R.S. Hawke

Summary of EM launcher experiments performed at LLNL

Performance results for three railguns are summarized. The system used a helium gas-driven injector and railgun launcher to accelerate 1- and 4-g polycarbonate projectiles intact up to 6.6 and 3.0 km/s, respectively. A 625 kJ capacitor bank powered the railgun, and an adjustable inductor provided pulse shaping and peak current control. Operation in hard and soft vacuum was reliably achieved. Projectiles were accelerated without blowby of injector gas or plasma. The diagnostic system measured the projectile position and launch velocity, verified that the projectile was launched intact in the desired direction, and identified system components where improvements could enhance performance. Flash x-ray radiography measured velocity and verified that projectiles were intact. Post-launch projectile travel along the axis of the launcher without tilt was recorded with flash radiographs and impact impressions or holes in witness plates. The system performed as expected up to 4-5 km/s but below expectations at higher velocities. Our diagnostics suggest that the decreased performance was probably caused by the restriking of a second arc in the breech of the railgun, which shunted the current from the propulsive arc. Estimates of ablated launcher mass, drag forces, methods of eliminating restrike, and suggestions for improving the performance of railguns are discussed.

MAGRAC--A railgun simulation program

We have developed and validated a computer simulation code at the Lawrence Livermore National Laboratory (LLNL) to predict the performance of a railgun electromagnetic accelerator. The code, called MAGRAC (MAGnetic Railgun ACcelerator), models the performance of a railgun driven by a magnetic flux compression current generator (MFCG). The MAGRAC code employs a time-step solution of the nonlinear time-varying element railgun circuit to determine rail currents. From the rail currents, the projectile acceleration, velocity, and position are found. We have validated the MAGRAC code through a series of eight railgun tests conducted jointly with the Los Alamos National Laboratory. This paper describes the formulation of the MAGRAC railgun model and compares the predicted current waveforms with those obtained from full-scale experiments.

Results of railgun experiments powered by magnetic flux compression generators

Researchers from the Lawrence Livermore National Laboratory and the Los Alamos National Laboratory initiated a joint railgun research and development program to explore the potential of electromagnetic railguns to accelerate projectiles to hypervelocities. The effort was intended to 1) determine experimentally the limits of railgun operation, 2) verify calculations of railgun performance, and 3) establish a data base at megampere currents. The program has led to the selection of a particular magnetic flux compression generator (MFCG) design for a set of initial experiments and the design of small- and large-square-bore railguns to match the expected MFCG power profile. The bore sizes are 12.7 and 50 mm, respectively. In this paper, we briefly describe the design of the railguns and the diagnostic and data reduction techniques, followed by the results of eight experiments with the two railgun types.

Railgun performance with a two-stage light-gas gun injector

Results obtained with the HELEOS (hypervelocity experimental launcher for equation of state) railgun, which uses a two-stage light-gas gun (2SLGG) as an injector, are presented. The high-velocity 2SLGG injector preaccelerates projectiles up to approximately 7 km/s. The high injection velocity reduces the exposure duration of the railgun barrel to the passing high temperature plasma armature, thereby reducing the ablation and subsequent armature growth. The 2SLGG also provides a column of cool, high-pressure hydrogen gas to insulate the rails behind the projectile, thereby eliminating restrike. A means to form an armature behind the injected projectile has been developed. In preliminary tests, the third-stage railgun has successfully increased the projectile velocity by 1.35 km/s. Extensive diagnostics have been used to determine the behavior of the armature and track the launchers performance. Insome cases, velocity increases in the railgun section have been achieved, which are in close agreement with theoretical predictions, whereas in other experiments deviations from theoretical have been observed. The reasons for and implications of these results are addressed. Recent tests are reported. >

STARFIRE: hypervelocity railgun development for high-pressure research

STARFIRE has included efforts to identify and solve the problems that have inhibited reliable attainment of velocities greater than the 8 to 9 km/s attainable with two-stage light-gas guns (2SLGG). Issues studied include plasma arc formation and stabilization, restrike inhibition, viscous drag, ratio of preload to operating stresses, barrel joint design, and barrel precision requirements. The system uses a 2SLGG as an injector to minimize barrel ablation and armature contamination. Hydrogen is used as the injection gas and will also serve to reduce the probability of forming secondary arcs. An optical Doppler system is used to measure the projectile velocity continuously and precisely from a standing start in the 2SLGG barrel, through several joints, the HELEOS (hypervelocity experimental launcher for equation of state) railgun barrel, and postlaunch. The STARFIRE program is focused on the combined use of precision diagnostics and new experimental techniques. Test results are presented. >

