Comparison of radiating divertor behaviour in single-null and double-null plasmas in DIII-D

‘Puff-and-pump’ radiating divertor scenarios, applied to both upper single-null (SN) and double-null (DN) H-mode plasmas, result in a 30–60% increase in radiated power with little or no decrease in τE. Argon was injected into the private flux region of the upper divertor, and plasma flow into the upper divertor was enhanced by a combination of deuterium gas puffing upstream of the divertor targets and particle pumping at the targets. For the same constant deuterium injection rate, argon penetrated the main plasma of SNs more rapidly and reached a higher steady-state concentration when theB× ∇B-ion drift direction wastowardsthe divertor (V∇B↑) rather thanaway fromthe divertor (V∇B↓). We also found that the initial rate at which argon accumulated inside DN plasmas was more than twice that of comparable SN plasmas having the sameB× ∇B-ion drift direction. In DNs, the radiated power was not shared equally between divertors during argon injection. Only when theB× ∇Bion drift direction was away from the divertor were both significant increases in divertor radiated power and an accumulation of argon in the divertor observed, based on spectroscopic measurements of Ar II. Our data suggest that an unbalanced DN shape where theB× ∇B-ion drift is directedawayfrom the dominant divertor may provide the best chance of successfully coupling a radiating divertor approach with a higher performance H-mode plasma.