Disruption Avoidance on TFTR

Disruptions on the Tokamak Fusion Test Reactor (TFTR), especially those occurring at high stored energy, result in lost experimental run time because many discharges are required to regain wall conditions necessary for good plasma performance. A variety of disruption types have been observed on TFTR. These include density-limit disruptions, those caused by a high influx of impurities, those occurring during the current ramp-down, those resulting from locked modes, and those occurring at high normalized β(β_N = β_TaB_T/I_p). A combination of operational experience and limiter development has helped to avoid many potential disruptions. However, the experimental goal of high fusion power production engenders the risk of high-β_N disruptions. A system to limit β_N by reducing the neutral beam power as a preprogrammed β_N limit is reached is now in use to help avoid high-β_N disruptions. Operational issues of disruption avoidance, the β_N feedback system, the limitations and possible improvements of the system are discussed.