A. K. Hansen

High confinement plasmas in the Madison Symmetric Torus reversed-field pinch

Reduction of core-resonant m=1 magnetic fluctuations and improved confinement in the Madison Symmetric Torus [Dexter et al., Fusion Technol. 19, 131 (1991)] reversed-field pinch have been routinely achieved through control of the surface poloidal electric field, but it is now known that the achieved confinement has been limited in part by edge-resonant m=0 magnetic fluctuations. Now, through refined poloidal electric field control, plus control of the toroidal electric field, it is possible to reduce simultaneously the m=0 and m=1 fluctuations. This has allowed confinement of high-energy runaway electrons, possibly indicative of flux-surface restoration in the usually stochastic plasma core. The electron temperature profile steepens in the outer region of the plasma, and the central electron temperature increases substantially, reaching nearly 1.3 keV at high toroidal plasma current (500 kA). At low current (200 kA), the total beta reaches 15% with an estimated energy confinement time of 10 ms, a tenfold ...

Locking of multiple resonant mode structures in the reversed-field pinch

Locking of a rotating mode by applying a resonant magnetic perturbation having the same helicity has been observed on various devices. Experiments have been carried out on the Madison Symmetric Torus reversed-field pinch (RFP) [Dexter et al., Fusion Technol. 19, 131 (1991)] which show that an externally applied magnetic perturbation can cause locking of the dominant magnetic modes (poloidal mode number m=1, toroidal mode numbers n=5–10) when the perturbation is resonant with them. A perturbation which is not resonant (m=0 or 2) produces no such effect. Thus, resonant torques may lock a stochastic magnetic structure arising from several modes, as likely exists in the RFP, as well as a distinct island as exists in tokamaks, although the details of the interaction are likely to be different.

Plasma flow in MST: Effects of edge biasing and momentum transport from nonlinear magnetic torques

Edge biasing in MST plasmas decreases electrostatic turbulent particle transport and increases the global particle confinement time. New Langmuir probe measurements in the edge identify decreased electric field fluctuations and increased anti-correlation of density and potential fluctuations to be responsible. Fast loss of momentum in the core of MST during sawtooth crash events can be explained as a result of nonlinear magnetic torques which allow viscous coupling over relatively distant regions of the plasma. Flow modifications resulting from biasing, plus other experiments, help reveal the nonlinear nature of this process, most directly measured by the triple product bispectral correlation between the nonlinearly interacting modes.