Formation of transient high-β plasmas in a magnetized, weakly collisional regime

T Byvank,K J Mccollam,C B Forest,D A Endrizzi,S J Langendorf,S C Hsu

doi:10.1017/s0022377820001488

T Byvank, K J Mccollam + Show 4 more

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https://doi.org/10.1017/s0022377820001488

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Abstract

We present experimental data providing evidence for the formation of transient ( ${\sim }20\ \mathrm {\mu }\textrm {s}$ ) plasmas that are simultaneously weakly magnetized (i.e. Hall magnetization parameter $\omega \tau > 1$ ) and dominated by thermal pressure (i.e. ratio of thermal-to-magnetic pressure $\beta > 1$ ). Particle collisional mean free paths are an appreciable fraction of the overall system size. These plasmas are formed via the head-on merging of two plasmas launched by magnetized coaxial guns. The ratio $\lambda _{\textrm {gun}}=\mu _0 I_{\textrm {gun}}/\psi _{\textrm {gun}}$ of gun current $I_{\textrm {gun}}$ to applied magnetic flux $\psi _{\textrm {gun}}$ is an experimental knob for exploring the parameter space of $\beta$ and $\omega \tau$ . These experiments were conducted on the Big Red Ball at the Wisconsin Plasma Physics Laboratory. The transient formation of such plasmas can potentially open up new regimes for the laboratory study of weakly collisional, magnetized, high- $\beta$ plasma physics; processes relevant to astrophysical objects and phenomena; and novel magnetized plasma targets for magneto-inertial fusion.

Highlights

Collisional plasmas with ratio of thermal-to-magnetic pressure β > 1 and, simultaneously, Hall magnetization parameter ωτ > 1, represent a frontier regime of laboratory plasma physics research
The head-on collisions (i) increase the duration for which the desired plasma state exists at a particular location and (ii) increase the magnitudes of the density and radial and toroidal magnetic field components, which widens the parameter space compared with that achievable with individual plasmas
As the plasmas propagate into the vacuum chamber from the magnetized coaxial guns, they expand into an applied background magnetic field that is oriented along the propagation direction (z axis)

Summary

Introduction

Collisional plasmas (particle mean free paths system size) with ratio of thermal-to-magnetic pressure β > 1 and, simultaneously, Hall magnetization parameter ωτ > 1 (i.e., gyro-frequency greater than collision frequency), represent a frontier regime of laboratory plasma physics research. Magnetically confined plasmas typically have β 1 and ωτ 1, whereas inertially confined and high-energy-density plasmas typically have β 1 and ωτ 1. If plasmas with both β, ωτ > 1 can be formed successfully, an interesting step will be to attempt to generate small-scale, tangled magnetic field with connection length much longer than the characteristic scale size of the plasma.

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