Self-consistent formation and steady-state characterisation of trapped high energy electron clouds in the presence of a neutral gas background

TL;DR

This paper investigates the formation and steady-state behavior of trapped high-energy electron clouds in coaxial geometries with neutral gas, using simulations and a fluid model to understand their dynamics under combined electric and magnetic fields.

Contribution

It introduces a self-consistent model of electron cloud formation considering strong radial electric fields and neutral gas collisions, supported by PIC simulations and a predictive fluid model.

Findings

Electron clouds remain axially trapped due to crossed fields.

Radial transport and ionisation are significant in these conditions.

The fluid model accurately predicts cloud density and current based on field parameters.

Abstract

This study considers the self-consistent formation and dynamics of electron clouds interacting with a background neutral gas through elastic and inelastic (ionisation) collisions in coaxial geometries similar to gyrotron electron guns. These clouds remain axially trapped as the result of crossed magnetic field lines and electric equipotential lines creating potential wells similar to those used in Penning traps. Contrary to standard Penning traps, in this study we consider a strong externally applied radial electric field which is of the same order as that of the space-charge field. In particular, the combination of coaxial geometry, strong radial electric fields and electron collisions with the residual neutral gas (RNG) present in the chamber induce non-negligible radial particle transport and ionisation. In this paper, the dynamics of the cloud density and currents resulting from electron-neutral collisions are studied using a 2D3V particle-in-cell code. Simulation results and parametric scans are hereby presented. Finally, a fluid model is derived to explain and predict the cloud peak density and peak radial current depending on the externally applied electric and magnetic fields, and on the RNG pressure.