Numerical analysis of runaway electron beam focusing with a homogeneous longitudinal magnetic field

Abstract

Numerical simulation of the “runaway” electron acceleration and transport in a stronglyinhomogeneous electric field and a homogeneous guiding magnetic field was carried out. The electrode system was “conical cathode – spherical anode”. The opening angle of the cathode cone was 40º, a minimum distance between the electrodes was 3 cm. The interelectrode gap was filled with molecular nitrogen at a pressure of 1 atm. The voltage applied was 80 kV. The magnetic field was considered to be directed along the axis of the electrode system. The magnetic field induction was varied from 0 T to 4.3 T. Using a developed numerical 3D Monte-Carlo model, the dependence of the “runaway” electron beam radius on the induction of the guiding magnetic field was studied. It was shown that, beside the magnitude of the magnetic field induction, the beam radius was strongly affected by the point of its generation in the near-cathode region and the approximation of the differential cross section for high-energy electron scattering. The assumption was made that, in the experiment, the radius of the “runaway” electron beam was determined rather by the size of the near-cathode plasma, which was formed at the initial stage of the discharge development.