Wall conditioning and density control in the TJ-II stellarator

Helium glow discharge cleaning (GDC) at room temperature, applied during the overnight periods between experimental days, has been used as standard method of wall conditioning in the TJ-II stellarator during the initial operation with ECRH plasmas in an all stainless steel scenario. However, the density control at medium and high power injection (more than 300 kW) becomes difficult due to the contribution of He implanted on the wall during GDC and desorbed by plasma discharges. He desorption rates of ∼4×1010 cm−3 ms−1 have been estimated from residual gas analyser (RGA) postpulse measurements. In order to overcome this effect, an Ar GDC of a duration of 30 min prior to the TJ-II operation, and after the overnight He GDC have been applied in the last campaign. The Ar GDC removes of the order of 1021 He atoms (about one monolayer) from the walls allowing a good control of the plasma density by means of external gas puffing. Values of Zeff∼2.0–2.5 in H2 plasmas and mass spectra measurements of thermal desorption show that the contribution of Ar implanted to the plasma fuelling is not important.