A.R. Susoeff

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.

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. >

Railgun rail gouging by hypervelocity sliding contact

A description is given of a recently resolved mechanism of gouging which occurs during hypervelocity sliding contact between two materials. A parameter study based on computer modeling of the gouging mechanism is presented in which gouging velocity thresholds are determined for several combinations of sliding materials. Materials which can gouge each other are found to do so only within a certain range of velocities. Related calculations of gaseous material ahead of railgun projectiles are also presented. Gun-bore gouging experience with the Lawrence Livermore National Laboratory railgun project is reviewed. >

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. >

A linear electric motor to study turbulent hydrodynamics

A linear electric motor has been built to study hydrodynamic instabilities at the interface between fluids subjected to acceleration. The facility is powered by 16 independent capacitor banks to provide arbitrary acceleration profiles up to 1000 times earth’s gravity. Electrical measurements include the current, magnetic field, acceleration, and projectile trajectory. The instability is observed with flash shadowgraphy. The shot turnaround time is <15 min and over 100 shots can be taken before replacing the armature and rails.

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. >

Measurements of experimental plasma brush behavior in hybrid armatures

Hybrid armatures used in railguns consist of a conducting link which is commutated to the rails by plasma brushes. The authors describe a set of experiments in which the effects of varying plasma brush parameters including the initial mass of metal foil used to form the plasma as well as the current were studied. The evolution of the plasma brush from a solid metal foil through its melt, vapor, and plasma phase was observed. The resulting brush length and voltage drop were measured. The results of these tests are presented, discussed, and compared to numerical simulations. The experiments have confirmed that a stable plasma can be formed and confined in a hybrid armature brush chamber by magnetic pressure. The experimental results support the numerical simulations which indicate that plasma brushes can be kept short enough to enable reasonably short chambers to be used in hybrid armatures. >

Design considerations for a passive magnetic induction signal generator for sensing hypervelocity projectile passage

Measurement of projectile passage after hypervelocity launch is an important measurement and/or trigger needed for electromagnetic launcher development. One method is to use a magnetic induction technique which takes advantage of the fact that a metal object passing through a magnetic field can move the magnetic flux aside and thereby produce a voltage in a coil. This type of system can be designed to use permanent magnets and thereby be totally passive. In addition, this passive system can be designed to be insensitive to the high electromagnetic fields generated during the operation of electrothermal guns and railguns. This diagnostic has been used to trigger other electrically sensitive data acquisition equipment including flash X-rays and to determine the velocity of the projectile. This report discusses the results of tests and a comparison of data with a simple model for calculating the expected signal output of such a device. >

Materials evaluation for use as plasma brush electrodes with hybrid armatures

There is a strong motivation to develop a railgun armature which will function as a nonarc, solid contact armature throughout the complete projectile acceleration to launch process. Solid contact armatures are more efficient in converting electrical to kinetic energy and generally cause less erosion of the rail. Considerable effort is being directed towards designing armatures capable of reaching 3+km/s while avoiding transition from a solid to an arcing contact. While nearly 2 km/s has been achieved in a nonarcing contact, it may be inevitable that an arcing contact will be unavoidable during the final acceleration to full desired velocities. At present most solid armatures are fabricated from aluminum or copper. While aluminum or copper might be good choices for solid contacts, this paper introduces the possibility that different materials will better serve as electrodes in arcing contacts. >