Inside-out mitigation by high-speed low-Z shells with solid cores

2023 Research Campaign, Thrust: Disruption Mitigation

Purpose of Experiment

Disruption mitigation is required for the safe operation of ITER and all future tokamak-based pilot plants. A successful disruption mitigation system must address the combined challenges of thermal quench, current quench, and runaway electron mitigation. Dispersive shell pellet (DSP) injection has been tested on DIII-D as a possible alternative to shattered pellet injection (SPI) and massive gas injection (MGI). DSP injection uses a low-Z shell surrounding a radiative payload (typically boron dust) to trigger an inside-out thermal quench by depositing impurities directly into the plasma core. In theory, this technique should result in higher assimilation fractions, enabling the use of low-Z radiators as the payload. The core deposition location should additionally result in improved radiation of the plasma thermal energy, and the increased density/magnetic field stochastization is predicted to reduce runaway electron populations. Previous DSP experiments in DIII-D utilized high density carbon (HDC) shells which proved to be too perturbative to the plasma, triggering the thermal quench before payload deposition. To reduce the radiation per atom and dilution cooling from shell ablation, lithium coatings have been proposed as the pellet surface material. Lithium was chosen as it has the lowest atomic number (Z = 3) of any solid material not requiring cryogenics. To further minimize ablation, these pellets will be fired at increased launch speeds (> 200 m/s), necessitating the use of solid plastic cores instead of the fragile HDC shells. To prevent the soft Li coating from ablation during launch, the shell pellet injector will be upgraded to utilize a sabot, enabling a “gentler” launch.

Experimental Approach

The shot plan consists of firing Li-coated solid cores and Li-coated HDC shells into Super H-mode discharges. The first step will be to verify that the new sabot design can successfully inject Li-coated pellets without ablating the coating in the launch tube. If successful, scans of lithium coating thickness and pellet velocity with the solid core pellets will be done to determine the optimum levels for future DSP experiments. Lastly, Li-coated HDC shells will be tested to compare the effectiveness of the sabot on more fragile shells and the difference in payload composition (plastic vs. boron).

See more details, including project leads, at U.S. Department of Energy, Office of Scientific and Technical Information (OSTI).