Energetic neutral particles behind the plt pumped limiter

A carbon film resistance probe was used to measure the energy and flux of energetic neutral deuterium in the pumping duct of the pumped limiter on the PLT tokamak. The particle flux at the back of the pumping duct increased from about 10¹²to 4 × 10¹³cm^-2s^-1as the limiter was moved into the plasma edge. With the limiter in the plasma edge, the mean particle energy measured at the back of the pumping duct was 160-300 eV. Gas pressures in the pumping duct up to about 1 mtorr were measured but were found to have little effect on the energetic neutrals reaching the back of the pumping duct. Particle transport calculations indicate that pressures more than an order of magnitude higher than observed would be necessary to stop the particles. The measured flux of energetic neutrals is estimated to be large enough to contribute significantly to the particle removal rate achieved by the pumped limiter.