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The Institute for Advanced Technology (IAT) is developing a plasma-driven railgun to launch low-mass projectiles of 5-10 g to a velocity in excess of 7 km/s. Accomplishing this goal requires overcoming the problem of bore ablation, which has been linked to an observed velocity ceiling of about 6 km/s in plasma-armature launchers. Bore ablation is a direct consequence of the intense heat radiated by plasma armatures. Controlling bore ablation requires a coordinated approach that includes magnetic augmentation to reduce power dissipation in the plasma, high-purity alumina insulators to raise the ablation resistance of the bore, and pre-acceleration to prevent ablation of the bore materials at low velocity. This paper describes the consequences of excessive bore ablation, the rationale for the IAT experiment, and results obtained from the hardware that has been designed and tested to date. Index Terms--Electromagnetic launch, hypervelocity, plasma, railgun.

Magnetized target fusion (MTF) is a means to compress plasmas to fusion conditions that uses magnetic fields to greatly reduce electron thermal conduction, thereby greatly reducing compression power density requirements. The compression is achieved by imploding the boundary, a metal shell. This effort pursues formation of the field-reversed configuration (FRC) type of magnetized plasma, and implosion of the metal shell by means of magnetic pressure from a high current flowing through the shell. We reported previously on experiments demonstrating that we can use magnetic pressure from high current capacitor discharges to implode long cylindrical metal shells (liners) with size, symmetry, implosion velocity, and overall performance suitable for compression of FRCs. We also presented considerations of using deformable liner-electrode contacts of Z-pinch geometry liners or theta pinch-driven liners, in order to have axial access to inject FRCs and to have axial diagnostic access. Since then, we have experimentally implemented the Z-pinch discharge driven deformable liner-electrode contact, obtained full axial coverage radiography of such a liner implosion, and obtained 2 112 dimensional MHD simulations for a variety of profiled thickness long cylindrical liners. The radiographic results indicate that at least 16 times radial compression of the inner surface of a 0.11-cm-thick AI liner was achieved, with a symmetric implosion, free of instability growth in the plane of the symmetry axis. We have also made progress in combining 21/2-D MHD simulations of FRC formation with imploding liner compression of FRCs. These indicate that capture of the injected FRC by the imploding liner can be achieved with suitable relative timing of the FRC formation and liner implosion discharges. Index Terms--Field-reversed configuration, liner implosion, magnetized plasma compression, magnetized target fusion.

The 'magnetic obturator' is an armature concept developed by IAP Research in the late 1980s. Advantages of the magnetic obturator include lower mass than conventional 'C-type' railgun armatures (approaching the theoretical minimum mass), compatibility with mid-drive sabots, and the potential for nontransitioning operation. The Institute for Advanced Technology (IAT) at The University of Texas at Austin began testing magnetic obturators in 1997. Our tests showed that magnetic obturators can operate very well, but not yet as reliably as C-type armatures. This paper discusses experiments conducted to understand and resolve several recurrent problems that include difficulties associated with start-up, high contact pressures leading to local yielding of copper rails, and susceptibility to develop intermittent arcing contact before transition. This paper reports our progress in addressing these three areas and presents recommendations for further research. Index Terms--Armature, railgun, transition, wear.

Hypervelocity gouging is a form of damage that can occur to surfaces in sliding contact at high relative velocity. Gouges, which are in the form of teardrop-shaped craters, have been observed on rocket sled tracks, in light gas gun barrels, and in the bore of railguns. One aspect of gouging that has not been adequately explained is the existence of a minimum velocity (or threshold velocity) for a given material pair below which gouging does not occur. This paper reports a series of experiments to test the hypothesis that the onset of gouging is governed by the hardness of the harder material and by the density and sound speed of both materials. In the tests, samples of various metals were accelerated to 2.2 km/s while in direct sliding contact with CD110 copper rails. The samples were carried in a Lexan polycarbonate forebody, modified to apply normal loads of 40 - 80 MPa to the sample/rail interface. The portion of the armature directly in line with the samples was cut away to avoid contaminating the gouge track. Visual inspection of the resulting gouges was used to establish a gouging threshold for each metal. The tests were conducted in the 40-mm square bore electromagnetic launcher at the Institute for Advanced Technology (IAT). Metals tested include AISI 1015 steel, silver, molybdenum, pure copper, tangsten, nickel, magnesium, and 7075 aluminum alloy. The results of the experiments show the existence of a straight line fit between hardness of the harder material and the shock pressure for a normal collision at the gouging threshold velocity. Index Terms - damage, gouging threshold velocity, high-speed sliding contact, hypervelocity gouging, railgun, wear.

