The Effect of Non‐Coulomb Scattering of Relativistic Electrons on the Generation of Runaways in Multicomponent Plasma

Highly energetic runaway electrons are able to penetrate the electron shell of partly ionized heavy ions during collisions, for which reason they may be scattered by a positive charge effectively larger than that of a shielded nucleus. This effect increases the Coulomb cross section and can be treated via an effective ion charge Z eff ( ε kin ) that depends on the energy of the incident electrons ε kin . The increase of effective charge number with increasing electron energy in multi‐component plasmas renders qualitatively the same result as high Z eff Coulomb plasmas. Since the generation rate of runaways depends on Z eff , its production during the mitigation of disruptions by massive gas injection could in some cases decrease owing to a heavy impurity concentration in the boundary tokamak plasma. This may explain why it has been observed that the runaway's avalanche is suppressed at electron densities below the so‐called “Rosenbluth density” (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)