Dynamics of a toroidal pure electron plasma using 3D PIC simulations

Abstract

Nonlinear dynamics of toroidally confined, initially cold, collisionless pure electron plasma has been numerically simulated in a tight aspect ratio, axisymmetric device, confined using a toroidal magnetic field, using a 3D3V particle-in-cell code PEC3PIC. A set of three numerical experiments are conducted by loading the toroidal electron cloud at varying radial distances from the central axis at the vertical midplane, and a comparative analysis of the progression of cloud dynamics and particle transport in the three experiments is carried out. In each experiment, the cloud is seen to initiate toroidal Diocotron oscillations with the following interesting features: (i) initial nonlinear reshaping and density peaking, (ii) elliptical orbital path in the poloidal cross section along with chirp or rotational frequency dynamics and the increase and decrease in the peak density of the filled electron cloud, (iii) cross-field transport and particle loss, and (iv) the measured wall probe signals showing close similarity to experimental signals. It is demonstrated that relatively better confinement of electrons in the toroidal configuration is achieved by loading the initial plasma at the vertical midplane, close to the inner wall of the chamber, supporting the mean-field theoretical predictions. For all cases, the density distribution profiles in the (r−θ) and (r − z) planes of the cylindrical coordinate system (r,θ,z) have consistent peaked density central profiles. The time dependency of the dominant frequencies of the dynamics, obtained from wall probe data using Hilbert–Huang transformation and windowed Fourier transformation, suggests toroidicity induced low poloidal number m (∼1−12) coupling and dynamical chirping.