Properties of the electrostatically driven helical plasma state

TL;DR

This paper investigates a newly discovered electrostatically driven helical plasma state with unique current and flow properties, exploring its potential for applications in electrical transformers and tokamak current profile control.

Contribution

It provides detailed analysis of the plasma state's properties, including current density, flow dynamics, and response to boundary conditions, advancing understanding for practical applications.

Findings

The plasma state exhibits a strong axial current density near the interior.

Perpendicular current components drive nearly Alfvénic helical flows.

The state shows sensitivity to boundary conditions and resistivity profiles.

Abstract

A novel plasma state has been found [C.~Ak\c{c}ay, J.~Finn, R.~Nebel and D.~Barnes, Phys.~Plasmas \textbf{24}, 052503 (2017)] in the presence of a uniform applied axial magnetic field in periodic cylindrical geometry. This state is driven by external electrostatic fields provided by helical electrodes, and depends on radius $r<r_w$ and $m\theta-n\zeta$, where $m=n=1$, $\theta$ is the poloidal angle, and $\zeta=z/R$ is the toroidal angle. In this reference, the strongly driven form of the state was found to have a strong axial mean current density, with a mean-field line safety factor $q_0(r)$ just above the pitch of the electrodes $m/n=1$ in the interior, where the plasma is nearly force-free. However, at the edge the current density has a component perpendicular to $\mathbf{B}$. This perpendicular current density drives nearly Alfv\'enic helical plasma flows, an notable feature of these states. This state is being studied for its possible application to DC electrical transformers and possibly tailoring the current profile in tokamaks. We present results on several issues of importance for these applications: the transient leading to the steady state; the twist and writhe of the field lines and their relation with the current density; the properties of the current density streamlines and length of the current density lines connected to the electrodes; the sensitivity to changes in the velocity boundary conditions; the effect of varying the radial resistivity profile; and the effects of a concentrated electrode potential.