Disruption mitigation by multiple shattered pellet injections in KSTAR

For disruption mitigation, ITER has adapted to use multiple SPIs simultaneously or sequentially to safely dissipate stored energies. However, sufficient experimental assessment has not been made on the disruption mitigation by multiple SPIs. Before full-scale operation of ITER, it is desirable to optimize the usage of the multiple SPIs. In KSTAR, two identical SPIs were installed symmetrically 180 degrees apart toroidally to verify the effectiveness of multiple SPIs. The KSTAR SPI can inject up to six pellets simultaneously or with specific time delays. The experiments have been focused on characterizing the impact of delays in the arrival of the pellet fragments when performing multiple injections either from different toroidal locations or from different barrels at the same location. The experiments demonstrated that synchronized pellet injection into H-mode plasmas is more effective compared to delayed injections. These findings provided important input to key design requirements such as the jitter in pellet release and spread in pellet velocity, both related to the pellet launching technique. Furthermore, we are developing an optimal injection scheme to achieve both high density and effective radiation by injecting pure deuterium pellets and neon-doped pellets sequentially.