Design, construction of magnetic mirror device, and initial experimental results

Abstract

Magnetic mirror device is a machine that traps plasma through mirror forces, generated by changes in axial magnetic field intensity. This device has been widely used in early nuclear fusion research due to its convenience of construction and comfort of accessibility to diagnostics from its simple structure, and is suitable for experimental research at the university level. For the analysis of plasma instability that could occur in mirror geometry and the study related to applications of mirror device to fusion and industrial fields, linear magnetic mirror plasma device has been constructed in the laboratory. Structure of vacuum chamber for this device is a cylindrical structure with a diameter of 0.5 m and a length of 2.48 m. The chamber is composed of three sub-chambers (source, center, and expander chamber) with the mirror nozzles connecting the sub-chambers. The primary rotary pump and the secondary turbo molecular pump were utilized to maintain a high vacuum of base pressure $~2×10^{-7}$ Torr, and the injection of gas required for discharge was controlled through piezoelectric valve instantaneously. To form a magnetic field of the mirror structure, 4 solenoid coils and 4 mirror coils are installed in the center chamber and the mirror nozzle to generate a magnetic field of up to 0.1 T in the center and 0.4 T in the mirror nozzle region. As a plasma source, a plasma washer gun was used, and it was designed to generate $10^{20} m^{-3}$ high-density pulsed plasma during 5 ms and via a Pulse Forming Network (PFN) power system. To construct the device, test of magnetic field system was conducted initially. Magnetic field intensity was measured in axial and radial locations by Gauss meter calibrated by Helmholtz coil. Generation of steady magnetic field was also confirmed by measuring coil voltage, current value and magnetic field by time. PFN power system was tested with dummy load, and the waveform was similar to the simulated value considering internal resistance of inductor and stray resistance. Experimental facilities were controlled and diagnostic data was obtained by implementing data acquisition and control system (DAQ). For plasma diagnostics, ion saturation current which is affected by electron density and temperature $(I_{sat}∝n_{e} \sqrt{T_{e}})$ has been measured with Langmuir probe diagnostics. Electron density and temperature were estimated from these measurements. It was confirmed that maximum 0.46 MW power was applied to the plasma source, and ion saturation current up to 0.45 A was measured at the center of the plasma. From the repeated experiments, current-voltage curve of Langmuir probe was measured, and the electron density $1.1×10^{20} m^{-3}$ and electron temperature 10 eV were estimated based on the measurements. Change of ion saturation current intensity by PFN voltage, magnetic field intensity and mirror ratio was observed to find variables to control plasma performance. Increase of ion saturation current was observed by increasing PFN voltage and magnetic field, but change of ion saturation current by mirror ratio was not shown clearly. Higher PFN voltage increases ion saturation current because plasma density generated in the plasma gun will be increased with higher PFN voltage. The reasons of ion saturation current increase by magnetic field intensity are likely to be enhancement of magnetization of plasma and suppression of instability. Mirror ratio was also expected to increase the ion saturation current by improving plasma confinement performance, but clear tendency was not observed, which could be due to degradation of mirror confinement efficiency by too frequent collision events in this experiment.