Flow reversal in the throat of the pump limiter ALT-II during high density conditions

Experiments have been performed to measure the conductance for backstreaming particles in the throat of the pump limiter ALT-II on TEXTOR under different discharge conditions. The flow inside the throat is analysed by injecting a steady helium flux towards the neutralizer plate and by measuring the helium partial pressure. The amount of helium escaping 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 medium densities its value is reduced, whereas at the highest densities (>5*1018 m-3) an increasing flow of escaping helium is found. It 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 high density discharges with additional heating