Radiation Dependence of Divertor Leg Length in Detachment on DIII-D

Experiments performed on DIII-D demonstrate that while increasing the outer leg length distributes radiation over a larger volume, the increased leg length does not prevent an eventual collapse of the X-point temperature at high density. Accommodating divertor radiation requires understanding the dissipation processes that takes place along a flux tube. Smaller core-volume plasmas in the open divertor were used to extend the poloidal leg length, L_pol, to 38 and 55 cm for two injected powers (8.3 MW and 5.4 MW). Gas puffing was used to increase the plasma density to the point of detachment to test the radiation volume required in the convective limit and extend beyond existing stable-detachment temperature gradient measurements (200eV/m for 15cm L_pol). The higher L_pol cases show a lower detachment onset density, but only by ~5%. The normalized confinement, H₉₈, drops from 1.2 before detachment and drops below one and continues to reduce as the divertor is driven deeper into detachment. Radiated power is found to extend along the outer leg in detachment for all cases. Despite the extended radiating volume, the CIII radiation pattern is found to peak strongly at the separatrix near the X-point in the detached limit, indicating a 7-8 eV electron temperature at the X-point.