Kuo Ta Hsieh

Advanced compulsators for railguns

In order to maximize the penetration of a projectile into a target, the acceleration on the projectile during the launch must be minimized. Low accelerations permit the design of long and slender projectiles which have better penetration capability. From this standpoint, power supplies for electromagnetic launchers must be able to provide rectangular current pulses with a high average to peak acceleration ratio. The authors discuss efforts to obtain the desired pulse shape from a compensated pulsed alternator (compulsator) when it is used as a power supply for railguns. A general theory of the pulse shaping technique is presented first. This is followed by a discussion on the tradeoffs between various equivalent generator configurations. Finally, the electromagnetic design of the compensated pulsed alternator being developed for task C of the Electromagnetic Gun Weapons System Program is presented. >

Numerical study on groove formation of rails for various materials

Recovered rails exhibit groove formation along the outer edge of the armature/rail contact surface in many experiments. Different causes are hypothesized, such as local melting resulting from the concentration of current density, material yielding due to high contact pressure, and material softening at elevated temperatures. Numerical simulations using the hybrid finite element/boundary element code EMAP3D were conducted to identify the cause of this damage. Various rail materials such as chrome copper, 7055 aluminum alloy, beryllium copper, and dispersion-strengthened copper are investigated. The results are presented in this paper, and they do show that groove formation is due to the effect of both material softening at high local temperature and material yielding.

Computation of magnetic fields using a 2-D hybrid finite-element/boundary-element algorithm and comparison with analytical solutions

Magnetic field computations using the finite-element (FE) method usually involve large air domains surrounding the conductors to satisfy boundary conditions at infinity. Elimination of these air domains saves computations and simplifies model generation-especially in large problems involving complex geometries like railguns and pulsed power generators, where fields in the air regions are calculated using fields at the air-conductor interface. The air domain can be avoided using the hybrid finite-element/boundary-element (FE/BE) method. A two-dimensional (2-D) hybrid FE/BE algorithm with the fundamental solution of Poissons equation as the weighting function is investigated here for applications to electromagnetic launch problems. Two examples with analytical solutions are used: a rectangular conductor carrying uniform current, and a quadrupole coil configuration. Solutions obtained using the hybrid algorithm match analytical solutions favorably in all regions, including geometric corners. The effects of the number of integration nodes in quadrature formulas and coupling schemes between the FE and BE formulations through the normal fluxes at the boundaries are also presented. These 2-D analyses serve as forerunners for three-dimensional investigations.

Edge Elements and Current Diffusion

Current flow in the neighborhood of sharp conducting edges and corners is encountered in many applications of computational electromagnetics. Some examples include resonant cavities and waveguides of rectangular shape, cracks in metal components in nondestructive testing, and the edges and corners of the rail-armature contact in an electromagnetic launcher. Although the number of analytical solutions to such problems is small, it is generally believed that such solutions often involve singularities, high gradients, or discontinuities in one or more field components near the edge or corner. It has been demonstrated that the so-called edge elements can provide much more accurate predictions of global quantities (like resonant frequencies and scattering parameters) in the case of full-wave electromagnetics when field discontinuities or singularities are present. They have also been shown to better represent field discontinuities at material interfaces in magnetostatics and reentrant corners in low-frequency electromagnetics. This feature of edge elements is due to their ability to allow necessary jumps in field components at geometric or material discontinuities, as opposed to nodal elements, with the Coulomb gauge enforced, which can overconstrain the fields in certain situations. The main question of interest in this paper is whether edge elements give a similar advantage in the representation of current diffusion around a sharp edge. Both edge- and node-based formulations are considered, and the results are discussed.

A Magnetofluid Mechanical Model to Describe Rail–Armature Interface Phenomena

When an armature slides between a pair of rails under the influence of Lorentz forces, the material at the interface will experience steep increase in temperature due to frictional and Joule heating. Experiments show condensed metal deposits on the rail, which indicates melting of the interface layer. As the temperature increases, the interface material progressively changes phase from solid to liquid and later ionizes to become a plasma. The charge transport from the rail to the armature has to occur through this layer and, hence, the magnetofluid mechanics (MFM) of this layer will affect essential railgun parameters, such as the inductance gradient. Initially, the armature will translate as a solid body. As an interface layer liquefies (and later becomes plasma), it will translate with the armature; it will also experience rotational motion through shear at the wall and through magnetic stirring by the jtimesB forces. The velocity field will influence the railgun through viscous dissipation and through VtimesB fields. In order to analyze and understand these phenomena, a model is proposed here. The armature is assumed to translate with an interface layer composed of aluminum liquid. A 1-D diffusing field with closed-form expressions for the magnetic potentials, fields, and current densities is imposed, enabling calculation of the accelerations and velocities of the armature. The secondary flow induced in the interface layer composed of liquid aluminum due to shear at the wall is calculated using MFM equations. The viscous dissipations are calculated and compared to Joule heating

