ACTIVE CONTROL OF 2 1 MAGNETIC ISLANDS IN A TOKAMAK/

Closed and open loop control techniques were applied to growing m/n=2/ 1 rotating islands in wall-stabilized plasmas in the High Beta Tokamak -Extended Pulse (HBT-EP) [J. Fusion Energy 12, 303 (1993)]. HBT-EP com bines an adjustable, segmented conducting wall (which slows the growth or stabilizes ideal external kinks) with a number of small (6 degrees wide toroidally) driven saddle coils located between the gaps of the conducting wall. Two-phase driven magnetic island rotation control fro m 5 to 15 kHz has been demonstrated powered by two 10 MW linear amplif iers. The phase instability has been observed and is well modeled by t he single-helicity predictions of nonlinear Rutherford island dynamics for 2/1 tearing modes including important effects of ion inertia and finite Larmor radius, which appear as a damping term in the model equa tions. The closed loop response of active feedback control of the 2/1 mode at moderate gain was observed to be in good agreement with the th eory. Suppression of the 2/1 island growth has been demonstrated using an asynchronous frequency modulation drive which maintains the inerti al flow damping of the island by application of rotating control field s with frequencies alternating above and below the natural mode freque ncy. This frequency modulation control technique was also able to prev ent disruptions normally observed to follow giant sawtooth crashes in the plasma core. (C) I998 American Institute of Physics.