Investigation of the eddy current effect on the high frequency response of the Mirnov probe on J-TEXT

This paper investigates the high frequency response of the Mirnov probe based on a test platform, which is capable of generating a uniform AC magnetic field within the frequency range of 1-300 kHz. The eddy current effect is quantitatively reflected by the phase shift ϕ_c and normalized amplitude δ of the measured magnetic field between cases with and without a conducting plate located near the Mirnov probe. This method compensates the resonant effect in the Mirnov probe circuit and hence reflects purely the eddy current effect. The eddy current effect increases with the decrease in the distance between the probe and the conducting plate. With the increase in frequency, the magnitude of δ decreases to a saturated value at 10 kHz but increases significantly above 100 kHz for 304-stainless steel, while the eddy current effect with graphite appears at around 10 kHz and the magnitude of δ decreases to the minimum at 125 kHz, followed by a significant increase above 125 kHz. With the increase in f, the magnitude of ϕ_c increased until 2.5 kHz and 40 kHz for steel and graphite, respectively, then decreased with a further increase in f. The phasor expression is introduced to describe the AC magnetic field and allows an easy expression of the eddy current field. The phase of the eddy current field decreases toward -180° with f. The amplitude of the eddy current field increases with f and reaches its maximum when the skin depth reduces to a critical value. The eddy current field decreases with a further increase in the frequency.