm.The dependence of the average value of the electrondensity in the focal volume, on the filling pressur efor a probe delay of 2.0 ns is shown in Fig. 3. Forcomparis on, the solid li ne gives the electron density

corresponding to fully stripped t$i trogen. In Fig. 3,

an electron density of 10 cm' is obtained for a

filling pressur e of 0.5 Torr with a degree of ioni-

zation Z = 3. For a filling pressure of greater than

1.0 Torr the gas molecules are fully stripped. Inthis pressur e range, the average value of the electrondensity is not very sensitive to laser energy. The

plasma dimension is also approximately independent Of

the filling pressure.

Observations of Electromagnetic Sidebands

Measurem ents of electromagnetic sidebands for

dual wavelength illumi nation provide further evidence

that resonant plasma density has been attained in agiven laser shot. The upshifted (anti-Stokes) anddownshifted (Stokes) sidebands are produced when the

low and high frequency purrp parametricall y couple tothe beat plasma wave (w9 + and -respectively). For laser intensities (;Pbove thresWhl:ldWPthis could be due to the stimulated forward Rama:scattering, whereas for the lower laser intensities it

could be due to Thomson scattering. In either case,the sidebands are forward propagating. However, inour experiments the intensity of the unfocused laserbeam is sufficient to generate a relatively strong

anti-Stokes sideband signal in air along the path

between the laser and the vac uum chamber, thusprecluding any measureme nts in the forward direction.We have therefore made measurem ents of the anti-Stokes

sideband signal in the backscatter direction, since'the backscatter signal cannot originate from reflected

laser light because of the low plasma reflectivity .The generation mechani sm of the anti-Stokes sideband(Fig. 4) is as follows. A low frequency (<< %p)long wavenumber (= 2 kg) ion acoustic wave generatedby stimulated Brill ouin scattering of the 9.6 lm~ pump,

parametrical ly couples to a high frequency ( )

short wavenumber (kp = ko-kl) beat plasma wave 7.0produce another forward propagating plasma wave with a

frequency ( )9.6 inn radia ion can now either Raman scatter or.

and wavenumber (2ks+kp). The

Thomson scatter off this plasma wave to produce the

backscatter ed radiation at 8.7 mm. Although this is

an indirect process, in that the forward propagating

plasma wave is driven by the two laser byproducts(beat plasma wave and ion acoustic wave) we believe

the backscatt ered signal provi des a relative measure

of the amplitude of the beat plasma wave.

Figure 5 shows the amplitude of the backscatter ed

8.7 mn radiation normalized to the backscatter ed9.6 em signal as a function of the initipi gas Jilting

pressur e for a laser intensity of 2 x 10 W/cm . The

curve is rather broad, with a peak in the pressurerange 0.4-0.5 Torr which is consistent with the inter-

ferometry measurements which show an average densityof 1 x 10" cmm3 in this pressure range. The ratherbroad nature of the curve is due to the dynamics ofthe ionization process, which has been modeled theo-retically. Essentiall y, at low filling pressures , theresonant density is reached late i n the laser pulse,where the low laser intensity cannot drive the beatplasma wave to a large ampl itude (resulting in lowbackscatter) . At high filling pressures, the rapidionization means that the electron density crosses the

resonance point at a rate too fast to allow the beatplasma wave to grow to a large amplitude (againresulting in low levels of backscatter) . The pressure

range in between these two limits is rather broad asindicated by the sideband measurements .

Estimate of Plasma Wave Amplitude

Accqr ding to the fluid treatment of Rosenbluth

and Liu , the early growth of the plasma waveamplitude is governed by the equation

g = qt)q(t) 4 (1)

where E = &t/n is the wave amplitude and a = eE/mcrt =

v,/c.

The wave amplitude grows linearly with time until

it saturates because of relati2vistic detuning. The

saturated amplitude is given by

Es = cq a+t)a@“3 (2)

and the time to saturation t, is determined from '

Assuming a linear risetime T for the laser

intensity, I(t) = It/T and a(t) = a(t/T)'/2, the

time to saturation is given by

"pts= 4.87[r up/a&

215(4)

The saturated electric field amplitude is then

calculated from' eE = mcwppcs.

For 10 J of energy in each of the two wavelengths

(9.6 m and 10.6 mm) in a 1.0 ns pulse focused to a120 un spot size, Is=110 = 4.4 x 1013 W/cm2, anda,=a2=0.06. With a laser intensity risetimeT = 300 ps, the time to saturation t, 2 80 ps andthe saturated amplitude es = 0.17 giving an electric

field gradient of 5.4 GVfm.

