HELIUM EXHAUST STUDIES WITH THE ALT-II PUMP LIMITER IN TEXTOR

In TEXTOR helium removal experiments with the pump-limiter ALT-II have started. To simulate the presence of helium ash in the plasma, helium is injected into the discharge (e.g. at t = 0.7 s) as a short pulse of DELTA-t = 20 ms. It is found that the He is removed from the discharge in an e-folding time of about half a second for neutral beam heated plasmas and in an e-folding time of about 1.5 s in an OH plasma. The exhaust efficiency of helium amounts to about 8% and is close to the one for deuterium. The fuelling efficiency for the injected helium is found to be in the range of 50-100%; the remaining part seems to be stored in the TEXTOR walls. An estimate of the surface density leads to a value of several times 10(13) cm-2. This helium can easily be liberated in succeeding discharges and can be removed efficiently when ALT is pumping.