Kyung-Seung Yang

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.

Overview of ETC program in Korea

Electromagnetic launcher (EML) technology had been investigated in South Korea since 1989. Muzzle velocities up to 4.3 km/sec were obtained using 1.3 g projectiles in 1991. Since 1992, the Korea Program has concentrated on electrothermochemical (ETC) technology instead of EML technology. A 20 mm caliber ETC gun was designed and set up together with the pulse-forming network of a capacitor bank. A 107 g projectile was accelerated to up to 1.24 km/sec. The new Korea ETC program is established to investigate ETC launchers, compulsators, power switches, etc. The whole program will be performed by the Agency for Defense Development with the help of some Korean institutes and industries.

Overview of ETC research in Korea

In late 2000, an experimental 30-mm electrothermal-chemical (ETC) gun, a data acquisition system, and a 2.4-MJ pulse-forming network (PFN) consisting of eight 300-kJ modules were constructed, and serious research projects were started. During the past years, basic studies on the effect of the electric pulse in the process of ignition and propulsion of the gun have been done. Two types of ammunition, an exploding wire cartridge and a capillary plasma injector cartridge, were used in the experiments. The load resistance should be estimated well in order to deliver electric energies into the combustion chamber as a desired form in time. For that purpose, there has been an effort to model the load resistance of the ETC gun, which shows the dependence on the pressure of the combustion chamber. A study on a capillary plasma itself is also an important topic and is now in the beginning stage of research using spectroscopic diagnostics. The capillary plasma discharged in the open air provides useful information through the comparison with that discharged in the ETC gun. In the development and operation of the PFN, some problematic characteristics such as a high-voltage surge leading to a destruction of crowbar diodes occurred. A countermeasure was presented to solve the problem. High-current switches are also being studied. Vacuum rotary arc gap switch, pseudospark switch, triggered vacuum switch, and inverse-pinch switch are being studied for better and reliable operations. The ETC program in Korea is currently concentrating on the detailed investigation on the interaction of a plasma with propellants.

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.

Transient Analysis of Circuit Containing Massive Conductors

Inductive electrical components made of massive conductors are usually adopted in the high-power circuit. Since they are not the types of filaments, the diffusion of the electromagnetic fields inside the conductors plays an important role in the transient discharge. Though the behavior of the circuit can be described reasonably using constant electrical parameters, the voltage drop in the massive conductor is not expressed accurately when constant resistance and inductance are used in the analysis of the transient discharge. To obtain a more accurate waveform of the pulsed current, we have used a comprehensive method calculating the correction voltage in the voltage drop. Once the resistive voltage drop and the magnetic flux for a step-function current are obtained either analytically or numerically, the voltage drop in the massive component for any current waveform can be calculated using the Duhamels integration. The equations of the circuit containing massive conductors can be solved through a few iterations using the method estimating the voltage drop. As an example, the method was applied to an analysis of an RLC circuit containing a massive pulse-forming inductor. Electromagnetic responses of the inductor for a step-function current were calculated numerically with the help of a finite element method. The current waveform calculated using the method of correction voltage showed a good agreement with the measured waveforms.

Experimental tests of a 25mm square-bore railgun

As a preliminary study on the electromagnetic propulsion, a 25 mm square-bore railgun with a travel-length of 1940 mm was fabricated and tested. Electrical circuit parameters of the PFN (Pulse Forming Network) and the railgun were determined from the discharges of the PFN with the railgun short-circuited at the breech or muzzle. Launch experiments were conducted to verify the basic performance of the constructed railgun. The aluminum armature of 30 gram in mass was accelerated up to 2000 m/s with a peak current of about 700 kA by simultaneous discharging of 8 capacitor bank modules. Contact problems and gouging phenomena were observed. Circuit equations coupled with the armature motion were modeled. By introducing appropriate friction forces the model showed a good prediction on the armature motion. This small scale railgun gave useful information for the preparation and design of the larger railgun with a medium caliber to be studied subsequently.

Capillary discharge in the open air

It is important to understand the physical processes contributed by the plasma injector in the development of an electrothermal-chemical (ETC) gun. To investigate basic properties of a capillary discharge, an injector was discharged in open air. Comparing the measured and the calculated plasma resistivities, the state of power balance was investigated. The capillary plasma discharged in open air showed large ejection of plasma without power-balancing, while the capillary plasma produced in the ETC gun experiment reaches near balanced steady-state as the pressure of the combustion chamber becomes appreciable. The ejected flames were monitored with a high-speed camera. Typically the observed flames were about 50 cm /spl sim/1 m long and 25 cm wide. To estimate temperature and density, a preliminary measurement of time-resolved spectra in the visible wavelength region was done.

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.

Comparison of Plastic Ablation Materials of Capillary Discharge

Summary form only given. Fast capillary discharge is well known as a soft X-ray radiation source in the near vacuum environment. In addition, with a long discharge of ms duration in the open air it also has been used as a simple and efficient plasma source of electrothermal ignition of propellants. Since the plasma materials are mainly provided from the wall ablation inside capillary, electrical property changes with different liner materials depending on the ablation property. In order to achieve robust discharge operations capable of energy transfer over 100 kJ under a limited capillary geometry of 5 mm in diameter and 25-30 mm in length, various plastic liner materials were tested. The discharge experiments have been conducted around a few tens kA current and several ms duration in the open air. A Mach cone appearing at the capillary exit region was used for the estimation of plasma pressure. Direct pressure measurements also have been done. For the estimation of electrical properties, plasma resistivities were calculated using Saha equation of state, and compared with experimental results, where the model by Kurilenkov and Valuev (1984) was more appropriate for the circuit simulation of our result. Among various materials polyacetal showed best result for a high power and energy transfer. Though the model using a wall ablation through blackbody radiation explains well the discharge properties, the experimental result of different resistivity values corresponding to the same rising and falling current is not clearly understood. The possible reasons will be proposed and discussed.

A 4.8-MJ Pulsed-Power System for Electromagnetic Launcher Experiment

A 4.8-MJ capacitive pulsed-power system (PPS) was designed and fabricated for electromagnetic launcher (EML) experiment. The PPS consists of eight 600-kJ segments, which can be operated independently. In each segment, six 100-kJ capacitor bank modules, a charging power supply, a dump and charge panel, and control circuit were integrated. The capacitor bank modules in the segment are charged at the same voltage up to 10 kV, but the trigger time of each module can be set up differently. The overall PPS system is controlled by a control program, which sets charging voltage and trigger time of each module and monitors the states of system component. All the control signals are transmitted through fiber optic communication. The 48 unit modules are connected in parallel to EML with high-voltage coaxial cables. A current amplitude of more than 1 MA and a pulsewidth of several milliseconds were achieved by the PPS. The PPS has been applied to several tens of firing experiments of EML successfully. The design, fabrication, and the test results of the 4.8-MJ PPS were described in this paper.