Kwang-Cheol Ko

Performance of pulsed power generator using high-voltage static induction thyristor

In present pulsed power generator systems using semiconductor switches, saturable magnetic switches are usually connected in series to compress the output pulse because the current rise time of semiconductor switches are generally not short enough. However, these magnetic switches are heavy and they reduce the energy transfer efficiency. Here, the authors propose a pulsed power generator system using 5500 V static induction thyristor (SI-Thy) and a Blumlein line for pulse formation, and its fundamental characteristics are evaluated. From experiments using only one SI-Thy, they obtained a turn-on time of several tens of nanoseconds and a maximum rise rate of the output voltage of 96 kV//spl mu/s. It is confirmed that SI-Thy will have sufficient performance as the main switch of the pulsed power generator for flue gas treatment and decomposition of hazardous gases if several devices are connected in series.

Voltage amplification effect of nonlinear transmission lines for fast high voltage pulse generation

The authors have proposed a new scheme of high voltage pulse generation utilizing a nonlinear transmission line (NLTL). The scheme is based on the voltage amplification effect of the process of the formation and collision of solitons in the NLTL with ferroelectric ceramic capacitors. Using the head-on collision scheme of two solitons, a fast high voltage pulse with 11 kV amplitude and 76 ns pulse width has been successfully obtained from a 3.6 kV, 300 ns initial pulse generated by a 3000 V class SI thyristor with a repetition rate of 1 kHz. In addition, they have employed stacked nonlinear transmission lines to obtain a much higher voltage amplification ratio. The scheme, known as stacked cable transformers, consists basically of equal lengths of transmission line connected in parallel at the input side, and in series at the output.

Active Clamping Circuit to Suppress Switching Stress on a MOS-Gate-Structure-Based Power Semiconductor for Pulsed-Power Applications

Metal-oxide-silicon (MOS)-gate-structure-based power semiconductors, such as MOS field-effect transistors, insulated-gate bipolar transistors, and MOS controlled thyristors, are widely used as high-voltage switch and power modulator components in pulsed-power applications. The power semiconductors are generally connected in series and in parallel in order to increase their maximum switching voltage and current, respectively. It is important to suppress overvoltage or switching stress on power semiconductors connected in series and parallel during an extremely short switching time and at fast operating frequency. Generally, gate drive control techniques and methods for the suppression of high voltage are required. To suppress overvoltage and switching stress, this paper proposes a simple and effective active clamping method rather than the use of a snubber circuit with free switching condition modulation. Based on comparative switching experiments, the active clamping method is expected to suppress switching stress and overvoltage while load and switching conditions are changed without modification of the high-side auxiliary circuit for pulsed-power applications.

Side-Extraction-Type Secondary Emission Electron Gun Using Wire Ion Plasma Source

A secondary emission electron gun applicable for gas treatment was designed, fabricated and tested. The gun is set beside an ion source, which is called a wire ion plasma source (WIPS). Ions extracted from WIPS are accelerated toward the cathode surface, which is set oblique to the ion loci. By collision of ions, secondary electrons emitted from this cathode surface are then accelerated toward the electron window and form a very wide electron beam. The electron window is set orthogonal to the ion extraction window and is on earth potential. In order to inject the electron beam perpendicular to the electron window, the ion and electron trajectories were numerically simulated for several conditions. A wide electron beam current distribution inside the gas treatment chamber was observed when a honeycomb board and an Al film were placed at the electron window. The electron beam current density of 6.4 mA/cm2 measured at cathode voltage of 80 kV possess sufficient energy to irradiate gaseous pollutants like NOx.

Characteristics of an ion induced secondary emission electron gun

This paper presents some fundamental characteristics and performance of a secondary emission electron gun (SEEG) using a pulsed glow discharge wire ion plasma source (WIPS). The positive helium ions extracted from WIPS are accelerated in vacuum toward a negatively biased cold cathode surface, which is set oblique to the ion loci in order to inject the secondary electrons emitted perpendicular to the foil window. The physical mechanisms governing the characteristics such as space charge, charge exchange and secondary electron emission have been reviewed. The dependence of such characteristics on the accelerating voltage of the incident ion and on the ion incidence position has been experimentally investigated. The experimental results are discussed together with available theoretical models of each characteristic to establish the relative understanding of such phenomena in a side-extraction-type SEEG. The experimental results are further discussed in the light of a self-developed numerical simulation using a finite element method, which presents a good understanding of particle trajectories as well as potential distribution inside the gun geometry.

