STUDIES ON IMPURITY CONTROL AND HYDROGEN PUMPING WITH CHROMIUM GETTERING IN ISX-B

Gettering inside the vacuum vessel has been employed in many tokamaks and has led to strongly reduced impurity radiation and expansion of the operating space. Excapt for one brief test with chromium, titanium has been used until now as getter material, mostly because sources for large-scale applications are commercially available. One of the characteristics of titanium is that it pumps large amounts of hydrogen by bulk diffusion even at room temperature. As a consequence, titanium gettering requires increased gas flow rates and entails large hydrogen inventories. In contrast, chromium pumps hydrogen only in monolayer quantities on the surface without subsequent bulk diffusion. Therefore, chromium gettering allows the control of impurities without increased gas flow rates or hydrogen retention at the wall.