Sanghyuk An

Fabrication and Testing of a 600-kJ Pulsed Power System

A 600-kJ pulsed power system (PPS) was built for various applications. The PPS consists of six 100-kJ unit modules. A single unit module is composed of a capacitor, a thyristor switch stack, a crowbar diode stack, and a pulse forming inductor. The module was designed to be operational for a maximum charging voltage of 10 kV and a peak current up to 75 kA. Each module can be triggered and controlled independently by a control computer via fiber optic communication. The six modules are combined into one frame and are charged to the same voltage by one capacitor charger. The design, fabrication and test results of the unit modules, and the 600 kJ PPS are presented in this paper.

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.

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.

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.

Modeling and circuit analysis of an electromagnetic launcher system for transient current waveforms

An electromagnetic launcher with a 4.8 MJ pulsed power supply has been fabricated, and experimental tests were started. Through the experiments and calculations, the effects influencing on the electrical properties and the performance of the launcher have been investigated. The electromagnetic launcher system includes pulse-forming inductors in the power supply and rails in the launcher which are made of massive conductors showing a prominent skin effect for a transient current. For these conductors constant impedance values or those defined per unit length have difficulties in calculating correct voltages appearing between the terminals of them. In addition, the frictional force between the rail and armature could not be ignored for a better estimation of the armature motion. To obtain solutions close to the measured ones, the diffusion of the magnetic flux density or current density was taken into account, and the history of the armature motion was reflected in the electrical modeling of the rails. These effects were implemented as voltage correction terms in the circuit equations with the help of the electromagnetic responses for a step-function current. In the equation of the armature motion a reasonable form of the frictional force was modeled considering the velocity skin effect and the structural analysis. In this paper, the application of the voltage correction method into rails and the calculated result of the modified circuit equations for the constructed electromagnetic launcher system are presented.

Modeling of 3-stage Electromagnetic Induction Launcher

Electromagnetic Induction Launchers (EIL) have been receiving great attention due to their advantages of noncontact between the coils and a projectile. This paper describes the modeling and design of 3-stage EIL to accelerate a copper projectile of 50 kg with 290 mm diameter. Our EIL consists of three independent driving coils and pulsed power modules to generate separate driving currents. To find efficient acceleration conditions, the appropriate shape of the driving coils and the position of the projectile have been calculated by using a finite element analysis (FEA) method. The results showed that the projectile can be accelerated more effectively as the gap between the coils is smaller; a final velocity of 45 m/s was obtained. The acceleration efficiency was estimated to be 23.4% when a total electrical energy of 216 kJ was discharged.

Modeling and Circuit Analysis of an Electromagnetic Launcher System for a Transient Current

An electromagnetic launcher with a 4.8-MJ pulsed power supply has been fabricated, and experimental tests were started. Through the experiments and calculations, the factors influencing the electrical properties and the performance of the launcher have been investigated. The pulse-forming inductors of the power supply and the rails of the launcher are made of massive conductors showing a prominent skin effect for a transient current. The circuit analysis using constant impedance values has a difficulty in the calculation of the correct voltages between the terminals of the conductors. In addition, the frictional force between the rails and the armature should be considered for a better estimation of the armature motion. To obtain the calculated solutions close to the measured ones, the diffusion of the magnetic flux or current was considered, and the history of the armature motion was reflected in the electrical modeling of the rails. These effects were implemented as voltage correction terms in the circuit equations with the help of the electromagnetic responses to a step-function current. In the equation of the armature motion, a reasonable form of the frictional force was modeled considering the velocity skin effect and the structural analysis. In this paper, the application of the voltage correction method into the rails of an electromagnetic launcher was introduced in detail, and the comparison of the results between the experiments and modified calculations for the constructed launcher system was described.

Numerical Analysis of the Transient Inductance Gradient of Electromagnetic Launcher Using 2-D and 3-D Finite-Element Methods

We present analyses of the transient inductance gradient of an electromagnetic launcher with the sliding contact between rails and armature. When the armature moves along the rails, electrical current concentrates on the contact trailing edge of the armature due to the velocity skin effect, affecting input current spreading into the rail. This effect also induces the temporal variation of inductance gradient. Using the 2-D finite-element method (FEM), the rail inductance gradient without an armature is calculated and compared with the propulsive inductance gradient with a moving armature calculated by 3-D FEM. Using the 3-D simulation, the influence of the step on the height difference between the rail and the transient moving armature is investigated. The result shows that the average of transient inductance gradients during acceleration is 10% larger than Kerrisk’s inductance gradient. Also, we found that the axial force on the armature is not significantly affected by the velocity of the armature, while the lateral force on it whose direction is outside the rail increases with the velocity. The circuit simulation using both inductance gradients shows good agreement with the measured current and velocity.