Design and fabrication of large- and small-bore railguns

A joint program between the Lawrence Livermore National Laboratory and the Los Alamos National Laboratory was conducted to establish whether railguns could be operated at megampere currents, to set operating limits, and to provide data to validate the modeling of railgun technology. This paper discusses the 12.7- and 50.0-mm-bore railguns designed and fabricated for this program. The design criteria, the materials and fabrication methods, and the success of the designs are discussed in detail.

Armature formation in a railgun using a two-stage light-gas gun injector

The authors summarize the problems encountered in attempts to achieve hypervelocities with a railgun. Included is a description of the phenomenology and details of joint Sandia National Laboratories, Albuquerque/Lawrence Livermore National Laboratory (SNLA/LNLL) work at SNLA on a method for forming the needed plasma armature. Attention is given to such problem areas as secondary arc formation through growth and separation of the propulsive plasma armature and arc restrike. Potential solutions to the problems are being incorporated in the STARFIRE railgun project. The primary improvement is to provide a high-injection velocity with a 2SLGG (two-stage light gas gun), which offers the additional benefit of filling the barrel behind the projectile with electrically insulating hydrogen. The resulting additional challenge of forming a propulsive armature behind the projectile has been met with an injected metal vapor and a projectile-mounted fuse which are used to initiate rail commutation and begin the formation of a hydrogen plasma armature. >

Rail accelerator development for ultra-high pressure research

The Lawrence Livermore National Laboratory is currently developing a rail accelerator system for launching hypervelocity projectiles suitable for ultrahigh pressure shockwave research. The primary goal is to accelerate 1 g projectiles with disk impactors to velocities in excess of 12 km/s and generate uniform, planar shockwaves at pressures above 0.5 TPa (5 Mbar) in metal targets. In order to generate precisely controlled impacts and shockwaves, several stringent requirements are imposed on the railgun system. During the last year, we have begun detailed development of a railgun launcher and power source. We are developing a launcher with a gas injector. The injector accelerates the projectile to more than 1 km/s reducing the dwell time of the plasma arc and the erosion of the rails. The injected projectile, with a fuse, also serves as the main switch in the power supply circuit. Current pulse shaping is used to control the applied stress to the projectile and launcher. Results of experiments with the new system will be reported and compared to computer simulations.

Plasma armature formation in high-pressure, high-velocity hydrogen

The use of a two-stage light-gas gun (2SLGG) as a preaccelerator in combination with a railgun is expected to reduce barrel ablation and improve overall performance significantly. To continue acceleration of the projectile, a plasma armature must be formed. Two methods of converting a portion of the fast-moving hydrogen gas into a plasma armature capable of supporting currents exceeding 100 kA are reported. This work is part of STARFIRE, a joint project to develop a hypervelocity railgun. Both fuse and spark discharge techniques were tested with low-velocity single-stage injectors, and a projectile-mounted fuse was tested and a 2SLGG and the HELEOS (hypervelocity experimental launcher for equation of state) railgun. Both aluminum foil and vapor-plated fuses formed plasma armature when vaporized by current provided by a 830- mu f capacitor charged to between 1 and 2 kV. Spark discharge armature formation was also successful in the nitrogen injector railgun. It is concluded that the advantages of 2SLGG injection can be realized only by preventing premature plasma formation in front of the projectile while forming an armature behind the projectile after entrance into a railgun. >

Analytic solutions to dynamic equations of plasma armature railguns

General governing nonlinear differential equations pertaining to the dynamic behavior of a plasma armature electromagnetic railgun are derived. Three different cases are then considered, and the corresponding governing equations are solved exactly by means of a set of nonlinear transformations. The cases correspond to no ablation, continuous ablation, and partial ablation for which an ablation threshold velocity plays a fundamental role. It is concluded that in order to achieve very high projectile velocities the projectile should be injected into the railgun at velocities higher than the ablation threshold velocity. Thus the ablation can be completely alleviated and the ensuing turbulent drag can be significantly diminished. It is shown that under these conditions projectiles can typically be accelerated up to 30 km/s or more while without hypervelocity injection, for the same railgun and typical operating conditions, the maximum projectile velocity could be severely limited. >