Runaway electron production in DIII-D killer pellet experiments, calculated with the CQL3D/KPRAD model

Runaway electrons are calculated to be produced during the rapid plasma coo ling resulting from "killer pellet" injection experiments, in general agree ment with observations in the DIII-D [J. L. Luxon , Plasma Physics and Cont rolled Nuclear Fusion Research 1986 (International Atomic Energy Agency, Vi enna, 1987), Vol. I, p. 159] tokamak. The time-dependent dynamics of the ki netic runaway distributions are obtained with the CQL3D [R. W. Harvey and M . G. McCoy, "The CQL3D Code," in Proceedings of the IAEA Technical Committe e Meeting on Numerical Modeling, Montreal, 1992 (International Atomic Energ y Agency, Vienna, 1992), p. 489] collisional Fokker-Planck code, including the effect of small and large angle collisions and stochastic magnetic fiel d transport losses. The background density, temperature, and Z(eff) are evo lved according to the KPRAD [D. G. Whyte and T. E. Evans , in Proceedings o f the 24th European Conference on Controlled Fusion and Plasma Physics, Ber chtesgaden, Germany (European Physical Society, Petit-Lancy, 1997), Vol. 21 A, p. 1137] deposition and radiation model of pellet-plasma interactions. T hree distinct runway mechanisms are apparent: (1) prompt "hot-tail runaways " due to the residual hot electron tail remaining from the pre-cooling phas e, (2) "knock-on" runaways produced by large-angle Coulomb collisions on ex isting high energy electrons, and (3) Dreicer "drizzle" runaway electrons d ue to diffusion of electrons up to the critical velocity for electron runaw ay. For electron densities below approximate to 1x10(15) cm(-3), the hot-ta il runaways dominate the early time evolution, and provide the seed populat ion for late time knock-on runaway avalanche. For small enough stochastic m agnetic field transport losses, the knock-on production of electrons balanc es the losses at late times. For losses due to radial magnetic field pertur bations in excess of approximate to0.1% of the background field, i.e., delt aB(r)/B greater than or equal to0.001, the losses prevent late-time electro n runaway. (C) 2000 American Institute of Physics. [S1070-664X(00)02111-X].