Optimization of initial SPI injection composition for ITER's staggered SPI mitigation scheme.*

Experiments on DIII-D investigated the efficacy of the envisioned staggered SPI injection scheme for ITER. The staggered scheme injects one or more pure H SPI followed by a H/Ne mixed species pellet to radiate the remaining energy. The experiments on DIII-D focused on understanding the physics behind the initial pure D₂SPI with an emphasis of maximizing initial assimilation. Initial analysis shows three distinct types of mitigation (1) plasma did not disrupt after D₂ SPI, (2) plasma disrupting abruptly after injection, due to existing MHD or high initial assimilation of D2 SPI, and (3) plasma had an extended cooling duration accompanied by fast n=1 mode growth. The non-disruptive case, (1), showed a slow loss of stored energy, upwards of 70%, with a global density rise and appears to be the ideal scheme for a staggered injection. Small amounts of Ne were added to the D₂ SPI in an effort to suppress the ejection of ablated material through grad-B drift effects by decreasing the temperature of the plasmoid surrounding the shards. Higher initial pre-thermal quench electron density rises were observed for SPI containing 0.9% Ne compared to other values, suggesting an optimal amount of Ne doping for an early density rise. However, post-thermal quench electron densities are similar for all cases, indicating that doping D₂ SPI does not increase overall assimilation.