BETA NORMAL CONTROL OF TFTR USING FUZZY LOGIC

Abstract

In TFTR plasmas heated by neutral beam injection, the fusion power yield increases rapidly with the plasma pressure. However, the pressure is limited by the onset of instabilities which may result in plasma disruptions that would have had an adverse effect on the performance of subsequent discharges and increase the risk of damage to internal components. The likelihood of disruption has been found to correlate with the normalized beta, defined as is the volume-average plasma perpendicular pressure, a the mid-plane minor radius of the plasma, BT the toroidal magnetic field and I P the plasma current. Other variables, such as the peaking of the plasma pressure and current profiles, have been found to influence the threshold of β N at which the probability of disruption begins to increase significantly. For TFTR plasmas with high fusion performance (TFTR “supershots”) the probability of disruption has been found to increase rapidly for β N > 1.8. Since confinement in this regime is affected by plasma-wall interaction, which can vary from shot to shot, operation at high β N with preprogrammed heating power pulses can produce an unaccept-ably high risk of disruption. To reduce the risk of producing beta-limit disruptions during neutral beam heating experiments, a control system, the Neutral Beam Power Feedback System (NBPFS), has been developed to modulate the total heating power by switching individual neutral beam sources on and off in response to the evolution of the normalized beta so that the limit will not be exceeded.