Previous efforts to obtain information from onboard sensors involved writing the data to an EPROM located in the launch package, performing a 'soft' catch of the projectile, and recovering the chip in order to download the data. This method requires soft catch capability and has been limited to velocities of less than 1 km/s. We have developed a method to measure in-bore quantities on board a railgun launch package and transmit the information out of the barrel in real time using an optical data link. The onboard transmitter, which is mounted on the front end of the launch package, is either a repackaged laser diode or a high intensity LED. During launch, light pulses are transmitted down the bore, reflected off a mirror just outside the muzzle, and received with a telescope and attached photomultiplier tube. Successful operation has been achieved for accelerations up to 105 kgees and velocities up to 2.0 km/s. An analog encoded version of the system has been built and used to perform an armature voltage measurement. Index Terms - g-hardened electronics, high-g acceleration, in-bore diagnostics, optical telemetry, railguns.

In 1994, Dreizin and Barber [1] presented an analysis of the muzzle voltage developed in a railgun with a solid contact armature. They showed that there is a significant contribution to the measured muzzle voltage arising from the current flow in the rails ahead of the armature. They further argued that this voltage is influenced by velocity skin effect and that it may become the dominant effect on muzzle voltage at high velocity. The results of that paper, although on solid theoretical foundations, generated some controversy because many previous investigations of armature behavior had obtained good agreement between theory and observation without taking this term into account. In this paper, a novel experimental technique is described that permits direct observation of the effect of precursor rail current on muzzle voltage. The observed magnitude of the voltage is in agreement with the estimates provided by Dreizin and Barber, including the contribution of velocity skin effect. Index Terms - experimental measurement, muzzle voltage, railgun, velocity skin effect.

The effect of a close-fitting laminated containment structure on propulsive L[prime] is studied both experimentally and computationally. It is shown that the interaction of axial magnetic fields near the armature with the laminations transfers a fraction of the propulsive force to the containment. This reduces the propulsive L[prime] without causing significant reduction in the rail L[prime]. The effect is very sensitive to the spacing between armature and containment. The loss of propulsive force may be as large as 20 to 25% for very close spacing. Index Terms - inductance gradient, laminations, railgun, reduced performance.

Achieving long rail life may require the use of a thin surface cladding of hard, refractory material. This paper reports numerical simulations of the effect of a resistive cladding on railgun efficiency. Simulations are performed using a three-dimensional (3-D) finite element code, Electro-Mechanical Analyses Program in 3 Dimensions (EMAP3D), that is capable of tracking moving conductors. Current and temperature distributions are calculated in two regions of interest: i) behind the armature - where current diffuses into the rail i n a two-dimensional manner, and ii) in the portion o f the rail adjacent to and under the armature - where current flow is fully three dimensional. The railgun model used for these simulations has a 40-mm square bore with a 1-mm thick cladding on the rail surface. The increase in electrical loss due to the cladding is a function of axial location and is largest where the armature velocity is low. We find that combined 2-D and 3-D effects cause a total additional thermal loss equal to about 5% of the muzzle energy when the cladding resistivity is 1 0 or 50 [micro][ohms][center dot]cm. The decrease in total launcher efficiency is less than 1%. Index Terms - cladding, finite element analysis (FEA), loss, railgun.

Railguns that do not ablate material into the bore may be subject to another force loss, electrical breakdown of the gas used to pre-accelerate the projectile. A 1-D calculation of this process for helium shows a 42% reduction in the driving force at 10 km/s. When two dimensional effects are taken into account the loss may be much greater.

Tests were conducted whereby a kinetic energy penetrator was launched from an electromagnetic launcher. The penetrator was fabricated from a tungsten alloy. An armature, which supported the penetrator during acceleration and conducted the rail current, was fabricated from 7075-T6 aluminum. The transition of the armature contacts from a low voltage to a high voltage was identified as the mechanism that caused the aft section of the subprojectile to fail. Transition occurred fairly consistently at an average velocity of 2050 m/s. Removal of armature material at the rod-armature interface increased the survivability of the subprojectile and was demonstrated with a launch package mass of 308.5 gm, yielding a useful mass fraction of 49%. Structural integrity of the subprojectile at a launch velocity of 2350 m/s was verified with an orthogonal flash X-ray image at the muzzle. Index Terms--Armature, railgun, sabot, tungsten penetrator.

Atlas is a high-energy, pulsed-power facility under development to study materials properties and hydrodynamics under extreme conditions. Atlas will implode heavy liner loads (m ~ 45 gm) with a peak current of 27-32 MA delivered in 4 [micro] s, and it is energized by 96, 240-kV Marx generators storing a total of 23 MJ. The power-flow system transports current from the Marx generators to the load. It includes a load protection switch to protect the load in case of a prefire; 24 oil-insulated, vertically oriented, radially converging, triplate transmission lines; a transition region that couples the transmission lines to the power-flow channel; and a radially converging, dielectric-insulated, horizontal and conical, power-flow channel. Proposed experiments, the design of the power-flow system, test results, and status are presented. Index Terms--Material testing, plasmas, pulse power systems, transmission lines.

Presents an experiment concerning magnetic obturators, an armature concept developed by IAP Research in Dayton, Ohio in the late 1980s. Features of the obturator; Limitations of the obturator; Analysis of starting an obturator without developing arcing contact.