Disruption thermal load mitigation with shattered pellet injection on the Joint European Torus (JET)

Abstract

Disruption mitigation remains a critical, unresolved challenge for ITER. To aid in addressing this challenge, a shattered pellet injection (SPI) system was installed on JET and experiments conducted at a range of thermal energy fractions and stored energies in excess of 7 MJ. The primary goals of these experiments were to investigate the efficacy of the SPI on JET and the ability of the plasma to assimilate multiple pellets. Single pellet injections produced a saturation in total radiated energy (W rad) with increasing injected neon content, suggesting total radiation of stored thermal energy. Further increases in injected neon quantities resulted in reduced cooling times and current quench (CQ) durations, indicating higher impurity assimilation. No significant variation in CQ duration or W rad was observed when varying the deuterium content at fixed neon quantities. Higher assimilation, inferred by shorter CQ durations, was measured when a mechanical punch was used to launch the pellets and this was attributed to a lower pellet velocity leading to higher solid content in the pellet plume and larger fragments penetrating deeper into the plasma. Radiation asymmetries averaged over the cooling time were inferred from Emis3D and ranged from 1.6 to 1.9. Asymmetries averaged over the entire disruption sequence were found to increase at higher thermal energy fractions. The radiated energy fractions decreased with increasing thermal energy fractions but this trend was eliminated when toroidal asymmetries were accounted for with Emis3D. Pure deuterium pellets were able to produce cooling times of up to 75 ms with a gradual loss in thermal stored energy of up to 80%. Experiments with multiple pellet injection indicated W rad can be increased through pellet superposition and density can be increased with an additional D2 injection without a reduction in W rad. KPRAD modelling accurately reproduced the cooling times and the CQ duration at high thermal energies. Assimilation estimates from KPRAD indicated CQ rates scale strongly whilst W rad scales weakly and saturates with assimilated neon content. Comparable W rad can be achieved with lower assimilated neon quantities as longer cooling times are attained. Thus reduced neon content can be preferential in a thermal load mitigation scheme as it may reduce radiation asymmetries and prevent flash melting.

Shattered pellet injection experiments at JET in support of the ITER disruption mitigation system design

“…A possible RE avoidance scheme is the combined injection of H and Ne. In [9] it has been reported that the radiated energy saturates for injected Ne quantities of more than ∼10 22 atoms for the disruptions shown in figure 2. However, from this analysis alone it is not possible to determine the amount of thermal energy being radiated.…”

Overview of JET results for optimising ITER operation

“…The JET SPI experiments results are presented in [83]. A first set of experiments demonstrated reduction of the thermal load, with complementary experiments with SPI injection into 3 MA/7 MJ H-mode plasmas providing a unique dataset for ITER in terms of magnetic and thermal energy [84]. By varying the neon content in the SPI pellets, the disruption current quench time can be controlled efficiently in JET, covering the range required by ITER.…”

MHD modeling of shattered pellet injection in JET

“…Experiments on the largest tokamak JET have been performed to assess the effectiveness of SPI disruption mitigation. These experiments scanned the amount of neon content in an SPI disruption with a variety of deuterium-neon mixtures [2], finding that cooling times and current quench duration are reduced as the impurity content is increased. While synthetic diagnostics were needed to estimate the total levels of radiation due to limited bolometry coverage, they concluded that toroidal peaking factor (TPF = the radiation at the injection location divided by the toroidal mean) had an average value 1.6-1.9 before the current quench and 2.0-2.6 at peak radiation.…”