Observation of non-thermal electron formation during the thermal quench of shattered pellet injection shutdowns in DIII-D

Abstract

Formation of non-thermal (hot) electrons is studied during the thermal quench of shattered pellet injection (SPI) discharge shutdowns in the DIII-D tokamak. Both pure neon and mixed neon/deuterium shutdowns are studied. High-energy radiation (SXR and EUV) brightness levels show indications of non-thermal electron formation during the TQ. The non-thermal densities are of order 1018 m−3 and non-thermal temperatures are estimated to be of order 10 keV. The non-thermal electrons appear to be mostly lost during the subsequent current quench, as no signal of relativistic runaway electrons is typically seen at the end of the current quench. The non-thermals appear to have an effect on the deposition profile of the SPI pellets, especially at the end of the TQ, due to increased pellet ablation.

3D radiation, density, and MHD structures following neon shattered pellet injection into stable DIII-D Super H-mode discharges

“…Notable measurements that cannot be explained by the Ne-I structure are the brightest feature in the AXUV contours and the highest density feature in the interferometers. Spectra from SPI terminated discharges on DIII-D show that the dominant radiated power exists at ultraviolet and extreme ultraviolet wavelengths [39] and supports the finding here that the Ne-I radiation structure need not be coincident with the dominant radiated power structure.…”

Estimate of pre-thermal quench non-thermal electron density profile during Ar pellet shutdowns of low-density target plasmas in DIII-D

“…The initial temperatures (up to 10 keV) studied here are comparable to ITER, while the electron densities studied here are far lower, and plasma dimensions are significantly smaller. The finding that impurity pellet injection into hot current-carrying plasma creates hot electrons ahead of the pellet, which increase the pellet ablation rate, could very well be relevant for ITER disruption mitigation using neon pellets, as already suggested in recent neon pellet analysis in DIII-D. 40 This would depend on the level of impurity ion and/or fast electron transport moving ahead of the strongly perturbing neon pellet and is unknown presently for ITER. The possible implication of this work that TQ hot electron loss in DIII-D may be more significant than expected may be relevant for ITER.…”

Thermal quench induced by a composite pellet-produced plasmoid

“…These calculations demonstrate that high-Z pellets may cause unacceptably fast thermal quenches in hot plasmas. The same concern comes from experimental observations that pure high-Z pellets facilitate runaway electron generation [20,21], presumably due to a much faster thermal quench than in the case of massive gas injection.…”