Anisotropy of the Runaway Electron Generation Process in Strongly Inhomogeneous Electric Fields

Abstract

An investigation of runaway electron (RE) kinetics under the influence of a strongly inhomogeneous electric field was carried out by means of 2-D analytical and 2-D numerical Monte–Carlo approaches. Several shapes of cathodes generating an inhomogeneous electric field were studied. Within the developed 2-D analytical model, blade-shaped and cone-shaped cathodes were considered. The voltage values which allow electrons to scatter at large angles close to <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula> /2 near a blade-shaped cathode were determined. Also, it was shown that the RE scatter angles are larger for the blade (in comparison with the ones for the cone) due to the electric field defocusing effect. Within the framework of the numerical 2-D Monte–Carlo model, a cathode was assumed to have a parabolic shape. Inapplicability of the classical runaway criterion in strongly inhomogeneous electric fields was demonstrated. Also, it was shown that the cathode surface domain of electron emission and an initial direction of electron motion have a strong influence on their energy balance and, hence, on the probability for electrons to transit into the continuous accelerating regime. Energy losses of REs were estimated. A correlation between the full-width at half-maximum of the RE energy spectrum and an inelastic energy loss value was proposed.