The High Beta Tokamak-Extended Pulse ͑HBT-EP͒ experiment ͓J. Fusion Energy 12, 303 ͑1993͔͒ combines an internal, movable conducting wall with a high-power, modular saddle coil system to provide passive and active control of long wavelength magnetohydrodynamic ͑MHD͒ instabilities. Systematic adjustment of the radial position, b, of the conducting wall elements in relation to the surface of the plasma ͑minor radius a͒ resulted in the suppression of -limiting disruptions for discharges in which b/aϽ1.2 and a positive plasma current ramp was maintained. Conducting wall stabilization of kink instabilities was observed in discharges with strong current ramps and in plasmas with  values near the Troyon stability boundary. The frequency of slowly growing modes that persisted in wall-stabilized discharges was controlled by applying oscillating mϭ2, nϭ1 resonant magnetic perturbations. A compact, single-phase saddle coil system permitted modulation of the rotation velocity of internal m/nϭ2/1 instabilities by a factor of 2.
The characteristics of external kink instabilities observed during wall stabilization studies in the HBT-EP tokamak have been compared with the predictions of ideal MHD theory, in order to examine the stabilizing role of a resistive wall that is segmented both toroidally and poloidally. The reconstructed equilibria, for discharges with different plasma-wall configurations, are consistent with external and internal magnetic measurements, measured soft X ray profiles and measured equilibrium wall eddy currents. The stability analysis of these equilibria predicts patterns of instability induced eddy currents for a model wall that is continuous and perfectly conducting, and these patterns are in good agreement with the ones observed on the HBT-EP segmented wall. These eddy currents account for the observed stabilization of fast ideal modes when the wall is fully inserted, consistent with the prediction of marginal stability.
It is widely believed that aberration, like the Doppler effect, depends on the relative velocity of source and observer. It is here shown that, if this were true, binary stars would mostly look widely separated and rapidly rotating. Not only is this not observed, but it would appear to conflict with Kepler's third law if it were. It is argued that aberration does not depend on the relative velocities of source and observer: it depends only on the change in velocity of the observer between the times when the two measurements from which the aberration is deduced are made. The misconception is due to a faulty customary interpretation of the correct standard treatment.
Research on the Alcator C-Mod tokamak is focused on exploiting compact high density plasmas to understand
core transport and heating, the physics of the H mode transport barrier, and the dynamics of the
scrape-off layer and divertor. Rapid toroidal acceleration of the plasma core is observed
during ohmic heated H modes
and indicates a momentum pinch or similar transport mechanism. Core thermal transport
observations support a critical gradient interpretation, but with gradients that disagree
with present theoretical values. High resolution measurements of the H mode barrier have
been obtained, including impurity and neutral densities, and the instability apparently
responsible for the favourable `enhanced D alpha' regime has been identified. Divertor
bypass dynamic control experiments have directly addressed the important questions
surrounding main chamber recycling and the effect of divertor closure on impurities
and confinement. Future plans include quasi-steady-state advanced tokamak plasmas using
lower hybrid current drive.
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