Study on Divertor Detachment and H-mode Pedestal Characteristics in DIII-D with Upper Closed Divertor

Experiments performed in DIII-D demonstrate that higher plasma current and heating power combining with impurity seeding facilitate the achievement of divertor detachment with a higher pedestal pressure and higher plasma performance in the H-mode plasmas with a closed divertor. With 3x variation in heating power or 2x in plasmas current and with only D₂ puffing, no significant difference in the separatrix density at detachment onset is found, which is inconsistent with theoretical predictions. Higher heating power requires higher N₂ puffing rate to achieve the same degree of detachment, while higher N₂ puffing leads to lower detachment onset separatrix density, both of which agree with 1-D detachment scaling theory. In contrast to the narrower and steeper pedestal in the open divertor approaching detachment, the pedestal density width in the closed divertor increases with separatrix density, while the peak gradient remains unchanged. At higher power the pedestal density gradient is much weaker, while the SOL density is significantly higher and wider. With different plasma current and heating power, the normalized pressure gradient remains identical. Hence, divertor detachment with a higher pedestal pressure and higher performance can be achieved with higher current and power.