Measurements of escaping fast particles using a thin-foil charge collector

Two screened, thin-foil charge collectors were mounted just beyond the plas ma edge at an outboard position (below midplane) in the Joint European Toru s to detect lost alpha particles during the 1997 high fusion power D-T expe riments. No convincing observations of alpha-particle collection were obtai ned, possibly because of the low level of alpha-particle losses but more pr obably because the positioning of the detector was not ideal for the high f usion power discharges that were run at high plasma current and toroidal fi eld. Under such conditions, alpha particles on escaping orbits leading towa rd the detector are highly likely to be intercepted by the nearby poloidal limiter Moreover, a small alpha-particle signal would have been obscured by interference from a large and unexpected signal attributed here to fast ne utrals, leaving the plasma and ionizing in the low density scrape-off regio n outside the plasma boundary. The interpretation of this unexpected signal is discussed. lit all probability, it will also be encountered in any futu re attempts to detect lost alpha particles in a current measuring detector unless suitable precautions are taken, e.g., provision of a thin first foil to remove light charged particles with energies below similar to0.5 MeV.