Identification and radiated power control of the divertor plasma in the DIII-D tokamak*

This is the first use of modern system identification for exhaust control on an American tokamak. With these experiments, we investigate the dynamics of, and successfully control, the divertor plasma exhaust in the DIII-D tokamak.

We identify the divertor plasma dynamics with frequency response measurements (FRMs) by perturbing the system with deuterium and nitrogen gas injection. The system's response is diagnosed with filterscopes and bolometers measuring Dα emission and radiated power. FRMs were done at different levels of radiated power, ▽B drift towards and away from the divertor, and resonant magnetic perturbations (RMPs). The phase of the FRMs is similar under these variations, indicating a process insensitive to plasma conditions. Contrarily, the gain of the FRMs varies with the level of radiation and magnetic field direction, suggesting a coupling with the plasma conditions. We hypothesize that these observations can result from rapid plasma and slow neutral dynamics. The FRMs of the radiated power with nitrogen remain comparable with and without RMPs. This result enables the design of a single radiated power controller for both cases, which is demonstrated experimentally.

Next to providing a dynamic model for fast and systematic controller design, the conducted system identification experiments on the DIII-D tokamak contribute to a database of benchmarks for dynamic divertor plasma models. These dynamic models are essential in designing exhaust controllers for future fusion power plants.