Turbulence and bias-induced flows in simple magnetized toroidal plasmas

Turbulence and bias-induced flows in simple magnetized toroidal plasmas are explored with global three-dimensional fluid simulations, focusing on the parameters of the Helimak experiment. The simulations show that plasma turbulence and transport in the regime of interest are dominated by the ideal interchange instability. The application of a bias voltage alters the structure of the plasma potential, resulting in the equilibrium sheared flows. These bias-induced vertical flows located in the gradient region appear to reduce the radial extent of turbulent structures, and thereby lower the radial plasma transport on the low field side.

Figure 1

Schematic cross section of the Helimak showing the geometry and a helical field line. The plates labeled 1–4 are isolated from each other and can be independently biased.

Figure 2

Typical snapshots of density $n$ (left), electron temperature $T_e$ (center), and plasma potential $\varphi$ (right) in the cross section for zero bias (upper panels) and bias $V_b=-3$ (lower panels).

Figure 3

Typical profiles of density $n$, electron temperature $T_e$, and plasma potential $\varphi$ for zero bias (solid line) and bias $V_b=-3$ (dashed line).

Figure 4

Typical experimental profiles of density $n$ and electron temperature $T_e$ for zero bias (solid line) and bias voltage $-20$ V (dashed line).

Figure 5

Typical profiles of radial particle flux ${\Gamma }_n$ and heat flux ${\Gamma }_T$ for zero bias (solid line) and bias $V_b=-3$ (dashed line).

Figure 6

Typical profiles of vertical $\mathit{E}\times \mathit{B}$ flow $v_E$ and velocity shear for zero bias (solid line) and bias $V_b=-3$ (dashed line).

Figure 7

Typical experimental profiles of vertical flow and velocity shear for zero bias (solid line) and bias voltage $-20$ V (dashed line). The flow is normalized to $c_s\simeq 4.9\times {10}^3$ m/s and shear to ${\Omega }_i\simeq 2.4\times {10}^5$ ${\mathrm{s}}^{-1}$.