| Summary: | 碩士 === 國立成功大學 === 太空與電漿科學研究所 === 106 === A 16 kJ pulsed-power system using a parallel plate capacitor bank (PPCB) is being built. The large and short current pulse provided by the pulsed-power system can ionize thin wires and produce plasma jets to simulate interactions between solar winds and planets. The expected peak output voltage and current are ∼ 80 kV and up to ∼ 800 kA, respectively, with a rise time of ∼ 1 μs. Nevertheless, the system will be charged to 20 kV when it is first built. The system consists of 2 groups of energy bank connected in parallel and placed in both sides of a vacuum chamber. The total capacitance of the system is 5 μs. Each group consists of 5 stages connected in parallel. In each stage, 2 capacitors are connected in series and the capacitance of each capacitor is 1 μ F. Parallel plate transmission line is used to guide the current to the chamber.
Since the PPCB needs a DC power source, a 2 kW high voltage DC power supply was built. The high voltage DC power supply consists of a first stage DC power supply, a pulse generator, a transformer and a voltage doubler. The first stage DC power supply provides a DC source up to 500 V to the pulse generator. The DC power source is converted to AC power by the pulse generator. The voltage is then raised by 60 times by the transformer and is doubler by using a voltage doubler. Therefore, a DC power source up to 60 kV is expected. However, a ∼ 30 kV is only provided at this point due to safety concern.
Switches are very important for PPCB. They need to be able to hold high voltage without breaking down when capacitors are being charged. On the other hand, the breakdown of switches must be controlled. The characteristics of switches are studied here. Several spark gaps including self-breakdown spark gap switches, controlled spark gap switches, and a rail gap switch were built and tested. The rail gap switch is chosen as the main switch of PPCB. To trigger the rail gap switch, a small pulsed-power system and a 3-stage transmission line transformer were built.
The small pulse power system consists of a spark gap switch, a 40 nF or 1 μF capacitor and a trigger pulse generator. The gap distance between the electrodes and the gas pressure in the switch are the main factors effecting the breakdown voltages of the spark gap switch. Totally three spark gap switch were built. After many tests, we selected a spark gap switch design with a total gap distance of 8 mm and pressurized up to 4 atm using nitrogen gas. A trigger pulse generator is used to control the spark gap switch. The trigger pulse generator has an output voltage of ∼ -20 kV and a rise time of ∼ 55 μ s. The trigger pulse generator is triggered by a function generator. In order to prevent the electromagnetic pulse (EMP) damaging the function generator when the spark gap switch is activated, the optical fiber system is connected between the trigger pulse generator and the function generator to convert the electrical signal into an optical signal and back to the electrical signal. In the small pulse-power system, the jitter was measured. The test is for the safety of the PPCB, and the time reference point for any experiments in the future.
In order to estimate the peak current of the system, the inductance of the rail gap switch must be measured and analyzed. In order to generate a multichannel discharge in the rail gap switch, the 5 kV/ns fast trigger pulse is needed. A pulse of ∼ -30 kV is provided the small pulse-power system. The voltage of the pulse is raised by the 3-stage transmission line transformer with a short rise time ( ∼ 150 ns). Therefore, the multichannel discharge in the rail gap switch can be triggered by the fast pulse output from the small pulse-power system with the 3-stage transmission line transformer.
In the future, discharge voltages of the rail gap switch with one to three stages. The inductance of the rail gap switch and the one-stage can be estimated by cross comparison and the PPCB can be built.
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