Three-dimensional Helical-rotating Plasma Structures in Beam-generated Partially Magnetized Plasmas

TL;DR

This paper investigates azimuthal plasma structures in beam-generated partially magnetized plasmas, revealing two distinct instability regimes and proposing analytical formulas for critical thresholds and rotation frequencies, supported by simulations and preliminary experiments.

Contribution

It introduces a detailed analysis of two instability regimes in such plasmas and provides analytical formulas for thresholds and rotation frequencies, supported by simulations and experiments.

Findings

Identification of two instability regimes at different pressures.

Development of analytical formulas for threshold pressure and rotation frequency.

Experimental verification of the simulated plasma structures.

Abstract

Azimuthal structures emerging in beam-generated partially magnetized plasmas are investigated using three-dimensional particle-in-cell/Monte Carlo collision simulations. Two distinct instability regimes are identified at low pressures. When the gas pressure is sufficiently high, quasi-neutrality is attained and 2D spiral-arm structures form as a result of the development of a lower-hybrid instability, resulting in enhanced cross-field transport. At lower pressures, quasi-neutrality is not achieved and a 3D helical-rotating plasma structure forms due to development of the diocotron instability. Analytical formulas are proposed for the critical threshold pressure between these regimes and for the rotation frequency of the helical structures. Preliminary experimental verification is provided.