VOLUME 65, NUMBER 19 PHYSICAL REVIEW LETTERS 5 NOVEMBER 1990 Resonant Wave-Particle Interactions in vp x B Acceleration Scheme Y. Nishida and T. Shinozaki'" Department of Electrical and Electronic EngineeringU, tsunomiya University, Utsunomiya, Tochigi 32i, Japan (Received 2 July 1990) Experimental investigations on the resonant wave-particle interaction in the framework of both a v~ x B acceleration and a conventional resonance-absorption scheme have been performed by injecting electron beams into the wave-particle interaction region. In the v~ &B acceleration scheme, the particles can interact with the wave for a longer duration and the final particle energy reaches larger values (about 1.5 times) than those observed in the resonance absorption. A simple model based on the vr x B acceleration scheme can interpret the results. PACS numbers: 52. 35.Mw, 52.40. Db, 52. 75. Di Several new concepts for high-energy particle ac- celerators using plasma and related phenomena have been proposed. ' One of these is a v~x8 accelerator' (also called a surfatron or a cross-field accelerator ), where v~ is a wave phase velocity and B is a static rnag- netic field. In this scheme particles are accelerated along the wave front of a driver wave propagating perpendicu- larly to a static magnetic field. Theoretical ' and ex- perimental ' ' investigations of the v~&&8 accelera- tion phenomena have already been reported. The experi- mental results, however, were not fully controllable, be- cause, in the experiments performed so far, only elec- trons of the background plasma, which have a velocity near the phase velocity of the excited plasma wave, were trapped and accelerated to high energy by the wave. Therefore, the number density of the trapped electrons and so the electron current fiux could not be controlled. Furthermore, the initial velocity of the electrons trapped in the wave trough was not clearly identified because the phase velocity of the plasma wave was not very precisely determined. The wave-particle interaction, which is a key phenomenon of the present v~ x 8 acceleration mech- anism, should be controlled very precisely for further un- derstanding of the v~ x B phenomenon and its application to a high-energy particle accelerator. In this paper, we present experimental results in which an electron beam with variable velocity and Aux, pro- duced from a small electron gun, is injected into a region of existing plasma wave, in order to show direct evidence of the wave-particle resonance interaction and particle acceleration. Specifically, the first demonstration of the acceleration of the injected electron beam by the v~ x B acceleration process is presented. The experiments are performed in a cylindrical stain- less-steel chamber with multidipole magnets on the out- side of the chamber wall [Fig. I (a)]. The argon plasma immersed in a weak magnetic field 8 ((11 G) is pro- duced by pulse discharges (discharge duration of 3 msec and 10 Hz repetition). Typical plasma parameters are plasma density n, ~ 1 & 10' ' cm, electron temperature T, =3-5 ev, ion temperature T; =T,/10, and density gradient scale length in the axial direction (z direction) 1, . -=100-150 cm and in the radial direction L, =100- coil (o) 00 z probe$++xxx kl ystron 800 r probe filaments anode electron gun grIcl p p — Vn, O O (0 z probe (b) microway e absorber I II I oal to pump 7V, V„ CO D C) 0 nance lO Zo ZO 40 Z (crn) FIG. I. (a) Experimental arrangement and (b) typical spa- tial profiles of electron density in steady state, n, high-energy electron emission, Ih, and absolute value of the rf electric-field pattern, E-', vs the distance measured from the horn-antenna edge. 50 200 cm. Here, the static magnetic field is produced by a pair of saddle-shaped coils with diameter of about 60 cm. p-polarized microwaves with frequency 2.86 GHz, maximum power of 10 kW, and typical pulse width of 5 psec are irradiated from the lower-density side through a 2386 1990 The American Physical Society