Electrostatically driven helical plasma state
A novel plasma state has been found in the presence of a uniform applied axial magnetic field in periodic cylindrical geometry. This state is driven electrostatically by helical electrodes, providing a driving field that depends on radius and $m\theta-n\zeta$, where $\theta$ is the poloidal angle and $\zeta=z/R$ is the toroidal angle. We focus on $m=n=1$. The radial magnetic field at the wall is taken to be zero. With weak driving, the resulting distortion is very small, but for stronger driving the mean field of the state has field line safety factor $q_0(r)$ just above the pitch of the electrodes $m/n=1$ except near the edge, where $q_0$ increases monotonically. This state is characterized as a single helicity Ohmic equilibrium with the helical symmetry of the applied field. The plasma appears to be close to force-free in the interior, but current density crosses the magnetic flux surfaces near the edge, where current must enter and exit through the helical electrodes. This perpendicular current density drives large helical plasma flows. Sensitivity of this state to flow boundary conditions, plasma resistivity profile, strength of the electrostatic driving, and parameters such as the loop voltage and the Lundquist number is explored. The magnetic helicity is calculated for both the transient period and time-asymptotic state. Possible applications to current drive in toroidal confinement devices and to electrical transformers are discussed.