Reduction of core-resonant mϭ1 magnetic fluctuations and improved confinement in the Madison Symmetric Torus ͓Dexter et al., Fusion Technol. 19, 131 ͑1991͔͒ 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 mϭ0 magnetic fluctuations. Now, through refined poloidal electric field control, plus control of the toroidal electric field, it is possible to reduce simultaneously the mϭ0 and mϭ1 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 kA͒. At low current ͑200 kA͒, the total beta reaches 15% with an estimated energy confinement time of 10 ms, a tenfold increase over the standard value which for the first time substantially exceeds the constant-beta confinement scaling that has characterized most reversed-field-pinch plasmas.
A biased electrode is used in the Madison Symmetric Torus ͑MST͒ reversed-field pinch ͓Fusion Technol. 19, 131 ͑1991͔͒ 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.
Large radial electric field gradients, leading to sheared E 3 B flow, are observed in enhanced confinement discharges in the Madison Symmetric Torus reversed-field pinch. The flow shear develops in a narrow region in the plasma edge. Electrostatic fluctuations are reduced over the entire plasma edge with an extra reduction in the shear region. Magnetic fluctuations, resonant in the plasma core but global in extent, are also reduced. The reduction of fluctuations in the shear region is presumably due to the strong shear, but the causes of the reductions outside this region have not been established. [S0031-9007(98)05480-5] PACS numbers: 52.55. Hc, 52.25.Fi, 52.25.Gj, 52.35.Qz
Improved confinement has been achieved in the MST through control of the poloidal electric field, but it is now known that the improvement has been limited by bursts of an edge-resonant instability. Through refined poloidal electric field control, plus control of the toroidal electric field, we have suppressed these bursts. This has led to a total beta of 15% and a reversed-field-pinch-record estimated energy confinement time of 10 ms, a tenfold increase over the standard value which for the first time substantially exceeds the confinement scaling that has characterized most reversed-field-pinch plasmas.
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 ͑1991͔͒ reversed-field pinch. Measurements in standard ͑low confinement͒ 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 ͑1͒ when auxiliary current is driven in the edge and ͑2͒ spontaneously, following sawtooth crashes. Both edge electrostatic and global magnetic fluctuations are reduced in these discharges, and both particle and energy confinement improve.
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