Fokker-Planck simulations of knock-on electron runaway avalanche and bursts in tokamaks

The avalanche of runaway electrons in an ohmic tokamak plasma triggered by knock-on collisions of traces of energetic electrons with the bulk electron s is simulated by the bounce averaged Fokker-Planck code, CQL3D. It is show n that even when the electric field is small for the production of Dreicer runaways, the knock-on collisions can produce significant runaway electrons in a fraction of a second at typical reactor parameters. The energy spectr um of these knock-on runaways has a characteristic temperature. The growth rate and temperature of the runaway distribution are determined and compare d with theory. In simulations of pellet injection into high temperature pla smas, it is shown that a burst of Dreicer runaways may also occur depending on the cooling rate due to the pellet injection. Implications of these phe nomena on disruption control in reactor plasmas are discussed.