2024 – Real time avoidance of the density limit via proximity control of an empirical edge density boundary

Real time avoidance of the density limit via proximity control of an empirical edge density boundary

2024 Research Campaign, Thrust: High Opacity and Density Operation

Purpose of Experiment

The mission of this experiment is to evaluate a potential path to exceeding the Greenwald limit by leveraging two recent advances at DIII-D: a proximity-to-density-limit metric and the “Proximity Control” algorithm. The experiment will first reproduce a L-mode density limit (LDL) disruption, either from 191793 or an LDL developed during MP 2024-36-01. Then, it will apply real-time feedback control to enable stable operation at high density. The main objective is to avoid a density limit disruption, and the “reach” goal is to exceed the Greenwald limit in a stable discharge via pellet fueling and feedback control on auxiliary heating.

Experimental Approach

The experiment will begin by reproducing the reference LDL disruption. This will be a density ramp based on either shot 191793 or a LDL scenario developed during 2024-36-01. Afterwards, we will utilize the STATIN algorithm and Proximity controller to avoid the LDL via real-time feedback control. If this goal is achieved early, the “reach” goal of the experiment is to achieve a stable plasma with Greenwald fraction > 1 for at least 0.5s during the flattop. The “reach” goal of the session will be to use this workflow to extend stable operation beyond the Greenwald limit. First, we will modify the scenario by augmenting the existing real-time gas fueling with aggressive pellet injection. The intent is to increase the central density while allowing the Proximity Control algorithm to keep the edge density away from the stability limit. Second, we will allow the algorithm to increase the input power through NBI as we approach the critical value of the stability metric, with the intention of raising the temperature in the edge to further increase distance to the density limit precursor regime.