Yun-Sik Jin

Design and performance of a 300 kJ pulsed power module for ETC application

In order to support the Korean electrothermal chemical (ETC) launcher program, a 2.4 MJ pulsed power supply system is scheduled to be constructed for the ETC launcher driver. The overall power system consists of eight capacitive 300 kT energy storage banks. In this paper we describe the design features, setup and test operation result of the 300 kJ pulsed power module. Each capacitor bank of the 300 kT module consists of six 22 kV 50 kJ capacitors. A triggered vacuum switch (TVS-43) was adopted as the main pulse power-closing switch and operation characteristics were investigated. Crowbar diode circuits, variable multi-tap inductors and energy dumping systems are connected to each high power capacitor bank via bus-bars and coaxial cables. A parallel crowbar diode stack is fabricated in coaxial structure with two series 13.5 kV, 60 kA avalanche diodes. The main design parameters of the 300 Kj module are a maximum current of 180 kA and a pulse width of 0.5-3 ms. The electrical performances of each component and current output variations into resistive loads have been investigated.

High speed scanning of terahertz pulse by a rotary optical delay line.

We demonstrate high speed scanning of a time-domain terahertz pulse by a rotary optical delay line (RODL) incorporated into a photoconductive antenna based terahertz system. The delay line of RODL consists of six rotating convex reflective blades with a rotating speed controlled for a wide range of scan repetition rates. It can perform path length scans of 2.1 cm at a scanning speed of 8.4 m/s while maintaining linearity. The comparison of the terahertz temporal waveform and the frequency spectrum measured by using the RODL and a linear stage optical delay line confirms the linearity of the RODL up to 140 ps at high scan repetition rates of 400 scan/s. A high signal-to-noise ratio, up to a few thousand, can be obtained by averaging multiple terahertz pulses at a high scan speed.

Design of high voltage capacitor charger with improved efficiency, power density and reliability

This paper describes the design of a 48 kJ/s high-voltage capacitor charging power supply (CCPS), focusing on its efficiency, power density, and reliability. On the basis of a series-parallel resonant converter (SPRC) that provides high efficiency and high power density owing to its soft-switching, the design of the CCPS is explained in detail, including its input filter, resonant tank parameters, high-voltage transformer and rectifier, as well as its protection circuit. By using two resonances per switching cycle, which provides a trapezoidal instead of a sinusoidal waveform of the resonant current, the proposed CCPS can take advantage of the lower conduction loss and reduced switching loss by improving the crest factor and allowing a higher value of the snubber capacitor, respectively. In addition, the compact design of an input filter without bulky components such as a DC reactor and an electrolytic capacitor allows for high power density, a high power factor, and low cost. In addition, the control loops for the voltage and current were optimized with a fast response time in order to compensate for the low frequency ripple of the input voltage, which results from the reduced filter component. Experiments on the developed charger were carried out with both resistor and capacitor loads in order to measure not only its efficiency and power factor with respect to the output power but also its charging time, in order to estimate the average charging current. The experimental results obtained with a resistor load showed a maximum efficiency of 96% and a power factor of 0.96 for a full-load condition. For the measured charging time of a 4 mF capacitor, with 9.68 s for 10 kV charging, the average charging current was estimated as 4.13 A. Moreover, to verify the reliability of the developed CCPS, a variety of tests, including opening and shorting of the output terminal as well as misfiring of the discharge switch during the charging operation, were performed with a 200 kJ pulsed power system. Finally, it was experimentally confirmed that the developed CCPS shows high performance in terms of efficiency (96 %), power factor (0.96), and reliability with a high power density (820 W/L).

Terahertz Pulse Detection Using Rotary Optical Delay Line

We report the first measurement of terahertz pulse using a rotary optical delay line (RODL). The delay line uses a rotating reflective surface that gives a linear delay length with respect to the rotation angle. Operation principles and the profile of the reflective surface are examined and the optical path as a function of the radius and the rotating angle is calculated. A comparison of the temporal waveform and frequency spectrum obtained by using the RODL and the linear stage optical delay line (LODL) confirms that the linearity of the RODL is up to 500 ps corresponding to a delay scan length of 7.5 cm, and the difference in time delay is less than 0.2 ps. The RODL is suitable for the high-speed scanning of terahertz pulses up to 400 Hz.