Electron Source and Acceleration

The source of electrons used in these experiments

554



was produced by irradiating an aluminuwith an auxiliary , high intensity (10

~,+slab targetW/cm2) CO2

laser beam focused to a 0.1 mn spot (Fig. 1). It hasbeen known for a long time that resonance absorptionof high intensity laser light at the critical surfacegenerates a nearly Maxwell ian high energy tail in the

electron distribution with a maxi mum energy ofapproximatel y 1.0 MeV, and a time duration on theorder of the laser ~~1st r isetime (1 - 300 ps).Exoerimental measurements of this emission aive afluence of approximately 10' electrons/keV-sr-at anenergy of 0.5 MeV, the emissi on being directed in a 5'cone about the target normal. A double-focusing

dipole magnet was used to energy select and injectthese electrons into the plasma in a directionparallel to the beatwave generating laser beam. With

a half-angle acceptance of 50 mrad for the dipole and

an energy resolution of b/E = 0.25% at the dipol$

output, we are transporting approximatel y 10electrons to the interaction region.

With the system shown schematically in Fig. 1 and

descri bed in this section, we have tested the beatwaveconcept by accelerati ng electrons. In order tosynchronize the various elements of the system, theoptical delay between the beatwave producing laser

pulse (main beam in Fig. 1) and the electron producingpulse (auxiliar y beam in Fig. 1) was adjusted so that

the time of arrival of the electrons at the plasma jet

overlapped with the duration of the beat plasma wavein the jet. A multichannel magnetic spectrometer wasused to measure both the injected and acceler ated

electrons in the energy r ange 0.3 MeV <E (4.5 MeV,the. electron signals being measure d with surfacebarrier detectors.

Preliminary measurements indicated that electronsinjected at 0.6 MeV were accelerated to 2.0 MeV.

Electron acceleration was observed only when a plasmadensity of 10" crnm3 was attained as determined from

the initial gas filling pressure and as supported bythe high level of backscattered anti-Stokes sideband

signal. Furthermore, acceleration was observed onlywhen the two laser lines (a = 9.6 wn and 10.6 em) wereincident on the plasma. Finally, energetic electronswere observed only when lower energy electrons were

injected into the plasma, confirming that the acceler-ated electrons observed in these experim ents did not

originate from the plasma jet as a result of some

non-linear process. Since the resonant plasma regionis approximatel y 1.5 mn long, the accelerati on

observed in these experiments would suggest anelectric field gradient of the order of 1 GV/m. Insome preliminary shots we have also observed signals

up to the 4.5 MeV channel, when electrons wereinjected at 1 MeV. If confirmed, these observati onswould suggest electric field gradients of the order of3 GV/m over a region of approximately 1.5 ma.

Conclusion

We have assembled an experiment to test the laser

Plasma beatwave accelerator concept. Our approach istotally laser-based,advantages.

and this gives us some importantAlthough it still remains a difficult

experiment, the complexiti es are greatly reduced bythis approach. Since the region over which the beat-wave is excited (approximately equal to twice the

Rayleigh length) is limited by the f-number of the

focusing Optics, we have reason to believe that w e canincrease this region quite considerably by using

longer f-number focusing optics. This would increase

the energy gain per stage, without changing the fieldgradients appreciably.optics, we can

With longer f- number focusingreasonably consider using several

Plasma jets (and hence accelerating regions) created

by the same laser beam in a multi-s tage configu:

ration. A principal shortcoming of our laser based

approach i s that the injection energy is limited toapproximately 1 MeV. However, for proof-of-princi ple

experiments and for studying the wave-wave and wave-

particle interactions, this does not appear to be a

serious drawback.. In any case a higher energy

injection source, such as a 10 MeV linac can beincorporated as an extension of this system in the

future.

With this system we have demonstrated the

acceleration process in a laser plasma beatwave, but

the process needs to be explored further and quanti-fied. Important theoretical aspects of wave-particl einteractions need to be verified and experiments need

to be devised to study important accelerator aspectssuch as transverse beam

$,a$:, g~fa~nve,ndco,::~P:,,,i ng 12.

Acknowledgments

We acknowledge the excellent technical support of

M. St-Pierre, F. Levesque, F. Poitras, D.D. Mercier

and J. Gauthier. One of the authors (N.A.E.) wishes

to thank Drs. J .H. Ormrod, J.C.D. Milton,

G.E. Lee-Whiting, L.W. Funk and K.C.D. Chan of AECL

for encouragement, support and many stimulatingdiscussions. This work was funded in part by NSERC

and FCAR grants.References

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co2HIGH PRESSURE

PRE-AMPLIFIER -co,

AMPLlFlER

Fig. 1 Schematic of the experimental arrangement.

Fig. 2 Measured electron density profile. The mainlaser beam is incident along the Z-axfs, atz = 0.

P (Torr 1

Fig. 3 Electron density as a function of fillfng

pressure for dry air.

Fig. 4 Generation mechanis m for 8.7 um

sideband in the backscatter.

Torr (Dry Air)

Fig. 5 Ratio of backscattered signals at8.7 un and 9.56 us as a function

of filling pressure for dry air.

556

n.a. ebrahim et al- first observations of acceleration of injected electrons in a laser plasma...

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