Characteristic test of dual shattered pellet injection systems of KSTAR

Dual shattered pellet injection (SPI) systems are installed in KSTAR (Korea Superconducting Tokamak Advanced Research) to study disruption mitigation in support of the ITER DMS (Disruption Mitigation System) design activities. KSTAR has two identical pellet injectors that can shoot pellets simultaneously from opposite toroidal locations. Each injector can shoot up to of three different diameter pellets. The pellet integrity and velocity after formation have been measured using microwave cavity (MWC) systems since 2019. However, this diagnostic provides only limited information on the pellet integrity, especially for partially broken pellets. Therefore, an optical pellet diagnostic (OPD) system monitoring the pellet has been installed in the first stage viewport of the differential pumping system and is operational since 2021. The KSTAR SPI systems offer a wide parameter range that must be characterized before the experimental campaign. To satisfy the experimental requirements, a proper procedure for pellet formation and acceleration must be commissioned. We carry out the commissioning by varying pellet parameters such as formation duration, temperature, heating, propellant gas pressure, and pulse length. The resulting pellet velocity, integrity, and tilt can be measured with the OPD system. These tests can be performed without plasma operation because no additional information is needed from the plasma response to the injection. This paper describes the results of the characteristic tests of single and multiple injections of dual SPI.