Amatuni, A Ts

Elbakian, S S

Laziev, E M

Nagorsky, G A

Petrossyan, M L

Sekhpossian, E V

CERN Document Server

Particle Accelerators, 1990, Vol. 32, pp. 221-227Reprints available directly from the publisherPhotocopying permitted by license only© 1990 Gordon and Breach, Science Publishers, Inc.Printed in the United States of AmericaDEVELOPMENT OF NEW METHODS FOR CHARGED PARTICLEACCELERATION AT YEREVAN PHYSICS INSTITUTEA.Ts.AMATUNI,S.S.ELBAKYAN, E.M.LAZIEV, G.A.NAGORSKY,M. L. PETROSSYAN E. V. SEKHPOSSYANYerevan Physics Institute,Yerevan 375036,Armenia,USSR.Abstract Theoretical and experimental works done at YPI are reviewed:the acceleration of charged particles by surface waves arising at totalinternal reflection, by inversion of the Vavilov-Cherenkov effect, byinversion of the free electron laser and by wake fields generated inpassive and active (plasma) structures by intense relativistic electronbeams. For the plasma case special attention was made on nonlinear eff-ects, when the acceleration rate and the transformation ratio are high.The possible self-acceleration of electrons in a relativistic bunchpassing through a plasma column is also considered.ACCELERATION BY SURFACE WAVES,ARISING AT TOTAL INTERNAL REFLECTIONThe main property of the surface waves(their propagation in vacuumwith a smaller than light velocity) makes them appropriate for accelerationof charged particles. This idea was first stated probably by Lohman in 1962.Later on, this phenomenon was independently· discussed at YPI in 1972. A morecomplete analysis providing a sufficiently high acceleration rate and at thesame time the fullfilment of vertical and phase equillibrium conditions wascarried out in reference l . High acceleration rate and phase equillibriumconditions are achieved by an additional vertical magnetic field. Verticalequillibrium is provided by a sign-changing magnetic field parallel to thesurface and perpendicular to the particle trajectory. In this case, as itfollows from the equations: of motion, jumps in radiation phase are needed.The full system of equations was studied and optimal conditions for the typeof the focusing system, angle of reflection and polarisation of the laserbeam were derived.. Finally, the breakdown properties of the dielectrics,which could be. used in future' experimental equipment were analysed.Acceleration gradients of the order of 100MeV/m are expected.[1663]/221222/[1664] A. TS. AMATUNI ET AL.INVERSE VAVILOV-CHERENKOV EFFECTIn 1962 K.Shimoda suggested in his pioneering work to use the inverseVavilov-Cherenkov effect for particle acceleration. Starting from 1972 thedetailed theory of this effect was developed in the works of the Yerevangroup. Special attention was made on the problem of the stability of motion.In particular, the role of the laser wave magnetic field, whose strength inthe medium is higher than that of the electric field, was considered. It wasshown that at relativistic velocities, the effect of the magnetic fielddisturbes the phase equillibrium. To ensure the stability it was suggestedto introduce a constant magnetic field directed along the wave magneticfield. A system of three Cherenkov waves was proposed and analysed, wherestable mode of particle acceleration with a large phase volume of the beamis also feasible2 • For both of these versions a method has been developed totake into account the multiple scattering effect on acceleration stabilityand it was shown, that the latter is possible only for sufficiently highstrengths (flux 1014W/cm2 ) of the wave electric field and short(picosecond) pUlses2 .In a series of works(see review2 ) an interesting feasibility to createhigh accelerating gradients using the double inversion of theVavilov-Cherenkov effect was proposed. First a particle beam, accelerated inan ordinary machine, is modulated by the inverse Cherenkov effect. Then itpasses into another specially chosen medium and produces Cherenkov radiationwith higher field strength which is used again for acceleration of anotherparticle beam by an inverse Cherenkov effect. Acceleration gradients of theorder of 10 GeV/m may be expected.ACCELERATION BY WAKE FIELDSThe first proposal on wake field acceleration in passive structuresbelongs to G.A. Voss and Th. Weiland. The experimental studies of the effectwere carried out at DESY with a device called by