William F. Weldon

Compact homopolar generator developed at CEM-UT

For electromagnetic launchers (EMLs) to become practical devices, they must evolve from laboratory test beds to field-portable systems. Such systems require the development of compact, lightweight, high-energy, high-current power supplies. Investigation of the candidate systems -- flux compressors, capacitors, inductors, batteries, and rotating machines -- showed the homopolar generator (HPG) to be a device with immediate potential for development. HPGs were selected because of their demonstrated ability to produce the high-energy, high-current electrical pulse required of an EML power supply from a relatively compact light-weight machine. By taking state-of-the-art HPG technology and integrating it with a machine designed specifically for high energy density, a field-portable HPG-powered EML system can be realized.

Testing of a high performance, precision-bore railgun

This paper discusses the results of high-pressure (up to 350 MPa) railgun experiments. The gun is designed both to be capable of high-pressure operation without structural damage and to be readily disassembled for inspection, maintenance and component testing. Considerable effort has been invested to develop techniques to produce and measure smooth, extremely precise (>5µm of variation) bores. We have not deliberately varied bore precision to determine the effect of precision on performance, but our experience suggests that if two otherwise identical railguns, one with a polished, high-precision bore, were fired under identical conditions, the precisely finished gun would achieve a higher muzzle velocity. Very high accelerations have been achieved (>107m/s2). A 2-g projectile has been accelerated to 5 km/s in a 13 mm square-bore gun only 1 m long. Projectiles with an L/D as small as 0.65 have been successfully accelerated. Projectiles with smaller L/Ds have not yet been tested to determine the minimum L/D which can be successfully accelerated. A number of different insulator materials ranging from common float glass to fused quartz have been tested. The best results have been obtained with fused quartz, which shows promise of being reusable. In the course of testing, the importance of gasketing the rail-to-insulator seams to prevent loss of plasma has become apparent. We have made progress in gasket design, but more work is needed. Rail gouging has been a continuing problem. Gouging may be dependent on bore precision, projectile fit, rail mechanical properties, projectile L/D, structural stiffness, operating pressure, velocity, and shape of the current pulse.

Driving Parallel Flashlamps with a Compensated Pulsed Alternator

Test results are given for a prototype Compensated Pulsed Alternator that was operated up to its 5400 rpm design speed. The machine has delivered approximately 140 kilojoules of energy in a single 1.3 millisecond pulse into a load comprising sixteen parallel flashlamps. The energy delivered to the lamps follows a W = (rpm/225)/sup 4/ scaling law to about 4200 rpm. Beyond that speed, eddy current losses become significant with the present design. New codes are able to model the machine parameters, and the prototype is presently being rebuilt to reduce the high speed losses predicted by the codes and verified by experiment.

Rotating machines-power supplies for the next generation of EM accelerators

The next generation of electromagnetic launchers (EMLs) will require compact, lightweight power supplies which can generate high-magnitude, repetitive current pulses of a very specific shape. Additionally, a simple, reliable system in which logistic concerns are minimized is required. Rotating machines, in particular compulsators, can meet these needs. The compulsator is a specialized alternator which has been designed to produce a series of high-power pulses. A number of advantages are inherent in the use of these devices for EML applications. In addition to high energy and power density, and the lack of additional power conditioning components, the machines are well-suited for the requirements of an electromagnetic (EM) gun circuit. The compulsator produces an alternating voltage which drives the current pulse through a current zero, achieving the desired pulsewidth. Conceptual designs of compulsator-driven pulsed power systems for use in 18 missions of interest were analyzed. These missions, which are summarized, include a wide variety of projectile masses and launch energies. Power supplies were designed to accommodate five types of launchers for each mission: railguns, coilguns, thermal-electric guns, hybrid guns, and the advanced electric gun. The hybrid gun represents a railgun with a thermal-electric injector and the advanced gun is a hypothetical far-term coilgun. The results of this analysis, including a general description of the compulsator, design assumptions, the methods used to achieve the desired pulse shape,s a discussion of magnetic energy recovery, gyroscopic effects, and scaling information are included. >

Rebuilding the Five Megajoule Homopolar Machine at the University of Texas

Abstract : The role of the 5 MJ homopolar machine at the Center for Electromechanics has changed from that of a pulsed power supply experiment to that of a power supply for various experiments. Because of this change in duty, it was necessary to modify the machine to allow more efficient operation and easier connection of the machine to the load. The experimental bearings which were on the machine were replaced with bearings of a more conventional design. These bearings exhibit a higher stiffness and lower loss than the original bearings, making the machine more reliable and reducing motoring time.