C.-S. Chiang

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 ...

Momentum transport and flow damping in the reversed-field pinch plasma

A biased electrode is used in the Madison Symmetric Torus (MST) reversed-field pinch [Fusion Technol. 19, 131 (1991)] to manipulate plasma flow in order to study flow damping and momentum transport. Finite radial conductivity allows a radial current, which provides the toroidal torque that spins up the plasma. The applied torque is balanced by a viscous force that opposes toroidal flow acceleration. From the plasma flow damping the viscosity is inferred to be anomalous. The radial transport of toroidal momentum is comparable to that of particles and energy, and is consistent with transport by stochastic magnetic field lines.

Modifications to the edge current profile with auxiliary edge current drive and improved confinement in a reversed-field pinch

Auxiliary edge current drive is routinely applied in the Madison Symmetric Torus [R. N. Dexter, D. W. Kerst, T. W. Lovell et al., Fusion Technol. 19, 131 (1991)] with the goal of modifying the parallel current profile to reduce current-driven magnetic fluctuations and the associated particle and energy transport. Provided by an inductive electric field, the current drive successfully reduces fluctuations and transport. First-time measurements of the modified edge current profile reveal that, relative to discharges without auxiliary current drive, the edge current density decreases. This decrease is explicable in terms of newly measured reductions in the dynamo (fluctuation-based) electric field and the electrical conductivity. Induced by the current drive, these two changes to the edge plasma play as much of a role in determining the resultant edge current profile as does the current drive itself.

E×B flow shear and enhanced confinement in the Madison Symmetric Torus reversed-field pinch

Strong E×B flow shear occurs in the edge of three types of enhanced confinement discharge in the Madison Symmetric Torus [Dexter et al., Fusion Technol. 19, 131 (1991)] reversed-field pinch. Measurements in standard (low confinement) discharges indicate that global magnetic fluctuations drive particle and energy transport in the plasma core, while electrostatic fluctuations drive particle transport in the plasma edge. This paper explores possible contributions of E×B flow shear to the reduction of both the magnetic and electrostatic fluctuations and, thus, the improved confinement. In one case, shear in the E×B flow occurs when the edge plasma is biased. Biased discharges exhibit changes in the edge electrostatic fluctuations and improved particle confinement. In two other cases, the flow shear emerges (1) when auxiliary current is driven in the edge and (2) spontaneously, following sawtooth crashes. Both edge electrostatic and global magnetic fluctuations are reduced in these discharges, and both particl...

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.

Confinement improvement in the MST reversed field pinch

Summary form only given. The energy loss in the reversed field pinch (RFP) predominantly results from parallel streaming in a stochastic magnetic field. This stochasticity results from B/spl tilde//B/spl sim/1% magnetic fluctuations which accompany m=1, n/spl sim/2R/a tearing (or resistive kink) instabilities in the plasma core. Major research goals in the MST are to understand fluctuation induced transport and to improve plasma confinement using this understanding. Magnetic fluctuation induced transport in the plasma core is studied during a period of high magnetic activity preceding sawteeth events. The flow dynamics of bulk plasma rotation is examined by measuring Doppler shifts of impurity ions spectral lines. Both, spontaneous and actively driven confinement improvement regimes have been observed. After machine conditioning with solid-target boronization, a high confinement regime, characterized by the absence of sawteeth, spontaneously appears during low-density discharges. Similar improvements result by actively applying a transient auxiliary inductive electric field to the MST plasma. The current density gradient is reduced, the growth of the m=1 tearing fluctuations slows, and the energy confinement time doubles. To sustain and enhance the improved plasma, electrostatic and Rf current drivers are being developed.

Fluctuation and transport reduction by current profile control in MST

Summary form only given. The dominant energy loss mechanism in the reversed field pinch (RFP) results from parallel streaming in a stochastic magnetic field. This stochasticity results from B~/B/spl sim/1% magnetic fluctuations which accompany m=1, n/spl sim/2R/a tearing (or resistive kink) instabilities in the plasma core. Inductive, electrostatic, and RF current drive are being explored as techniques to reduce the tearing fluctuation and the associated energy loss. For example, by applying an auxiliary poloidal inductive electric field to the MST RFP plasma, the current density gradient is reduced, the growth of the m=1 tearing fluctuations slows, and the energy confinement time doubles. Sawteeth associated with the m=1 instabilities are also suppressed. Since a toroidal flux change linking the plasma is required, inductive current drive must be transient to avoid excessive perturbation of the equilibrium, To sustain and enhance the improved plasma, electrostatic and RF current drivers are being developed. The novel electrostatic current drive scheme utilizes a plasma source for electron injection.