Relaxation of runaway electron energies due to excitation of high frequency waves

Relaxation process of the runaway electron energy in a plasma is studied numerically by using the quasi-linear equations combined with a Fokker-Planck collision term. It is assumed that the relaxation is caused by the interactions of the runaway electrons with the plasma waves of ωsub(k) = ωsub(pe)ksub(z)/k. It is shown that the perpendicular energy of the runaway electrons increases rapidly with time, while that the parallel one decreases relatively slowly. Time de velopments of the wave spectrum and the electron distribution function are in detail examined. The relaxation process is closely correlated with the asymptotic value of the energy density of the plasma wave excited. (author)