Chuhyun Cho

Effects of the medium on synthesis of nanopowders by wire explosion process

Silver nanopowder was produced in air and water media by wire explosion process. Size of the particles formed in water was smaller than that in air. In water medium, compared with air, the delay of plasma formation resulted in a higher energy deposition in the wire. As a consequence, there were no micrometer size particles in the powder, which is attributed to the complete vaporization of the wire. It is considered that the wire explosion in liquid medium meets three conditions to form smaller particles, namely, higher energy deposition in the wire, sufficient expansion volume, and quick cooling of the particles.

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.

Electromagnetic Launch Experiments Using a 4.8-MJ Pulsed Power Supply

After preliminary tests using a small 25 mm-caliber electromagnetic launcher, a larger mid-caliber launcher has been designed and fabricated. The launcher has a rectangular bore of 40 mm × 50 mm where the rails are separated by 50 mm each other and are 5.6 m long. To deliver an electrical current to the launcher, a new 4.8 MJ pulsed power supply (PPS) consisting of eight 600 kJ segments has been constructed. The 600 kJ segment is a basic building block of PPS operated independently. It contains a controller, a charger, a safety circuit, and six 100 kJ unit modules. Each unit module in the segment is composed of a 100 kJ capacitor bank, a thyristor switch, a crowbar diode, and a pulse-forming inductor. The modules in a segment are charged to the same voltage, but they are designed to have different triggering times to make a flexible shape of current waveform. Electrical parameters of the PPS were determined through the discharges of the unit modules, and those of the rails were done by launch experiments or short circuit tests at the ends of the rails. Launch experiments have been done using several current waveforms. The current of 1 MA in a few ms made the armatures of several hundred grams in mass accelerate to velocities near 2 km/s. In this paper, the design and basic performance of the constructed PPS and the electromagnetic launcher are presented.

Fast high-voltage pulse generation using nonlinear capacitors

Many pulsed power applications require short high-voltage pulses with a high-repetition rate. Conventional high-voltage discharge pulse-switches such as thyratrons, spark gap switches, and vacuum tube switches have a short lifetime, whereas the semiconductor switches have a long lifetime and high reliability. The semiconductor switches, however cannot be directly applied to fast high-voltage pulsed power generation due to their limited operating voltage ratings despite their relatively long switching times. Therefore, they are used with voltage amplification and a pulse compression stage. This paper describes two pulse generators that use the semi-conductor switches and nonlinear capacitors: one is based on an opening switch (IGBT) and inductive energy storage, the other is a combination of a closing switch (RSD) and capacitive energy storage.

Fast high voltage pulse generation using nonlinear capacitors

Many pulsed power applications require short high voltage pulses with high repetition rate. Conventional high-voltage discharge pulse-switches such as thyratrons, spark gap switches, vacuum tube switches have short lifetime, whereas the semiconductor switches have long lifetime and high reliability. The semiconductor switches, however, can not be directly applied to fast high voltage pulsed power generation due to their limited operating voltage ratings despite their relatively long switching times. Therefore they are used with a voltage amplification and a pulse compression stages. This paper describes two pulse generators using the semiconductor switches and nonlinear capacitors: one is based on an opening switch (IGBT) and an inductive energy storage. The other is a combination of a closing switch (RSD) and a capacitive energy storage.

Development of a 30-kW Plasma Gun System With a Long Lifetime

A hollow cathode that has extremely stable discharge characteristics has been developed. It is composed of two separated lanthanum hexaboride (LaB6) disks in the tube as the electron emitters. The design accomplishes great advantages by extending the surface discharge area of LaB6, an extension that is also useful for optimal fixing of LaB6. The hollow cathode is capable of producing 30 kW (100 V and 300 A) of power continuously. The plasma beam generated by the hollow cathode passes through holes in the centers of two intermediate electrodes covered with tungsten, which is water cooled due to the high plasma temperature entailed (greater than 3000degC ). The combination of the hollow cathode and the two intermediate electrodes is practically useful for an ion plating plasma beam source.