authors the wake fieldtransformer. Later on, the method was also studied experimentally at ANL.From 1985 theoretical studies were carried out at YPI in search ofways to enhance the acceleration rate and the transformation ratio invarious deaccelerating passive structures. In particular, calculations weremade for cylindrical waveguides with elliptical cross section, when theNEW METHODS OF ACCELERATION AT YPI [1665]1223driving beam passes through one of the focuses and generates the wake fieldsalong the axis going through the other focus; the problem of increasing thetransformation ratio and the acceleration rate by using specially shapedsequences of the driving beam bunches, with various distributions of chargein each bunch, was also studied in detaiI3 ,4,5. Acceleration rate ~ 100MeV/mand transformation ratio ~ 10 may be obtained.The ideas of using longitudinal wake fields excited by electrons orelectron bunches in a plasma to accelerate charged particles were initiatedby B.M.Bolotovsky and Ya.B.Fainberg in early fifties. At nearly the sametime A. I.Akhiezer and R. V. Polovin formulated and exactly solved theequations for free nonlinear longitudinal waves in an infinite relativisticplasma. Since the end of seventies and up to now in works of the Yerevangroup both of these ideas were used in consideration of the generation oflongitudinal nonlinear waves in plasma by extended bunches of relativisticelectrons. Some of the results of these investigations (see ref. 6 ,7,9 andreviews ) are as follows. The essential parameters in question for the caseof a fixed bunch are the ratio n In , where n - is the density(fixed) ofbObthe electron bunch, n - is the electron density of a neutral cold plasma inoequillibrium, ?fb - the Lorentz factor of the bunch, d - longitudinal lengthof the bunch and w - the plasma frequency. It was shown that whenpthe maximum of the electric field behind the bunch in the nonlinearlongitudinal wake field isr mcwE = v2 P_ ?f1/2max eFor the case when2n ( n fl nb 1 v n ( nbfi/2?fb » n~ 1-2 n: » 1, -<- , d 2~ 1-2 - (2)n - 2 w n n0 p 0 0the maximum of the field is224/[1666] A. TS. AMATUNI ET AL.r mcw (nb ) 1/2 ( nb ) -1/4E = v2 --p -- 1-2-m~ e n no 0In both cases (1,2) the deaccelerating electric field inside the bunch is ofthe order of mcw Ie , and therefore the transformation ratio could bepsufficiently high along with a high value of the acceleration rate.The above mentioned results follow from the exact solution of theequations of magnetic hydrodynamics for bunches which are infinite intransverse directions. Tpe results for cylindrical bunches wilth a finiteradius ~ show, that the conditions for the fullfilment of the abovementioned results are the following:2TrvbwpA,2 2P Db nb--2- -n- « 1,a 0n 2_b ~ « 1, 0 ~ r ~ an 2o aFor n In «1 the influence of ion motion is not essential, but forb 0n In ~ 1/2 a restriction, namely - Db < 1/2(M IZm )1/4 exists where M isbOle 1the ion mass.The Yerevan group has taken into account in a selfconsistent way theinverse action of the beam and plasma electron fields on the longitudinaldensity and momentum distributions in the electron bunch. It is assumed thatsome quasistationary state of the plasma-beam system arises for a while afterthe injection of the electron bunch into the plasma, when the energyh b h d 1 1 · . 11 l' 'bl 10exc ange etween the bunc an p asma e ectrons IS practlca y neg Igl .e .In this situation the velocity of the buneh front (at z = d) is assumed tobe fixed and exceeds the phase velocity of the excited quasistationarylongitudinal wave. Then in the extreme relativistic case the longitudinaldistribution of the bunch electrons is practically not changed, but in themomentum distribution at the end of the bunch (at z = (j)} one findsaccelerated electrons. For case (1) we have for the dimensionless momentump (0) == P (O)/me = R (d) + 4i3,b z b bp. (d) == P (d)/mcb zNEW METHODS OF ACCELERATION AT YPIand for case (2)[1667]/225(n )-1P (0) ~ p (d) + 2'02 1-2_b ;b b b no'Ob ~ '0 »1phFor the case which is nearest to existing laboratory conditionsn nvb bn « 1, 'Ob » 1, d = 2W ' Pb(O)o pThe relations, which define the phase velocity and wave vector of thewake through '0 , n In , d and ware derived (dispersion relation).b bOpThe increase of the momentum of bunch electron inside the plasmafollows from conservation of the Hamiltonian