Anisotropy-driven collisional separation of impurities in magnetized compressing and expanding cylindrical plasmas

TL;DR

This paper investigates how velocity-space anisotropy, generated during plasma compression or expansion, significantly influences impurity transport in magnetized plasmas, especially affecting impurity distribution in plasma edges.

Contribution

It introduces a new understanding of impurity transport driven by pressure anisotropy in magnetized plasmas undergoing compression or expansion, independent of temperature or density gradients.

Findings

Anisotropy can reverse impurity transport direction.

Impurity transport depends on magnetic field pitch and radial velocity.

Anisotropy effects are prominent in plasma edge regions.

Abstract

When a cylindrically-symmetric magnetized plasma compresses or expands, velocity-space anisotropy is naturally generated as a result of the different adiabatic conservation laws parallel and perpendicular to the magnetic field. When the compression timescale is comparable to the collision timescale, and both are much longer than the gyroperiod, this pressure anisotropy can become significant. We show that this naturally-generated anisotropy can dramatically affect the transport of impurities in the compressing plasma, even in the absence of scalar temperature or density gradients, by modifying the azimuthal frictions that give rise to radial particle transport. Although the impurity transport direction depends only on the sign of the pressure anisotropy, the anisotropy itself depends on the pitch magnitude of the magnetic field and the sign of the radial velocity. Thus, pressure anisotropy effects can drive impurities either towards or away from the plasma core. These anisotropy-dependent terms represent a qualitatively new effect, influencing transport particularly in the sparse edge regions of dynamically-compressing screw pinch plasmas. Such plasmas are used for both X-ray generation and magneto-inertial fusion, applications which are sensitive to impurity concentrations.