Raed Kombargi

Observation of wall stabilization and active control of low-n magnetohydrodynamic instabilities in a tokamak*

The High Beta Tokamak‐Extended Pulse (HBT‐EP) experiment [J. Fusion Energy 12, 303 (1993)] combines an internal, movable conducting wall with a high‐power, modular saddle coil system to provide passive and active control of long wavelength magnetohydrodynamic (MHD) instabilities. Systematic adjustment of the radial position, b, of the conducting wall elements in relation to the surface of the plasma (minor radius a) resulted in the suppression of β‐limiting disruptions for discharges in which b/a<1.2 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 β values near the Troyon stability boundary. The frequency of slowly growing modes that persisted in wall‐stabilized discharges was controlled by applying oscillating m=2, n=1 resonant magnetic perturbations. A compact, single‐phase saddle coil system permitted modulation of the rotation velocity of internal m/n=2/1 instabilities by a f...

Stabilization of kink instabilities by eddy currents in a segmented wall and comparison with ideal MHD theory

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.

Initial high beta operation of the HBT-EP Tokamak

HBT-EP is a new research tokamak designed and built to investigate passive and active feedback techniques to control MHD instabilities. In particular, HBT-EP will be able to test techniques to control fast MHD instabilities occurring at high Troyon-normalized beta, βN ≡ βBa/Ip [Tm/MA], since it is equipped with a thick, close-fitting, and adjustable conducting shell. The major goals of the initial operation of HBT-EP have been the achievement of high beta operation (βN ∼ 3) using only ohmic heating and the observation of MHD instabilities. By using a unique fast startup technique, we have successfully achieved these goals. A variety of MHD phenomena were observed during the high beta operation of HBT-EP. At modest beta (βN ≤ 2), discharges have been maintained for more than 10 msec, and these discharges exhibit saturated resistive instabilities. When βN approaches 3, major disruptions occur preceded by oscillating, growing precursors. During start-up, one or more minor disruptions are usually observed. A 1-D transport code has been used to simulate the evolution of the current profile, and these early minor instabilities are predicted to be double tearing modes. The simulation also reproduces the observed high beta operation when saturated neo-Alcator energy confinement scaling is assumed.