Beta normal limiting of TFTR neutral beams

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 increase the risk of damage to internal components. The likelihood of disruption has been found to correlate with the normalized beta (β_N). For TFTR plasmas with high fusion performance (TFTR “supershots”) this probability of disruption has been found to increase rapidly. 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 unacceptably 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. The value of β_N is calculated in real time and transmitted to the NBPFS. The value of β_N and its calculated time derivative are input to a fuzzy logic controller which implements a proportional-derivative control based on the difference between β_N and a programmed reference level β_NREF which is programmed as a function of time. The output of the controller is a set of twelve signals, one for each neutral beam source, which can block injection. These signals are updated every 10 ms in response to the plasma behaviour. The tokamak operators specify the order in which sources will be turned off and on as the β_N limit is approached. The system has been in operation since May 1994 and has shown that the β_N can be controlled and that the neutral beam sources can operate reliably with this 10 ms modulation