Thin foil bolometer development for shattered pellet injection studies on KSTAR tokamak

Abstract

The shattered-pellet injection (SPI) is widely believed as an effective tool for mitigating plasma disruption in magnetic confinement fusion devices, as the frozen pellets can penetrate deep into the core. The investigation of SPI and consequent radiation profile on KSTAR tokamak was initiated since 2019 experiments. In conjunction with the existing radiation diagnostics such as electron cyclotron emission imaging (ECEI) and radio frequency (RF) spectrometers [1-2], two types of thin foil bolometers are developed along with an extreme ultra-violet bolometer for measurement of high radiative flux during SPI in KSTAR. The first type is a thin foil infrared (IR) bolometer consisting of a graphite coated platinum foil and an IR detector. The foil is sandwiched between two copper rings for good thermal conduction, from the foil to the metal support structure. The calibration and time response of the foil are obtained by the IR detector facing the foil, which is illuminated with a laser beam on the other side. The decay time of the IR detector signal from saturation after turning-off the laser is found comparable (~100 ms) for varying foil thicknesses (0.5 m and 2.5 m). Multiple layers of EMI shields were proven critical for preventing high amplitude EMI noises both in the laboratory tests using a Tesla coil and in the commissioning test on the KSTAR. The second type is a resistive foil pulsed current bolometer. Each bolometer unit has a pair of resistive foils for enhanced signal contrast and reduction of system noises. A prototype is underway which shall be compared with the IR bolometer. Work supported by the Korea Institute of Fusion Energy under the KSTAR collaboration program.