Taichi Takezaki

We propose an electric circuit that generates a high magnetic field with a variable pulse width using insulated gate bipolar transistor (IGBT) switching. One method to realize a magnetic field with a variable pulse width is to switch the circuit open and short using semiconductor devices. However, in the case of a coil load used to generate a high magnetic field, inductive energy is released as a surge voltage during high-current switching, which can lead to the failure of switching devices. Therefore, we proposed connecting an external circuit in parallel with the coil to absorb and dissipate the inductive energy and analyzed the surge voltage and current waveforms generated in the circuit. The analytical solution based on circuit analysis and the circuit simulation results showed good agreement, indicating the transient response waveforms of the current and surge voltage in the proposed circuit. Moreover, the results reveal a trade-off between the surge voltage and the current decay time constant, depending on the resistance and capacitance values of the external circuit. A switching circuit with anti-series connected IGBTs demonstrated the interruption of the discharge current (500 A peak, 1 ms FWHM pulse width, 3 kV surge voltage) at arbitrary timing for driving the magnetic field. From these results, a pulsed magnetic field with a variable pulse width was implemented.

The mechanism of generating collisionless shock in magnetized gas plasma driven by laser-ablated target plasma is investigated by using one-dimensional full particle-in-cell simulation. The effect of finite injection time of target plasma, mimicking the finite width of laser pulse, is taken into account. It was found that the formation of a seed-shock requires a precursor. The precursor is driven by gyrating ions, and its origin varies depending on the injection time of the target plasma. When the injection time is short, the target plasma entering the gas plasma creates a precursor; otherwise, gas ions reflected by the strong piston effect of the target plasma create a precursor. The precursor compresses the background gas plasma, and subsequently, a compressed seed-shock forms in the gas plasma. The parameter dependence on the formation process and propagation characteristics of the seed-shock was discussed. It was confirmed that the seed-shock propagates through the gas plasma exhibiting behavior similar to the shock front of supercritical shocks.

We investigated the behavior of a mixed-gas plasma flow driven by a compact plasma focus device into a perpendicular magnetic field, by using a one-dimensional hybrid particle-in-cell simulation. When the plasma flowed into the perpendicular magnetic field, some accelerated ions were generated in front of the plasma flow owing to the compression of the magnetic field. The behavior of the compressed magnetic field changed by mixing heavy ions with the collisionless plasma. The maximum velocity of the accelerated light ions was proportional to the square root of the average ion mass of the plasma. These results indicate that the effect of the multi-ion species strongly affects the injection of light ions.