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A pellet injection system operating at 20 Hz has been operated in KSTAR (Korea Superconducting Tokamak Advanced Research) since 2016. This system can inject pellets with various size, velocity, and frequency to the KSTAR vacuum vessel during plasma experiments. Various experiments such as peak density control, enhancement of confinement by increasing core beta, and NTM stabilization were carried out with the pellet injection system in 2018. Most of all, the characteristics of the pellet should be studied to better understand the interaction with plasma. The pellet trajectory is one of the interesting topics in KSTAR and therefore a related investigation was carried out outside of the tokamak first. We introduce the preliminary results of the trajectory test, the pellet velocity tendency, and a plasma experiment in this paper.

20 Hz pellet injection system was installed for improving a fueling system of KSTAR (Korea Superconducting Tokamak Advanced Research) in 2016. Several things of hardware such as a liquid helium transfer line, an extruder nozzle and an extension part of pellet guide tube are changed for 2017 campaign. For plasma experiments, the pellet injection system uses the smallest pellet (1.5 mm of length, 2 mm of diameter). The pellet velocity can be variable from 150 m/s to over 200 m/s by controlling of propellant gas pressure. The clear increase of plasma density observed during KSTAR experiments is also presented in this paper.

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.

Shattered pellet injection is of the most attractive way to mitigate the plasma disruption in fusion research facilities up to now. DIII-D has already utilized it and achieved very positive results. ITER has decided to adopt this technology for the DMS (disruption mitigation system) for PFPO-1 (Pre-Fusion Power Operation phase 1). The validation between simulation code and experiment, and continuous engineering development need to be carried out to meet the DMS’s requirement and reliability. KSTAR (Korea Superconducting Tokamak Advanced Research) is a possible candidate to test the urgent issues of plasma disruption for ITER. KSTAR can install two injectors in toroidal opposite positions. For this work, ORNL (Oak Ridge National Laboratory) will provide the two injectors, the shatter tubes and auxiliary systems. NFRI (National Fusion Research Institute) is preparing the infrastructure of a pumping system, control and data acquisition system, and arranging the location of diagnostic and the heating systems. This presentation describes the basic requirements and the engineering challenges to be solved for successful deployment and operation of multiple SPI injectors in 2019.

Efforts have been made to obtain vacuum condition that is essential for the plasma experiments. Under certain situations, for example, the vacuum vessel should be vented to repair in-vessel components such as diagnostic shutter, exchange of window for diagnostic equipment, and PFC damaged by high energy plasma. For the quick restart of the campaign, a recovery process was established to make the vacuum condition acceptable for the plasma experiment. In this paper, we present the recovery process of wall condition in KSTAR after temporal machine-vent for repair. It is found that an acceptable vacuum condition has been achieved only by plasma based wall conditioning techniques such as baking, GDC, and boronization. This study was that the proper recovering method of the vacuum condition should be developed according to the severity of the accident.

KSTAR has reached a plasma current up to 630 kA, plasma duration up to 12 s, and has achieved high confinement mode (H-mode) in 2011 campaign. The heat flux of PFC tile was estimated from the temperature increase of PFC since 2010. The heat flux of PFC tiles increases significantly with higher plasma current and longer pulse duration. The time-averaged heat flux of shots in 2010 campaign (with 3 s pulse durations and Ip of 611 kA) is 0.01 MW/m2 while that in 2011 campaign (with 12 s pulse duration and Ip of 630 kA) is about 0.02 MW/m2. The heat flux at divertor is 1.4–2 times higher than that at inboard limiter or passive stabilizer. With the cryopump operation, the heat flux at the central divertor is higher than that without cryopump. The heat flux at divertor is proportional to, of course, the duration of H-mode. Furthermore, a software tool, which visualizes the 2D temperature distribution of PFC tile and estimates the heat flux in real time, is developed.

The thermal shield intercepts thermal radiation from the components at room temperature to superconducting magnet at 4.5 K. The thermal shield of KSTAR is divided into three categories: vacuum vessel thermal shield, port thermal shield, and cryostat thermal shield. Tube-on-panel is the basic configuration of KSTAR thermal shield. The layout of the tubing has been designed to get the temperature distribution of 80 K. The shield is designed to be cooled down by flowing gaseous helium at 18 bar in the tube. After the installation, helium leak test was intensively performed to confirm leak tightness of cooling pipes of which length are about 703 and 2544 m for vacuum vessel thermal shield and cryostat thermal shield, respectively. On April 3, 2008, the cool down of the KSTAR started and during the operation the temperature was monitored by PT-100 temperature sensor. This paper describes the overalls of thermal shield work from design to installation, and reports the results of the KSTAR thermal shield during initial operation.

The installation of the Korea Superconducting Tokamak Advanced Research (KSTAR) device vacuum pumping system for both the vacuum vessel and the cryostat was finished in the middle of 2007. After the successful self-commissioning of the main vacuum pumping system by August 2007, the system successfully operated in the integrated system commissioning and 1st plasma experiments of the KSTAR device. The vacuum pumping system has successfully operated and achieved the target vacuum pressure for the vacuum vessel and the cryostat during the machine commissioning and 1st plasma experiments with no severe troubles and problems, which is one of the main factors that led the KSTAR project to achieve successful 1st plasma. Furthermore, the vacuum pumping system has been satisfactorily operated by remote control and synchronization in gas puffing experiments. This paper describes the final configuration of the KSTAR vacuum pumping system, the results of vacuum commissioning, and the other results of the vacuum conditions that relate to wall conditioning.

The wall conditioning of the KSTAR vacuum vessel has been performed in order to remove various kinds of impurities including H2O, carbon and oxygen for the first plasma. The first step of the wall conditioning was baking to 100 by hot pressurized water which circulates through the water channels between inner and outer wall of the vacuum vessel. The baking process continued for more than 200 h. Next step of the wall conditioning was the R-GDC with both hydrogen and helium. The R-GDC was operated in low level of 10−3 mbar range. The partial pressure analysis showed that the R-GDC in the baking process is satisfactory and efficient and the outgassing rate is remarkably reduced. In addition to the R-GDC, the ICRH discharge cleaning under 1.5 T of the toroidal field is performed between plasma shots.

The initial phase wall conditioning in KSTAR is depicted. The KSTAR wall conditioning procedure consists of vessel baking, glow discharge cleaning (GDC), ICRH wall conditioning (ICWC) and boronization (Bz). Vessel baking is performed for the initial vacuum conditioning in order to remove various kinds of impurities including H2O, carbon and oxygen and for the plasma operation. The total outgassing rates after vessel baking in three successive KSTAR campaigns are compared. GDC is regularly performed as a standard wall cleaning procedure. Another cleaning technique is ICWC, which is useful for inter-shot wall conditioning under a strong magnetic field. In order to optimize the operation time and removal efficiency of ICWC, a parameter scan is performed. Bz is a standard technique to remove oxygen impurity from a vacuum vessel. KSTAR has used carborane powder which is a non-toxic boron-containing material. The KSTAR Bz has been successfully performed through two campaigns: water and oxygen levels in the vacuum vessel are reduced significantly. As a result, KSTAR has achieved its first L–H mode transition, although the input power was marginal for the L–H transition threshold. The characteristics of boron-containing thin films deposited for boronization are investigated.