Comparison of Velocity Shear with Turbulence Reduction Driven by Biasing in a Simple Cylindrical Slab Plasma

Abstract

In an experimental realization of the sheared cylindrical slab, the level of plasma turbulence is strongly reduced by application of a sufficient bias potential difference in the radial direction. Density fluctuation levels ∆nrms/n decrease by more than a factor of five. The ion flow velocity profile is measured spectroscopically from the Doppler shift of an argon ion line. Comparison of the shearing rates with the turbulent amplitudes as a function of bias show no relation between the shearing rate and turbulence reduction, contrary to expectations.

Analysis of the influence of external biasing on Texas Helimak turbulence

“…[1][2][3][4] In the last years, several works have investigated how to control plasma turbulence and improve the confinement by imposing an external electric potential that changes the radial electric field profile. [5][6][7][8] Although there has been reported progress, further investigations are necessary to complete the experimental description, of the reported turbulence alterations obtained by controlling the electric field, and to better understand the influence of the biased electric potential on the turbulence spectral properties.…”

Turbulence and bias-induced flows in simple magnetized toroidal plasmas

“…When the bias voltage is made sufficiently large (here V b = −3), the biased state is reached in which the bias-induced vertical E × B flows pass through the plasma interior from the top and bottom boundaries. These equilibrium sheared flows appear to reduce the radial extent of turbulent structures; that is, the bias tends to reduce the radial correlations and increase the vertical correlations, as observed in the experiment [3]. Following the experiment, the transitions of plasma dynamics between the biased and grounded states are also simulated.…”

“…profiles are quite similar to the simulations. In the biasing experiment, the plates second from the inside were connected together and biased over the radius R from 0.86 to 1.06 m. Applying a negative bias voltage ∼− 20 V leads to the transition to a state in which plasma fluctuations decrease to a much lower level [3]. The global 3D simulations exhibit similar transitions: the negative bias V b = −3 (corresponding to a physical voltage ∼−20 V in the Helimak) leads to the biased state in which radial convective transport is reduced.…”

“…The shear magnitude (the gradient) of the flow is often expected to increase to suppress plasma turbulence. However, recent Helimak experiments observed that turbulence reductions can be driven by negative biasing and the bias-induced equilibrium flows with shear magnitudes comparable to the unbiased case are sufficient to reduce plasma fluctuations [3]. This unexpected observation is possibly because the nature of the interchange instability in the Helimak device is radially global, and the mean E × B flows are generated by an equilibrium electric potential arising from the sheath boundary conditions [4,5].…”

“…Simple magnetized torus (SMT) experiments [6][7][8] such as the Helimak device [3,9,10] provide opportunities to study the basic physics of plasma turbulence and transport in a simplified magnetic configuration that is most similar to the scape-off layer (SOL) of tokamaks. In the SMT configuration, a small vertical field B v is added to a dominant toroidal field B ϕ .…”