An improved unimodal absorbing boundary condition for waveguide problems

An improved unimodal absorbing boundary condition (ABC) is proposed by using one-dimensional (1-D) mode finite-difference time-domain (FDTD). In the unimodal ABC, an uniform auxiliary waveguide of input-output port should be introduced so that evanescent waves attenuate sufficiently. In this letter, the uniform auxiliary waveguide is simulated by 1-D mode FDTD rather than three-dimensional (3-D) FDTD which is used in the conventional unimodal ABC. Memory storage and CPU time are significantly reduced by applying the proposed ABC. A WG-90 rectangular waveguide with a thick asymmetric iris is analyzed by FDTD with the conventional modal ABC and the proposed ABC, and scattering parameters and computational efficiency are compared.

Optimization of a grounded electrode shape in gas insulated switchgear with a reversely elliptical permittivity graded insulator

An application of a functionally graded material (FGM) to the solid spacer in gas insulated switchgears (GISs) can reduce the electric field concentration at the specific region, for example, the anode or the interface between the spacer and the gas. However, the electric field stress near the triple junction of the grounded electrode with a rounded shape, which remarkably affects the insulation capability of the GIS, inversely related. Therefore, in order to prevent this, it is necessary to modify the grounded electrode geometry in the common Cubicle-GIS and apply the elliptical FGM spacer with a reverse-direction variation unlike the permittivity variations of the existing unidirectional or bidirectional distributions. In this paper, we found the proper distribution of dielectric permittivity, dug a groove in the grounded electrode near the triple junction, and performed the optimization of this grounded electrode configuration by using the design of experiments (DOE). Consequently, the electric field stress in the triple junction of the grounded electrode can be effectively reduced by using the optimally designed grounded electrode shape applying the FGM spacer.

High rep-rate operation of pulsed power modulator using high voltage static induction thyristors

A repetitive pulsed power modulator, which uses high voltage static induction thyristors as main switching devices, has been designed and constructed for application to discharge light source. The main components of the power modulator are a pulse forming network (PFN) and a semiconductor switch. The PFN consists of 100 ceramic capacitors (2000 pF, 30 kV) connected in parallel. The measured impedance and output pulse width of PFN are 0.75 ohm and 427 ns, respectively. The semiconductor switch is made of 3 high voltage static induction thyristors connected in series, which is able to withstand 10 kV. The significant feature of the static induction thyristor is that it has very low ON-state voltage. This feature is especially suitable for high rep-rate operation of pulsed power modulators, since energy loss by switch can be remarkably reduced.

Performance of pulsed power generator using high voltage static induction thyristor

In present pulsed power generator systems using semiconductor switches, saturable magnetic switches are usually connected in series to compress the output pulse because the current rise time of semiconductor switches are generally not short enough. However, these magnetic switches are heavy and they reduce the energy transfer efficiency. Here, the authors propose a pulsed power generator system using 5500 V static induction thyristor (SI-Thy) and a Blumlein line for pulse formation, and its fundamental characteristics are evaluated. From experiments using only one SI-Thy, they obtained a turn-on time of several tens of nanoseconds and a maximum rise rate of the output voltage of 96 kV//spl mu/s. It is confirmed that SI-Thy will have sufficient performance as the main switch of the pulsed power generator for flue gas treatment and decomposition of hazardous gases if several devices are connected in series.

Propagation Model of High-Power Electromagnetic Pulse by Using a Serial–Parallel Resistors Circuit

The risk of high power electromagnetic (HPEM) pulse is dramatically increasing as the output of HPEM pulse increases and the affected electronic devices becomes smaller [1], [2]. There are largely two ways of coupling HPEM pulse: 1) front door coupling, which is through antennas and sensors to receive signals and 2) back-door coupling, which is through an unintended way, such as punctures and slots. In this paper, we modeled the loss phenomenon of HPEM pulse which happens when HPEM pulse radiated from HPEM system propagates to the receiving antenna. The loss circuit consists of serial-parallel resistors, each value of resistor is formularized as distance. Our modeling consists of two parts: 1) one-stage model, which means initial value for various applications and 2) nth-stage model, which means additional distances. Therefore, we could figure out the effect of HPEM on antenna as the propagation distance changes. After the due simulations, we confirm our hypothesis as the HPEM pulse which reaches to receiving antenna is consistent with the existing theory. This circuit modeling is very remarkable in that it understands two different HPEM system and propagation/receiving antenna as one whole system.