Noble crowbar circuit for compact 50 kJ capacitor bank

This paper presents a novel crowbar circuit for a 50 kJ capacitor bank. The crowbar circuit consists of a Rogowski coil, a trigger signal generator, and a triggered vacuum switch (crowbar switch). In our crowbar circuit, the Rogowski coil senses the load current and generates trigger signals at the correct time for the crowbar switch operation. At full capacitor charge voltage of 22 kV, it was possible to keep the reverse voltage of the capacitor below 15% of charging voltage. In this paper, we report the features of our crowbar switch system and the electrical performance of the crowbar system.

Fast scanning of a pulsed terahertz signal using an oscillating optical delay line

We describe a fast measurement of a pulsed terahertz signal generated by a femtosecond laser and a photoconductive antenna using an oscillating optical delay line. The method to measure the amplitude of the retroreflector in the oscillating optical delay line is proposed and the displacement of the retroreflector is exactly calculated to acquire the optical delay time in the fast scan mode. With the different oscillation frequency and amplitude of the retroreflector, the pulsed terahertz signals are measured and analyzed. The comparison of the temporal waveform and frequency spectrum between the fast scan mode and the slow scan mode shows a good agreement with the decrease in the scanning time from 60 to 1 s at a signal to noise ratio of 430.

Analysis of Output Pulse of High Voltage and Nanosecond Blumlein Pulse Generator

A high voltage and nanosecond Blumlein pulse generator has been developed to produce an output pulse whose voltage level is greater than 250 kV and pulse duration 5 ns. The generator consists of various components such as a charging circuit, a pulse transformer, and a spark gap switch. As a heart of the generator, a Blumlein pulse forming line has been constructed in the cylindrical form using three cylindrical aluminum electrodes that are placed concentrically. Unlike the ideal Blumlein line, the output pulse of an actual Blumlein line may be affected by stray inductances and capacitances of switching and charging components, thereby degrading the performance of the generator. In this paper, PSPICE simulations have been performed to examine effects of stray inductances and capacitances on waveforms of output pulses. Simulation results show that the pulse waveform is significantly distorted mainly by the stray inductance of the spark gap switch.

High voltage ultrawide band pulse generator using Blumlein pulse forming line

A high voltage ultrawide band pulse generation system has been developed to radiate intense and ultrawide band electric fields for the examination of effects of the electric fields on the operation of electronic devices. As major components of the system, a helical strip∕wire type of air-cored pulse transformer and a triaxial type of Blumlein pulse forming line have been designed and fabricated to amplify and shape the output pulse, respectively. For the construction of a compact system, the pulse transformer and the Blumlein line are installed in a single cylindrical container. An ultrawide band TEM horn antenna has been fabricated to radiate the Blumlein output pulses to electronic devices. A number of experimental results demonstrate that the system is capable of providing an output pulse whose voltage is greater than 300 kV, pulse duration is ~5 ns, and rise time is ~500 ps with repetition rate of 10 Hz. The peak-to-peak value of electric field intensity of a radiated pulse is also measured to be approximately 42 kV/m at a distance of 10 m away from the antenna.

Three-Dimensional Particle-in-Cell Simulation of Fast Oscillation Startup in Strapped Magnetron Using Electrically Primed Electrons

Fast oscillation startup in a strapped magnetron using electrically primed electrons is demonstrated by a three-dimensional particle-in-cell simulation. The electrons are electrically primed by five-fold perturbation of the radial electric field along the azimuthal direction in a 10-vane strapped magnetron, and this leads to fast oscillation startup in the desired π-mode. The start of oscillation is advanced from 62 to 52 ns, and the time at which the steady state is reached from 130 to 85 ns when the radial variation of the protrusion and recession of the central region of the anode is 0.3 mm with an angular width of 0.5 mm.

Performance of 2.4-MJ pulsed power system for electrothermal-chemical gun application

In order to maximize electrothermal-chemical (ETC) gun performance, the power supply for the ETC gun driver needs to have a capability of producing a flexible current pulse. This current pulse forming can be realized by a combination of current pulses from multiple modularized capacitor banks. For this purpose, a 2.4-MJ pulse power system (PPS) has been developed. The 2.4-MJ PPS consists of eight modular 300-kJ capacitor banks (CBs). The 300-kJ CB modules are designed to operate independently and have a charging voltage of 22 kV and a current capability of 150 kA. Operation characteristics of 2.4-MJ PPS have been investigated. Various shapes of current pulse have been tried for a dummy resistive load and an ETC gun load. In this paper, we discuss some problems that take place during multiple modules operation and suggest solutions for these problems. We also show some experimental results when the 2.4-MJ PPS was applied to resistive dummy load and a 30-mm ETC gun load.