2025 – Effect of Detachment and Reattachment on Edge Density Pedestal Structure and Neutral Ionization in DIII-D

Effect of Detachment and Reattachment on Edge Density Pedestal Structure and Neutral Ionization in DIII-D

2025 Research Campaign, Thrust: High Opacity and Density Operation

Purpose of Experiment

a)Evaluate and characterize the high electron density in the HFS SOL, and its connection to inner and outer divertor detachment. b)Determine the effect of different poloidal puffing locations in the onset of the outer leg detachment, the characteristics of the ionization profiles/fueling measured by LLAMA and the effect on the pedestal performance when reaching detachment on the outer leg. c)Evaluate the relation between pedestal and separatrix density with the divertor condition for the different puffing locations through the process of detachment and reattachment, evaluating any hysteresis effect on the relation between core/edge profiles and divertor conditions.

Experimental Approach

The experiment will consist of two phases: Phase 1: Detachment and reattachment of the inner divertor leg. This was achieved in shot 200668 (black) with the strike point in the floor with a line integrated density between 5 and 6 x 10^13 cm^-3 (No puffing during the discharge, reaching the low density phase in early USN and then moving to LSN, some wall conditioning the previous day may have helped the goal). This phase requires low density, and this is why it is in the first few shots of the experiment. As detachment cannot be reached with puffing (it would make it difficult to reattach), we will perform X-point sweeps, extending and reducing the inner leg hoping to produce detachment and reattachment. We may also move the X-point up and down, to hopefully characterize the 2D structure of the HFSHD with LLAMA. This phase focuses on purpose a). Phase 2: Outer strike point detachment and reattachment with different gas puff locations. We will perform gas puff ramps from GAS A (top), CPBOT (bottom, strike point) and CPMID (HFS), flat puffing keeping detachment with strike point sweep and then stop puffing to try to reattach. This phase focuses on purpose b) and c).