Development of the pulse RAG switch

Summary form only given. A rotary arc gap (RAG) switch has been designed as a pulse closing switch to discharge the 500 kJ of stored energy in a capacitor bank. It has two ring electrodes that are maintained axially parallel each other with gap between them joined by insulating spacing element. One of the electrodes has a hole for generation of plasma jet. Triggering of RAG switch is realized by plasma jet from the plasma gun connected to controller output. Both electrodes have cuts dislocated relatively each other. The current feedlines to electrodes are placed in such manner that after closing of the interelectrode gap by pulse plasma jet from plasma gun, and the arc, ignited by triggering, is forced to move along the ring electrodes by self-magnetic field. That is why wearing of electrodes is uniform on all over their surfaces and the switch has quite long lifetime. This switch operates in air atmosphere and needs no additional gas and pumping equipment. It ensures RAG switch trigger in wide range of voltage applied to switch (from 0.7k V up to 11 kV). Peak current discharged through RAG switch reaches up to 400 kA. Transferred charge quantity per pulse reaches 200 Coulombs.

Electromagnetic launch experiments using a 4.8 MJ pulsed power supply

After preliminary tests using a small 25 mm-caliber electromagnetic launcher, a larger mid-caliber launcher has been designed and fabricated. The launcher has a rectangular bore of 40 mm × 50 mm, where the rails are separated by 50 mm from each other and are 5.6 m long. To deliver an electrical current to the launcher, a new 4.8-MJ pulsed power supply (PPS) consisting of eight 600-kJ segments has been constructed. The 600-kJ segment is a basic building block of PPS operated independently. It contains a controller, a charger, a safety circuit, and six 100-kJ unit modules. Each unit module in the segment is composed of a 100 kJ capacitor bank, a thyristor switch, a crowbar diode, and a pulse-forming inductor. The modules in a segment are charged to the same voltage, but they are designed to have different triggering times to make a flexible shape of current waveform. The electrical parameters of the PPS were determined through the discharges of the unit modules and those of the rails were measured by launch experiments or short circuit tests at the ends of the rails. Launch experiments have been done using several current waveforms. The current of 1 MA in a few milliseconds accelerated the armatures of several hundred grams in mass to velocities near 2 km/s. In this paper, the design and basic performance of the constructed PPS and the electromagnetic launcher are presented.

Nanosecond high-voltage pulse generator using a spiral blumlein pfl for electromagnetic interference test

High-power electromagnetic (HPEM) pulse can be delivered to electronic equipment in two ways. One is through the application of radiated fields, and the other is through conduction along cables and wires. In this paper, a high-voltage pulse generator using a spiral Blumlein pulse forming line (PFL) was developed for the test of HPEM effect by radiation and conduction. For the conduction test, a spiral strip Blumlein PFL filled with transformer oil is employed to generate high voltage (>300 kV) and long duration (50 ns) pulse. High-power ultrawideband pulse for the radiation test was generated by combining a TEM horn antenna with the high-voltage generator. The peak-to-peak value of electric field intensity of a radiated pulse is 53.3 kV/m at a distance of 10 m away from the antenna. The pulse generation system was used for the examination of effects of the electric fields and conducted pulses on the operation of electronic devices.

Increased lipid productivity of Acutodesmus dimorphus using optimized pulsed electric field

An alternative stress-inducing method, a low-energy pulsed electric field (LE-PEF), was used to improve the lipid productivity of microalgae cultures. A large shift in the Nile-Red-stained peaks toward a higher intensity in fluorescent-activated cell sorting, suggestive of an increased neutral lipid content, was observed when 10 kV LE-PEF pulses were applied to 800 mL batch cultures of Acutodesmus dimorphus. The optimal LE-PEF on-off cycle treatment for A. dimorphus was 2xa0s on–60xa0s off for 15xa0min with 6 cycles per day. Under these optimal LE-PEF treatment conditions, A. dimorphus showed an overall 28.8xa0% increase in lipid productivity. Therefore, based on these results, LE-PEF can be regarded as a promising tool to avoid the lipid content and growth rate trade-off and improve the lipid productivity of microalgae in various cultivation systems.