EVALUATION OF AN ITER COMPATIBLE, THIN FOIL FARADAY COLLECTOR AS A LOST ALPHA-PARTICLE DIAGNOSTIC FOR HIGH-YIELD D-T FUSION PLASMAS

We have examined the concept of a thin foil Faraday collector as a los t alpha particle detector capable of operating under ITER-like conditi ons. A prototype detector consisting of a single set of four 2.5 mu m Ni foils was installed on the JET first wall and operated during a var iety of deuterium plasma conditions during the 1995 JET run period. Al though there was no significant production of alpha particles during t hese plasmas, the prototype demonstrated the expected resistance to th e high temperature and x-ray backgrounds, as well as moderate neutron and gamma ray backgrounds characteristic of these plasmas. In addition , this prototype showed no significant response to neutral beam, rf, o r lower hybrid plasma heating. The device did pick up a low level sign al when neutral beams were injected simultaneously with heavy gas puff ing. Strong intermittent correlations were seen with excursions in the H alpha edge brightness signal. In addition, the detector produced a significant signal in response to a roughly 250 ms disruption precurso r. A similar prototype detector was installed immediately outside the Tokamak Fusion Test Reactor Vacuum vessel during the 1994 D-T run peri od to test the expected insensitivity to neutron backgrounds. No signa l was seen above background during D-T plasmas for which the fast neut ron production was in excess of 2x10(18) n/s. (C) 1997 American Instit ute of Physics.