Characteristics of Interaction of Energetic Electrons with Heavy Impurity Ions in Tokamak Plasma

Collisions of the fast electrons with partially stripped high-Z impurity ions in a tokamak plasma are considered. The Thomas–Fermi model is used in calculation of the form factor which describes the screening of the potential of impurity nuclei. The calculated dependence of the form factor on the transferred momentum of the fast electron at elastic collisions allows accurate evaluation of the differential scattering cross section and transport collision frequency accounting for the relativistic effects. The transport collision frequency for the energetic electrons in the plasma with partially stripped impurity ions is shown to exceed strongly the frequency calculated for the fully ionized plasma with equivalent effective charge. This is apparently the situation occurring in the tokamak plasma disruptions accompanied by generation of runaway electrons. An algorithm to calculate collisional energy loss of the fast electrons is suggested as a generalization of that for electron drag in neutral media. Analysis of the collisional drag of the fast electrons on the partially ionized plasma demonstrates that formal replacement in the collision integral of the electron plasma density with the sum of the free electron density and half of the bound electron density provides sufficiently high, ~10%, accuracy in evaluating the fast electron energy losses. The presented results allow accurate evaluation of collisional effects in the simulation of the fast electron dynamics.