Experimental results of multiple shattered pellet injection systems in KSTAR

Abstract

Shattered pellet injection (SPI) is the technology chosen for the ITER Disruption Mitigation System and is explored at several fusion research devices, like DIII-D and JET and J-TEXT. The ITER disruption mitigation strategy relies on multiple injections to achieve RE (runaway electron) avoidance with optimum TQ (thermal quench), CQ (current quench) durations to adequately reduce wall loads. To demonstrate the feasibility of the multiple injection and to extrapolate to ITER, experiments with two identical injectors toroidally opposite to each other are needed urgently.

KSTAR (Korea Superconducting Tokamak Advanced Research) can be a unique testbed to study the plasma disruption mitigation for ITER. KSTAR has installed two identical injectors in 180 degrees of toroidal opposite positions in 2019. For this system, ORNL (Oak Ridge National Laboratory) provided the two injectors, the shatter tubes, and auxiliary systems. NFRI (National Fusion Research Institute) provided the infrastructure of a vacuum pumping system, control & data acquisition system, and installed additional diagnostic systems for SPI in collaboration with the ITER Organization. This paper describes the engineering achievements during installation on KSTAR and the initial results of single and multiple SPI experiments in the 2019 campaign.

Introduction

The mitigation of plasma disruption is one of the main challenges for future tokamak experiments. A large thermal load occurs during the thermal quench (TQ) and a large mechanical load occurs on the vacuum vessel and in-vessel components during the current quench (CQ). The impact of runaway electrons (RE) that can develop in the CQ on plasma facing components must be avoided. The disruption loads in ITER require a reliable and effective disruption mitigation strategy [1]. This strategy relies on the dissipation of thermal and magnetic energy through radiation and on the avoidance of RE formation through a massive increase in plasma density before the thermal quench.

The mitigation methods such as the gas injection, intact pellet injection, and solid particle injection have been studied for the mitigation of plasma disruptions. The ITER Disruption Mitigation System (DMS) will use Shattered Pellet Injection (SPI). The technology forms and accelerates large cryogenic pellets that are shattered into small fragments before entering the plasma to achieve high assimilation. The DMS of ITER can inject up to 24 pellets from three toroidal locations at the equatorial level [2]. For the validation of the ITER DMS design, the ITER Organization has established the DMS Task Force, a multi-year, multi-partner undertaking to enhance the physics and technology basis for the ITER DMS. The KSTAR SPI project is part of this programme.