Electromagnetic Field Effect and Analysis of Composite Structure

The electromagnetic and thermal response of composites subjected to magnetic fields is simulated by solving Maxwell and heat transfer equations simultaneously. The developed analysis accounts for the anisotropic nature of the electrical and thermal properties in three dimensions. A finite element code is developed to predict the response of composite structures subjected to transient magnetic fields. The analysis has been validated against a closed form solution and applied to simulate the induction heating process of composite cylinders. The developed analysis can be applied to the design of modern electrical weapons and used to simulate composite manufacturing processes such as induction cure.

Electromagnetic field effect and analysis of composite structure

The electromagnetic and thermal response of composites subjected to magnetic fields is simulated by solving Maxwell and heat transfer equations simultaneously. The developed analysis accounts for the anisotropic nature of the electrical and thermal properties in three dimensions. A finite-element code is developed to predict the response of composite structures subjected to transient magnetic fields. The analysis has been validated against a closed-form solution and applied to simulate the induction heating process of composite cylinders. The developed analysis can be applied to the design of modern electrical weapons and used to simulate composite manufacturing processes such as induction cure.

Stray field perturbations induced by the poloidal field magnet leads in IGNITEX

Summary form only given. The vacuum stray fields produced by the PF (poloidal field) leads in IGNITEX were calculated in detail. In the proposed design, each PF coil is fed by extended parallel flat plates close together so that they significantly self-cancel the magnetic field produced by the current flowing in opposite directions. The shapes of the terminals are modified based both on design considerations of the toroidal field (TF) magnet and on the perturbed fields produced by the terminals. Emphasis is given to the analysis of the plasma breakdown phase of the IGNITEX discharge. Stray field errors throughout the discharge are analyzed. This provides the basis for the evaluation of boundary conditions to study the magnetic configuration in the plasma as modified by stray field errors

Design and scaling of a simple and inexpensive fusion ignition experiment

Summary form only given. The basic characteristics of the ignited plasma in IGNITEX were obtained from considerations of thermal energy production and losses in the discharge. Ample ignition, margin, path to ignition conditions. and thermal runaway control were shown. A high possibility of ignition was obtained with the IGNITEX characteristics. Ignition is possible far from marginal stability, with ohmic heating alone and without the need for enhanced energy confinement. The high level of stress present in the magnet system can be tolerated by a single-turn toroidal coil with a very high filling factor. Impedance matching requirements are met by homopolar generators. Iron-core and reinforced epoxy-composite flywheel technologies are used for the power supplies. Nuclear shielding requirements for the primary shielding and for the diagnostic system have been estimated. The first wall system was studied. Scaling relationships for the principal design characteristics of an ignition, single-turn tokamak have been obtained. The scalings were used to analyze the small and large versions of the IGNITEX experiment

Electromechanical analysis of the toroidal field magnets in the small and large versions of IGNITEX

Summary form only given. The Ignition Technology Demonstration (ITD) program was initiated to design, build, and test the operation of a single turn, 20-T, TF (toroidal field) coil powered by an existing 60-MJ HPG (homopolar generator) power supply system. The ITD TF coil is a 0.06 scale of the 1.5-m major radius IGNITEX baseline design. A finite-element program (TEXCOR) which solves a set of coupled electrical circuit, magnetic diffusion, and thermal diffusion equations with temperature-dependent properties was developed under the ITD program. TEXCOR provides temperatures and magnetic body force densities for a stress analysis of the magnet structure. The stress analysis is performed using a 3-D finite-element code (ABAQUS). The feasibility of the TF magnets of the small and large versions, based on the electromechanical aspect, was evaluated using TEXCOR. The temperature distribution stresses and axial preload specifications of the TF magnets were investigated