and the decrease of thepotential energy of electrons at the rear part of the bunch. This decreaseis due to the motionless ion excess at the end of the bunch. Considerationof ion motion gives the same restrictions as for the previous.Results obtained show the possible longitudinal contraction of the beam.For example if n In = 10-3 n = 101gm-; '0 = 104 , d = 3mm the maximalb 0 '0 benergy at the end of the bunch approaches the value ~ 100GeV, and the numberof accelerated particles is ~3 108 when the cross section of the bunch is1cm2 • When '0 b= 10 in this example the distance needed for noticeablecontraction of the bunch is a few meters long.For the experimental investigation of various schemes of wake fieldgeneration in passive and active structures and demonstration of the effectsof self-acceleration of the electron beams in plasma a linac is underconstruction at YPI with the following parameters: energy - 30MeV(50MeV athalf the current), pulse current - 1.0 - 1.5A, pulse duration - 8 10- 6sec,bunch duration - 3.6 10-11 sec, repetition rate - 50 - 100Hz, particledensity in a bunch - (2.2 - 3.3)109cm3 , bunch length - 0.5 - 1.0cm, bunchdiameter - 0.5 - 0.7cm, average power - 30 - 45KW. The new linac will havealso a beam monochromatization system.INVERSE FREE ELECTRON LASER(IFEL)The accelerators based on IFEL may become in the future a promisingsource of intense electron beams with energies up to hundreds of GeV.226/[1668] A. TS. AMATUNI ET AL.Having in mind this final goal, experiments were carried out at YPI toobserve laser acceleration in undulators and for elaboration of experiencefor future developments. The first experimental results were delivered atthe previous conference11. A 8MeV microtron 10MW CO laser, undulator with2period 5. 25mm, 20cm length and magnetic field up to lKOe were used. Later onthe energy of the microtron was increased up to 12MeV and the laser powerwas doubled. The results12 with the new arrangement are presented in Fig. 1,where the maximum change of electron energy is given as a function of theundulator magnetic field H. The curve shows the standard interactionfunction F(H) of electrons and the laser wave.~E(KeV)+20°-20J/f r'( tIIt ~ ~~ IHI ~V ~F(H)+0,0675o-0,0675°4000 8000Fig. 1H[Oe]12000At present the collaboration YPI - Troitsk branch of Kurchatov Institute hasassembled a new experimental setup at Yerevan where a 5GW CO laser,2developed at the Kurchatov Institute is used. Now the measurements areunderway, and an acceleration rate up to IV 30 MeV/m is expected.At the next stage it is planned to construct the prototype of theIFEL-accelerator with electron energy up to IGeV, and current up to lKA. Tothis end,as the estimates show, IV 1013W laser power will be needed.A more detailed description of the ideas and results presented in thistalk are given in a recent reviews.REFERENCESNEW METHODS OF ACCELERATION AT YPI [1669]/2271. G.A. Nagorsky, A.Ts. Amatuni and V.M. Harutiunyan,Proe. XII Int. Conf. on High Energy Ace., p. 488, Fermilab, 1983.2. G.A. Nagorsky, Nucl. Instr. Meth., A248, 31(1986).3. E.M. Laziev and V.M.Tsakanov, Proc. Conf. Linac-86,Canada,1986.4. E.M. Laziev and V.M.Tsakanov, Proc. XIII Int. Conf.on High Energy Acc., v.1, p.141, Novosibirsk, 1987.5. P. SchUtt, T. Weiland and V.M. TSakanov, Preprint DESY M-88-13,1988.6. A.Ts. Amatuni, S.S. Elbakyan, M.R. Magomedov andE. V. Sekhpossyan, Plasma Physics(in Russian), ~, 85(1979)7. A.Ts. Amatuni, 5.5. Elbakyan and E.V. Sekhpossyan,Plasma Physics(in Russian), 12, 1145(1986).8. A.Ts. Amatuni, S.S. Elbakyan, E.M. Laziev, G.A. Nagorsky,M.L. Petrossyan and E.V. SekhpossyanElementary Part. and Atomic Nuclei (in Russian), 20, No.5,p. 1246, Dubna, JINR, 1989.9. A.Ts. Amatuni, 5.5. Elbakyan and E.V. SekhpossyanProc XIII Conf. on High Energy Acc.. v.1, p. 175, Novosibirsk, 1987.10. L.E. Thode and R.N. Sudan, Phys. Fluids, 18, 1552(1975).11. A. Ts. Amatuni; M.L. Petrossyan, B. V. Petrossyan,L.V. Khaehatryan, A.S. Hovsepyan and R.Ts. Harutiunyan,Proe. XIII Conf. on High Energy Ace., v.1, p.138,Novosibirsk, 1987.12. A.Ts. Amatuni, M.L. Petrossyan, B.V. Petrossyan et aI,Review of Sci. Instr., Issue devoted to Synchrotron Rad.Instrument. Conf. (in print),1989.

Development Of New Methods For Charged Particle Acceleration at Yerevan physics Institute

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