2025 – Demonstration of Non-Inductive Vertical Position Control in DIII-D

Pellet dynamics and impurity transport resulting from PFC slag in DIII-D

2025 Research Campaign, Diagnostics and Actuators

Purpose of Experiment

This experiment will demonstrate the viability and performance of non-inductive plasma vertical stability control using Faraday-effect polarimetry measurements. It will also demonstrate the viability of the Radial Interferometer Polarimeter (RIP) diagnostic being able to provide real-time signals at control relevant speed. It will advance TRLs of current diagnostics and validate FPP-relevant diagnostics for control and operation use. It will also entail the implementation and testing of control algorithms based on non-inductive vertical stability control, and demonstrate their viability and achieve performance comparable to traditional control using external magnetic probes. This experiment will provide the first demonstration of real-time, non-inductive plasma vertical stability control, and inform necessary FPP diagnostics development.

Experimental Approach

The initial reference shot is 172959 (elliptical, limited,) but the experiment will be conducted in several plasma shapes at low power. The experiment will start in limited plasma shapes and aim to move on to diverted L-mode plasma shapes L mode plasmas have lower resource needs (no beam heating required) and are harder to control in the context of vertical stability. Therefore, they make ideal plasmas for our demonstration of vertical stability control. Before the experiment, the choice of plasma shape and condition will be verified by the control simulation above and, if necessary, changed. The plasma start-up and ramp-up phases will use the default 𝑍0 control setting, and switch over to the RIP-based algorithm once flattop conditions are established Control demonstration using the non-inductive, RIP-measured 𝑍0 will be performed at the flattop phase, same as expected for FPPs. The experiment has two stages: Stage 1: demonstrate steady 𝑍0 control using RIP-measured 𝑍0 , and perform controller tuning as necessary for the new vertical control implementation. Stage 2: explore control range, accuracy, and time response using RIP-measured 𝑍0. Including the response to sudden changes to z reference value, and to elongation scans. The main goal of this experiment, i.e. demonstrating the control using non-inductive 𝑍0 measurement, will be accomplished in Stage 1. Nevertheless, Stage 2 is essential to understand the capabilities and limitations of existing measurement and control, critical to raising TRL further. We aim to demonstrate that RIP-based vertical stability control can perform vertical stability control for DIII-D’s operating range. Assuming the above-mentioned control simulation will be completed successfully, we expect the experiment to have a very high chance of success. The main uncertainty is that the gain in the actual experiment may differ from that used in the simulation. A portion of the experiment will be dedicated to finding a proper control setting in Stage 1. Phase 2 will scan 𝑍0 during the shot using a triangle waveform to test the spatial range of 𝑍0 control, as well as scan the plasma elongation to challenge the controller’s performance. This is aimed to test the two anticipated limits of this control technique. The first is the relatively poor performance of RIP-based Z position estimation when Z is far from 0 (due to the geometry of installation), and the second is the high vertical instability growth rate conditions in highly elongated, high 𝑙𝑖 conditions. If time permits, we will also test the controller with step waveforms to test the time response characteristic of this control algorithm. The actual number of scan steps is preliminary and subject to change based on the control simulation as well as the outcome from Stage 1.