Development and preliminary application of high-frequency magnetic probe array on quasi-axisymmetric stellarator CFQS-T

For magnetic confined fusion devices, magnetic probe diagnostic is a basic but very important diagnostic tool for studying plasma magnetic fluctuations. The first experimental phase of the Chinese First Quasi-axisymmetric Stellarator (CFQS), which is also called CFQS-T, needs magnetic probe diagnostics to provide plasma magnetic fluctuation measurements, especially the high-frequency (<inline-formula><tex-math id="M1">\begin{document}$50 \leqslant f \leqslant 300$\end{document}</tex-math></inline-formula> kHz) magnetic fluctuation measurements. In this paper, a newly developed high-frequency magnetic probe array (HFMPA) diagnostic on the CFQS-T is reported. This array consists of 8 identical three-dimensional high-frequency magnetic probes, each of which can simultaneously measure magnetic fluctuations in the poloidal, radial and toroidal directions. The HFMPA magnetic probes are carefully mounted on the inner vacuum vessel wall of the CFQS-T, and their positions are precisely measured by the laser tracker system. The HFMPA can be used to study the poloidal and toroidal propagation characteristics of magnetic fluctuations due to the optimized spatial arrangement, and its maximum toroidal mode number resolution is improved to <i>n</i> = ±16 compared with <i>n</i> = ±6 of the low-frequency magnetic probe array (LFMPA, used for the <inline-formula><tex-math id="M3">\begin{document}$f \leqslant 50$\end{document}</tex-math></inline-formula> kHz magnetic fluctuation measurements). The main subsystems of the HFMPA diagnostic, such as the mechanical system, signal transmission lines, acquisition and control systems, and the challenges overcome in the development of each subsystem, will be briefly introduced in this paper. The effective areas of the HFMPA magnetic probes are calibrated by the relative calibration method, which shows that their areas are all around 0.02 m<sup>2</sup>. The in-situ frequency response of the HFMPA magnetic probes is calibrated with an LCR digital bridge with a maximum working frequency of 10 MHz. The resonance frequency of the HFMPA magnetic probe in each measurement direction is greater than 400 kHz, which meets the design requirements for measuring 50–300 kHz high-frequency magnetic fluctuations in CFQS-T. Preliminary applications of the HFMPA diagnostic in studying the low-frequency (1.5–16.0 kHz) magnetic fluctuations and high-frequency (65–105 kHz) magnetic fluctuations in CFQS-T are briefly introduced, which shows that the HFMPA diagnostic works well for providing the spectrogram, poloidal, and toroidal propagation information of low-frequency and high-frequency magnetic fluctuations. It is worth noting that the measurement and analysis results of high-frequency (65–105 kHz) magnetic fluctuations in CFQS-T are reported for the first time in this paper. The successful development of the HFMPA diagnostic will help to carry out in-depth research on plasma magnetic fluctuations in CFQS-T stellarator.