Beta normal limiting of TFTR neutral beams

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 βN (normalized beta) is calculated in real time and transmitted to the NBPFS. The value of βN and its calculated time derivative are input to a fuzzy logic controller which implements a proportional-derivative control based on the difference between βN and a programmed reference level βNREF 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 behavior. The tokamak operators specify the order in which sources will be turned off and on as the βN limit is approached. The system has been in operation since May 1994 and has shown that the βN can be controlled and that the neutral beam sources can operate reliably with this 10 ms modulation