STABILIZATION BY RESISTIVE WALLS AND Q-LIMIT DISRUPTIONS IN TOKAMAKS

Stabilization of the M=2/n=1 tearing mode by resistive walls is effective in tokamaks at low-q operation, 2a&le2.6, when the mode rotates quickly compared with the time constant of the wall and with resistive growth times. A sharp stability limit at qa2 results from the loss of wall stabilization when the q=2 surface moves out of the conducting plasma. This stability limit correlates well with the experimentally observed disruptive limit in plasma current. Simulations of wall locking initiating a disruption at the qa=2 limit show very short precursors in comparison with density limit disruptions at high qa.