Mineral insulated cable assessment for inductive magnetic diagnostic sensors of a hot-wall tokamak

Abstract

The COMPASS-U tokamak, designed to be a 5 T magnetic field device with a full-metal first wall and operating at plasma-facing component temperatures up to 500°C, will start its operation in 2022 at IPP Prague. This device will address ITER and DEMO relevant plasma exhaust physics, including operation with liquid metal divertor. Inductive magnetic diagnostics based on conductive loops of different geometry and orientation are crucial for magnetic confinement fusion devices. Due to the high temperatures of the vacuum vessel upon which they will be operated, a suitable cable insulation needs to be chosen carefully. Mineral-insulated cables (MIC) have proven to be compatible with high baking temperatures. However, the steel sheath of MIC attenuates the response of the sensor at higher frequencies which could affect real-time plasma control feedback and magnetic equilibrium reconstruction. In this work, characterization and testing of multiple MgO MIC of different diameters was conducted. A variety of electrical property measurements, such as frequency attenuation, resistance and capacitance, for each cable is presented, both at low and high temperatures up to 300°C. Cutoff frequencies from 65 kHz to 335 kHz were identified and attributed to the shielding in a flux loop configuration. Using an external RLC circuit, the frequency response of MIC coils is compared to an electrical model for shielded coils, yielding an useful calibrated model for future probe prototypes with different geometries in the frequency range of interest.