Ion-driven destabilization of a toroidal electron plasma -- A 3D3VPIC Simulation

TL;DR

This study uses 3D3V PIC simulations to investigate ion resonance instability in toroidal electron plasmas, revealing how small ion populations destabilize the plasma and transfer energy, affecting mode behavior and temperature profiles.

Contribution

It demonstrates the destabilizing effect of ion resonance instability on toroidal electron plasmas using high-fidelity 3D3V PIC simulations, highlighting the role of ion populations and temperature transfer.

Findings

Ion populations destabilize the electron plasma and increase mode coupling.

Wall probe current growth saturates over time for ion fractions ≥ 0.005.

Electron temperature scales as 1/R^2 along the flux tube.

Abstract

Ion resonance instability of toroidal electron plasmas in a tight aspect ratio axisymmetric toroidal device is reported for ${Ar}^+$ ions of different initial density values using a high fidelity 3D3V PIC solver. Stability of a recently discovered quiescent quasi-steady state (QQS) of a toroidal electron plasma obtained from "seed" solution as a result of entropy extremization at zero inertia, is addressed to the presence of a small ion population. An ion fraction ($f$) and corresponding number of secondary electrons are preloaded into the system after the electron plasma attains a QQS state. Driven by the ions, the electron plasma exhibits destabilized "center of charge motion" ($m = 1$) along with increased poloidal mode coupling ($m = 1$ to $9$) with dominant $m = 2$ mode. The growth in wall probe current is algebraic in nature and increases for $f$ $\geq$ $0.005$, showing saturation at later time. Higher values of ion temperatures than the electron temperatures indicate a resonant energy transfer from electron plasma to ions via ion-resonance and concomitant ion heating. The volume averaged temperature value of the electron plasma rises with simulation time, attaining a quasi-steady nature near the end of the simulation time. As can be expected from conservation of adiabatic invariants, the flux tube averaged electron temperatures along parallel and perpendicular directions are found to scale as $1/R^2$ and $1/R$ respectively, where $R$ is the major radial variable, though the plasma is nearly collision-less.