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.