Cyclotron-Resonance Accelerations by a Generalized EM Wave

K. Akimoto, H. Hojo1)

Teikyo University, Utsunomiya, Tochigi 320-8551, Japan
1) University of Tsukuba, Tsukuba, Ibaraki 305-8577, Japan

Though particle accelerations by a nondispersive EM wave/pulse have been studied extensively,1 here, net accelerations by a generalized wave, i.e., 1D, EM dispersive (group velocity being different from phase velocity), pulse are investigated analytically and numerically. The emphasis is laid on the case when an external magnetic field is applied to the system, and initially both the pulse and particles propagate along the field. Dispersion effects on particle accelerations via various Gaussian-shaped pulses of arbitrary group- as well as phase-velocities, covering ultra-short impulses, such as sub-cycle pulses, as well as ordinary wave packets are studied systematically. For these pulses, several mechanisms have been recently identified for producing net particle accelerations.1
1. Transit-time acceleration that works at velocities well away from the central group velocity.
2. Quasi-trapping that works at around the central phase velocity.
3. Reflection due to quasi-trapping at around the central phase velocity.
4. Reflection by ponderomotive force that occurs near the group velocity.
Processes 1 and 2 are respectively linear and nonlinear transit-time effects, while 3 and 4 are reflections. Processes 1 is linear and the rest nonlinear. Process 1 has been studied extensively for a standing-wave pulse with extreme dispersion.1
In the presence of an external magnetic, however, another pair of acceleration mechanisms are found to emerge.1 The transit-time accelerations are replaced with cyclotron-resonance accelerations [CRA], and the quasi-trapping by the cyclotron-resonance trapping, which at times leads to cyclotron-resonance reflections [CRR]. Some effective mathematical expressions for perpendicular and parallel velocity shifts etc. have been obtained to characterize the CRA mechanism, and properties of CRR have been explored mainly numerically. It was found that the efficiencies of these acceleration mechanisms exceed those of the other ones. CRA in particular is much more efficient than TTA. The possibilities of utilizing CRA and CRR to particle accelerators are also explored, taking wave dispersion, which are important for waves propagating inside a waveguide, into consideration. Preliminary results indicate high efficiencies of these mechanisms.

References

[1] K. Akimoto, Phys. Plasmas, 4, 3101 (1997); ibid. 9, 3721(2002); ibid. 10, [in press]