FLOW REVERSAL IN THE THROAT OF THE PUMP LIMITER ALT-II DURING HIGH-DENSITY CONDITIONS

Experiments have been performed to measure the conductance for backstr eaming particles in the throat of the pump limiter ALT-II on TEXTOR un der different discharge conditions. The how inside the throat is analy sed by injecting a steady helium flux towards the neutralizer plate an d by measuring the helium partial pressure. The amount of helium escap ing into the plasma volume has been found to be strongly dependent on the local electron density and electron temperature. At low densities the backstreaming conductance corresponds to the classical value, at m edium densities its value is reduced, whereas at the highest densities (> 5 x 10(18) m(-3)) an increasing how of escaping helium is found. I t is shown that a reversal of the plasma flow in the throat of ALT-II can be established by exposing one blade (the other 7 retracted) to hi gh density discharges with additional heating.