Geometric scalings for the electrostatically driven helical plasma state

TL;DR

This paper investigates a new electrostatically driven helical plasma state in a periodic cylinder, focusing on how its properties scale with geometry and how different helicities affect its behavior, with potential applications in electrical transformers and tokamak current shaping.

Contribution

It analyzes the geometric scalings and helicity variations of a novel plasma state driven by helical electrodes in a periodic cylinder.

Findings

State exhibits strong axial current density with safety factor near electrode pitch.

Scaling properties depend on plasma aspect ratio and length.

Different helicities alter the plasma's magnetic and current profiles.

Abstract

A new plasma state has been investigated [C.~Akcay, J.~Finn, R.~Nebel and D.~Barnes, Phys.~Plasmas $\textbf{24}$, 052503 (2017)], with a uniform applied axial magnetic field in a periodic cylinder of length $L=2\pi R$, driven by helical electrodes. The drive is single helicity, depending on $m\theta+kz=m\theta-n\zeta$, where $\zeta=z/R$ and $k=-n/R$. For strong $(m,n)=(1,1)$ drive the state was found to have a strong axial mean current density, with a mean-field safety factor $q_0(r)$ just above the pitch of the electrodes $m/n=1$ in the interior. This state has possible applications to DC electrical transformers and tailoring of the current profile in tokamaks. We study two geometric issues of interest for these applications: (i) scaling of properties with the plasma length or aspect ratio; and (ii) behavior for different helicities, specifically $(m,n)=(1,n)$ for $n>1$ and $(m,n)=(2,1)$.