Two-fluid biasing simulations of the large plasma device

Dustin M Fisher,Barrett N Rogers

doi:10.1063/1.4975616

Abstract

External biasing of the Large Plasma Device (LAPD) and its impact on plasma flows and turbulence are explored for the first time in 3D simulations using the Global Braginskii Solver code. Without external biasing, the LAPD plasma spontaneously rotates in the ion diamagnetic direction. The application of a positive bias increases the plasma rotation in the simulations, which show the emergence of a coherent Kelvin Helmholtz (KH) mode outside of the cathode edge with poloidal mode number m≃6. Negative biasing reduces the rotation in the simulations, which exhibit KH turbulence modestly weaker than but otherwise similar to unbiased simulations. Biasing either way, but especially positively, forces the plasma potential inside the cathode edge to a spatially constant, KH-stable profile, leading to a more quiescent core plasma than the unbiased case. A moderate increase in plasma confinement and an associated steepening of the profiles are seen in the biasing runs. The simulations thus show that the application of external biasing can improve confinement while also driving a Kelvin-Helmholtz instability. Ion-neutral collisions have only a weak effect in the biased or unbiased simulations.

Highlights

Over the past decade, researchers have explored the effects of plasma biasing and its impact on flows and turbulence in the Large Plasma Device (LAPD).1 Without external biasing, the LAPD plasma spontaneously rotates in the ion diamagnetic direction, expected theoretically and reproduced by our simulations as the plasma charges slightly positive to counterbalance the larger electron thermal streaming particle losses along the magnetic field to the end walls
External biasing of the Large Plasma Device (LAPD) and its impact on plasma flows and turbulence are explored for the first time in 3D simulations using the Global Braginskii Solver code
The LAPD plasma spontaneously rotates in the ion diamagnetic direction, expected theoretically and reproduced by our simulations as the plasma charges slightly positive to counterbalance the larger electron thermal streaming particle losses along the magnetic field to the end walls

Summary

INTRODUCTION

Researchers have explored the effects of plasma biasing and its impact on flows and turbulence in the Large Plasma Device (LAPD). Without external biasing, the LAPD plasma spontaneously rotates in the ion diamagnetic direction, expected theoretically and reproduced by our simulations as the plasma charges slightly positive to counterbalance the larger electron (than ion) thermal streaming particle losses along the magnetic field to the end walls. Our numerical model employs a simple limiter shape near the cathode-anode source end of the simulation domain that aims to partially model the experiments of Schaffner et al., in which an annular limiter near the cathode end of LAPD is biased with respect to the core plasma and anode. This limiter is encompassed by a second electrically isolated biasable annulus that terminates radially at the side walls and can be separately biased during experiments or left to float.

BIASING SETUP

BIASING RESULTS

CONCLUSIONS