Relationships between impurities and mitigated/unmitigated disruptions in JT-60U/SA*

The factors determining the early current quench (CQ) time in JT-60U have been discussed focusing on radiation loss caused by impurities. In this study, mitigated disruptions induced by neon gas-puff and unmitigated disruptions that seem to be caused by MHD instabilities without neon gas-puff in high-performance plasma, which aim for high beta or high bootstrap current fraction, have been compared. The early CQ time has been interpreted by considering neon/carbon impurity amount for mitigated/unmitigated disruptions, respectively. The impurity amount that causes these disruptions has been estimated with plasma resistance derived from the CQ time considering the temporal change in plasma inductance. The estimation of the impurity amount has been compared with the model of CQ using the axisymmetric disruption code INDEX. Finally, we refer to the massive gas injection experiments in JT-60SA. Disruption mitigation is one of the major issues addressed in the JT-60SA project, and it is required to evaluate the characteristics of mitigated disruptions caused by the massive gas injection (MGI) in preparation for the coming experiment. The accessible range of the CQ time in JT-60SA MGI experiments is discussed for the injected amount of noble gas as well as its deuterium mixture with INDEX.