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Measurements of Escaping Fast Particles Using a Thin-Foil Charge Collector

Owen N. Jarvis, Pieter Van Belle, Malcolm A. Hone, Guy J. Sadler, G. A. H. Whitfield, F. Edward Cecil, Douglass S. Darrow, Basilio Esposito

Fusion Science and Technology / Volume 39 / Number 1 / January 2001 / Pages 84-95

Technical Paper /

Two screened, thin-foil charge collectors were mounted just beyond the plasma edge at an outboard position (below midplane) in the Joint European Torus to detect lost alpha particles during the 1997 high fusion power D-T experiments. No convincing observations of alpha-particle collection were obtained, possibly because of the low level of alpha-particle losses but more probably because the positioning of the detector was not ideal for the high fusion power discharges that were run at high plasma current and toroidal field. Under such conditions, alpha particles on escaping orbits leading toward 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 neutrals, leaving the plasma and ionizing in the low density scrape-off region outside the plasma boundary. The interpretation of this unexpected signal is discussed. In all probability, it will also be encountered in any future 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 ~0.5 MeV.

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