(The) development of the diamagnetic loop at KAIMIR for diamagnetic flux measurements

Abstract

The diamagnetic loop, wound in the azimuthal direction on a cylindrical plasma device, is a magnetic diagnostic that measures the diamagnetic flux, which is the reduced axial magnetic flux due to the plasma. It is widely utilized in evaluating the performance of the plasma since the measured diamagnetic flux can be used to estimate the stored energy within the plasma. In most laboratories, including the one utilized in this thesis, the generated plasma has significantly lower plasma pressure than the externally applied magnetic pressure. In this case, the diamagnetic flux is significantly lower than the externally applied background flux. Therefore, the removal of background flux is crucial in measuring diamagnetic flux, and a method to eliminate background flux is employed for the diamagnetic flux measurements. This thesis presents the installation of three diamagnetic loops in the central chamber of the magnetic mirror device, KAIMIR, to measure the diamagnetic flux. The background flux removal method applied in this study utilizes two diamagnetic loops with different radii on the same plane to eliminate the background flux. Each diamagnetic loop measures the diamagnetic flux induced by the same plasma, while only the externally applied background flux is measured with varying magnitudes. Therefore, it is possible to remove the background flux by utilizing the differences in these measurements. As mentioned earlier, since the diamagnetic flux in the plasma used in this study is expected to be a very small signal compared to the background flux, even subtle noise compared to the background flux can have a significant impact on the measurements. Therefore, efforts to reduce noise are necessary to ensure that the signal measured by the diamagnetic loop is sufficiently larger than the background noise. To reduce the background noise in the signal measured by the diamagnetic loop, two identical loops were measured with opposite polarities. Through this method, the induced signal in the loops has been successfully amplified by a factor of 2, and the parasitic capacitance noise arising from the external insulation of the loops has been reduced. The diamagnetic flux was measured from three pairs through the installation of three diamagnetic loops, and the diamagnetic flux obtained from each pair was observed to be consistent within the uncertainties. This supports the validity of the diamagnetic flux measurements. Furthermore, the ion saturation current measured by the Langmuir probe was compared with the trend of the diamagnetic flux measured by the diamagnetic loop. Since the ion saturation current is proportional to n_e √(T_e ) and the diamagnetic flux is proportional to ⟨nT⟩, it was predicted that the values measured by the two diagnostics would have similar trends, and indeed, the observed similar trends confirmed the qualitative validity of the diamagnetic flux measurements. Additionally, the stored energy per length measured by the Langmuir probe was compared with the stored energy per length measured by the diamagnetic loop, based on the power applied to the plasma, the central magnetic field strength, and the changes in the magnetic field configuration. By confirming that the estimated values from each diagnostic were within their uncertainties, the plasma stored energy measurements using the developed diamagnetic loop in this study were quantitatively validated. Meanwhile, under all experimental conditions conducted in this study, the internal energy per unit length measured by the diamagnetic loop was consistently larger than the energy measured by the Langmuir probe, although within their uncertainties. Possible causes for these results could include the influence of azimuthal magnetic energy and rotational energy in the azimuthal direction which are not considered when estimating stored energy from the diamagnetic flux, and ion energy. Additionally, the impact of energy from the plasma's outer region, which is ignored during Langmuir probe measurements, could also be a possible cause. The fact that the fractional difference between the two measurements stays at the similar level regardless of the applied power and central magnetic field strength suggests that the effects of azimuthal magnetic energy and rotational energy in the azimuthal direction, which are expected to vary with these two parameters, are relatively small. Therefore, it is speculated that the consistently larger measurements by the diamagnetic loop are attributed to the influence of ion energy and the energy from the plasma's outer region, which is ignored during Langmuir probe measurements.