Evaluation of an ITER compatible, thin foil Faraday collector as a lost alpha narticle diagnostic for high yield D-T fusion plasmas

We have examined the concept of a thin foil Faraday collector as a lost alpha particle detector capable of operating under ITER-like conditions. A prototype detector consisting of a single set of four 2.5 μm Ni foils was installed on the JET first wall and operated during a variety of deuterium plasma conditions during the 1995 JET run period. Although there was no significant production of alpha particles during these plasmas, the prototype demonstrated the expected resistance to the 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, or lower hybrid plasma heating. The device did pick up a low level signal when neutral beams were injected simultaneously with heavy gas puffing. Strong intermittent correlations were seen with excursions in the Hα edge brightness signal. In addition, the detector produced a significant signal in response to a roughly 250 ms disruption precursor. A similar prototype detector was installed immediately outside the Tokamak Fusion Test Reactor vacuum vessel during the 1994 D-T run period to test the expected insensitivity to neutron backgrounds. No signal was seen above background during D-T plasmas for which the fast neutron production was in excess of 2×10¹⁸ n/s.