Diamagnetic measurements by concentric loops in the HL-2A tokamak

Development of a diamagnetic loop in KAIMIR

We developed a diamagnetic loop for the estimation of plasma stored energy in the KAIST Magnetic Mirror magnetic mirror device [Oh et al., J. Plasma Phys. 90, 975900202 (2024)]. Diamagnetic loops are used to estimate the plasma stored energy from measurements of the diamagnetic flux in plasma with an applied external magnetic field. However, diamagnetic flux measurements are accompanied by the vacuum flux, which generally exceeds the diamagnetic flux by over 10 000 times. Therefore, it is critical to attain a high signal-to-noise ratio with minimized noise in diamagnetic flux measurements. In this study, we employed a novel method to reduce background noise and improve the signal-to-noise ratio. Using two identical loops with opposite polarities, we successfully removed parasitic capacitive noise from the external insulation while amplifying the inductive signal two times. To eliminate the vacuum flux, we utilized two coaxial loops with different radii positioned at the same axial location. Results obtained from six paired loops confirmed the successful removal of the vacuum flux. The plasma stored energy was also found to agree well with Langmuir probe measurements, which verifies the diamagnetic flux measurements using the developed loop.

Diamagnetic measurements based on the compensation of TF current diffusion in J-TEXT

Due to the existence both of toroidal ripples and toroidal field (TF) current diffusion, the toroidal flux changes with time when the TF current is at the flat-top. A diamagnetic measurement based on the compensation of TF current diffusion has been built in J-TEXT to solve this problem. The measurement system includes a double-loop installed in the vacuum vessel and an array of small printed circuit board (PCB) magnetic probes placed on the mid-plane of one TF coil. A model was proposed to analyze and compensate the effect of TF current diffusion. Experiment results show that the residual flux is about 1 × 10^-4 Wb after the compensation and it can meet the need of diamagnetic measurement in J-TEXT.

Development of a low-drift integrator system on the HL-2A tokamak

In this work, we developed a new integrator system with low-drift and small integration time constant less than 1 ms, which applies to the weak signals from magnetic measurements. This integrator system is designed on the basis of the analog drift compensation and the real-time digital correction of residual drift. The analog drift compensation is achieved by the subtraction between two integrators and the digital correction method is available due to the stability of integral drift in short time scale. The algorithm of the residual drift calculation and correction is implemented by the field programmable gate array. The integral drift can be well compensated within 10 mV/10 s at RC = 0.5 ms and meet the requirements of magnetic diagnostic on HL-2A.