Particle-in-cell simulations of anomalous transport in a Penning discharge (1711.09790v3)

Abstract: Electrostatic particle-in-cell simulations of a Penning discharge are performed in order to investigate azimuthally asymmetric, spoke-like structures previously observed in experiments. Two-dimensional simulations show that for Penning-discharge conditions, a persistent nonlinear spoke-like structure forms readily and rotates in the direction of ExB and electron diamagnetic drifts. The azimuthal velocity is within about a factor of two of the ion acoustic speed. The spoke frequency follows the experimentally observed scaling with ion mass, which indicates the importance of ion inertia in spoke formation. The spoke provides enhanced (anomalous) radial electron transport, and the effective cross-field conductivity is several times larger than the classical (collisional) value. The level of anomalous current obtained in the simulations is in good agreement with the experimental data. The rotating spoke channels most of the radial current, observable by an edge probe as short pulses.

• The paper demonstrates that azimuthally rotating spoke structures drive enhanced radial electron transport in Penning discharges.
• The paper employs a modified PPPL-LSP code in 2D PIC simulations to capture critical kinetic effects across varied gas species and conditions.
• The paper finds that effective cross-field conductivity far exceeds classical predictions, aligning well with experimental data.

Anomalous Transport in Penning Discharges: A Simulation Study

The paper "Particle-in-cell simulations of anomalous transport in a Penning discharge" presents an in-depth exploration of the behavior and characteristics of anomalous transport phenomena within Penning discharges using electrostatic particle-in-cell (PIC) simulations. The research primarily focuses on the elucidation of azimuthally asymmetric, spoke-like structures manifested within a Penning discharge, a configuration commonly observed in various crossed-field plasma devices such as Hall thrusters.

Simulation Methodology and Setup

The paper employs two-dimensional PIC simulations to investigate the nonlinear dynamics and anomalous transport effects in the Penning discharge. Utilizing the Large-Scale Plasma (LSP) code, modified here as PPPL-LSP, the simulations capture kinetic effects essential for understanding plasma behavior under Penning-discharge conditions. These simulations were conducted for different gas species, including helium, argon, and xenon, at varying pressure and magnetic field strengths tailored to reflect experimental conditions observed at the Princeton Plasma Physics Laboratory (PPPL).

Key Findings and Results

The simulation results consistently demonstrate the formation of persistent rotating spoke-like structures that significantly influence radial electron transport. The spokes exhibit an azimuthal velocity approaching the ion acoustic speed, approximately scaling with the square root of the ion mass, indicative of the essential role of ion inertia in spoke formation. Moreover, the effective cross-field conductivity within the discharge significantly exceeds classical predictions, with the simulations providing numerical agreement with experimental values for the anomalous current levels.

The spoke structures predominantly channel radial leakage current, characterized by short pulses observable at the edge of the simulation domain. The interplay between ExB and electron diamagnetic drifts is highlighted, with the simulation results suggesting that neither drift dominates completely, and localized variations exist depending on plasma conditions.

Theoretical Implications and Comparative Analysis

The simulations reflect the nonlinear and multimodal nature of the spoke structures, with their characteristics aligning with predictions made from modified Simon-Hoh instability theory. Furthermore, the paper discusses the challenges inherent in scaling the simulation domain to resolve the Debye length while addressing the computational constraints posed by the large domain sizes required for realistic device dimensions.

The dispersion relation provided in the simulations offers a theoretical framework to compare and predict the behavior of the spokes and associated instabilities. Although the simulated frequencies do not precisely match theoretical predictions, they align well with experimental observations, providing a robust basis for future explorations into plasma device configurations and performance optimization.

Conclusion and Future Directions

This paper advances our understanding of anomalous transport in crossed-field plasma devices by leveraging the sophisticated computational capabilities offered by PIC simulations. The results underscore the significance of these spokes and their impact on plasma confinement and transport, providing a valuable tool for future experimental and theoretical studies. The insights drawn from this research not only illuminate the complex physics of Penning discharges but also hold implications for enhancing the performance and longevity of plasma devices like Hall thrusters.

The methodological advancements and findings in this paper pave the way for future work to incorporate ionization effects, extend the scalability of simulation parameters, and ultimately transition towards comprehensive three-dimensional modeling of Hall-effect thruster configurations, as the computational resources and techniques continue to evolve.