Comparison of transient and stationary neutral pressure response in the DIII-D advanced divertor

The DIII-D divertor baffle system was designed to facilitate density control in long pulse H-mode discharges by removing a particle flux equal to the neutral beam fueling rate (∼20 Torrl/s) with a ∼1 mTorr neutral pressure under the baffle (p₀). Initial measurements of the baffle pressure indicated that p₀∼10 mTorr (without pumping or biasing), a value much in excess of that required for long pulse density control. Radial sweeps of the X-point position have been employed to determine the maximum p₀, as well as to establish the dependence of this pressure on geometry. An estimate of the particle equilibration time for the baffle system has been made by studying the baffle pressure response to "giant" ELM effects. "Steady state" experiments in which the X-point position was fixed for ∼2.5 s have also been carried out and steady baffle pressures were observed. The scaling of baffle pressure with plasma parameters has been found to be similar under transient and "steady state" conditions. Detailed modeling of these experiments with the B2, DEGAS, and WDIFFUSE (wall model) codes has been performed.