Geometric scalings for the electrostatically driven helical plasma state

Abstract

A new plasma state has been investigated [Akcay et al., Phys. Plasmas 24, 052503 (2017)], with a uniform applied axial magnetic field in a periodic cylinder of length L=2πR, driven by helical electrodes. The drive is single helicity, depending on mθ+kz=mθ−nζ, where ζ=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 q0(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).