The DES raligun facility at CEM-UT

The Center for Electromechanics at The University of Texas at Austin (CEM-UT) has constructed a facility for the operation of electromagnetic (EM) launcher experiments. The facility was specifically designed to investigate distributed-energy-store (DES) railguns. Experiments conducted in the facility have demonstrated the DES railgun concept using a 1-m long, four-stage DES railgun. Investigations have begun on a 4-m, ten-stage DES railgun to demonstrate operation of such a system at higher projectile velocities. The capabilities and design of the major components of the facility are described. Also presented is a review of the experimental development of the railgun system. The DES railgun facility is a versatile laboratory test bed facility for EM acceleration experiments.     

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.     

Hypervelocity impact experiments in surrogate materials

A requirement to perform experiments for hypervelocity impacts in substitute materials arises from the need for data at velocity ranges inaccessible in the laboratory. The role of melt and vapor in hypervelocity shielding designs cannot be assessed with the maximum velocity of approximately 8 km/sec that is achievable. As a consequence, there is interest in performing experiments in materials where the melt and vapor regimes occur at lower velocities.

Such surrogate experiments can in principle be exact. A process in a surrogate material will be dynamically similar at one in the material of interest if the constitutive equations are the same to within three arbitray scale factors given by the ratios of the natural mass densities, sound speeds and viscosities. The experiments can ben performed at scaled size, velocity and time .

Cadmium and zinc are considered as candidates to substitute for aluminum to allow velocity scaling. Their thermodynamic equations of state are constructed from existing data and the ANEOS analytical equation of state model. They are recast in a scaled form in which the satisfaction of the scalign requiremens can be assessed. Reasonable matching is attained. Testing in cadmium at a velocity of 6 km/sec is approximately dynamically similar to experiments in aluminum at 18.6 km/sec, a velocity factor of 3.1.     

Hypervelocity impact simulation experiments on LDEF°-foils

A variety of space environmental effects can be studied on many experiments having been exposed on the LDEF-satellite.

Among others the thermal blankets of the Ultra-Heavy Cosmic Ray Nuclei Experiment (“UHCRE”, Exp. A0178) displayed many micrometeoroid / space debris impact features.

In an effort to understand their nature and characteristics, an experimental impact simulation program has been carried out.

UHCRE-spare foils have been impacted by glass, aluminium, and iron projectiles with masses ranging from about 30 nanograms up to several milligrams. Impact velocities range between about 3 km/s and 13 km/s.

Characteristic impact craters and perforation holes have been produced. Their sizes and morphologies have been related with respective projectile impact parameters.

“Halo zones” around perforation holes, as they had been observed in the exposed LDEF-foils, have also been obtained experimentally. They were found to be delamination effects within the foil layers caused by the propagation of impact shock waves.     

Hypervelocity acceleration and impact experiments with the LLNL electric guns

In the electric gun, the explosion of an electrically heated metal foil and the accompanying magnetic forces are used to accelerate a thin flyer plate to velocities as high as 18 km/s. Here we report preliminary results of a study to extend this capability to the acceleration of projectiles or particles ranging in size from micrometeorites to chunky projectiles with a mass as high as 0.5 g. We also have started code calculations of projectile impacts on thick aluminum witness plates for comparison with observations of experimentally produced cratering.     

Microparticle impacts at ultra-high velocities: Their relation to macroparticle impacts

In recent years the Hypervelocity Microparticle Impact (HMI) project at Los Alamos has utilized electrostatically accelerated iron spheres of microscopic dimensions to generate ultra-high velocity impact experiments to about 100 km/sec, about an order of magnitude beyond the data range for precisely controlled impact tests with ordinary macroscopic projectiles. But the extreme smallness of the micro impact events brings into question whether the usual shock-hydrodynamic size scaling can be assumed. It is to this question of the validity of size scaling (and its refinement) that the present study is directed.

Impact experiments are compared in which two comparable impact events at a given velocity, a microscopic impact and a macroscopic impact, are essentially identical except that the projectile masses and crater volumes differ by nearly 12 orders of magnitude—linear dimensions and times differing by 4 orders of magnitude. Strain rates at corresponding points in the deforming crater increase 4 orders of magnitude with the size reduction. Departures from exact scaling, by a factor of 3.7 in crater volume, are observed for cooper targets—with the micro craters being smaller than scaling would predict. This is attributed to a factor 4.7 higher effective yield stress occurring in the micro cratering flow. This, in turn, is because the strain rate there is about 10⁸/sec as compared to a strain rate of only 10⁴/sec in the macro impact.

The measurement of impact craters for very small impact events leads to the determination of metal yield stresses at strain rates an order of magnitude greater than have been obtained by other methods. The determination of material strengths at these exceedingly high strain rates is of obvious fundamental importance. Results are compared to recent theoretical models by Follansbee, Kochs and Rollett.

Finally, the problem is addressed of predicting crater sizes in a target material with strain rate effects. First some basic results are recalled pertaining to the late stage equivalence of hypervelocity impacts. It is then seen, for a strain rate dependent material, that the curve of dimensionless crater volume versus impact velocity is replaced by a family of curves, each member of which is for one final crater size. The spacing of the curves is determined by the stress versus strain properties of the material.     

Hypervelocity crater formation in aluminum alloys at low temperatures

The penetration and perforation of three kinds of aluminum alloys at room temperatures and low temperatures in the velocity range from about 0.5 to about 3.7 km/s were investigated experimentally. Main interests were focused on the depth and diameter of craters and their relations to the impact velocity. As a result, very distinctive features of the temperature effect on the shape and size of craters were found. Also, the effect of impact-induced phase transition of projectiles on the crater formation was examined about carbon steel, aluminum alloy and NaCl projectiles.     

Hypervelocity penetration of tungsten alloy rods into ceramic tiles: experiments and 2-D simulations

A series of terminal ballistics experiments and 2-D simulations, with small scale tungsten alloy penetrators, was performed in order to quantify the ballistic efficiency of confined ceramic tiles. The data includes both depth of penetration (DOP), into thick steel backing and X-ray shadowgraphs during the penetration process. Impact velocities ranged between 1.25 to 3.0 km/s. The size of the tiles varied in order to check their performance as a function of thickness and lateral dimensions. We found that the differential ballistic efficiency of alumina tiles is practically independent on impact velocity and tile thickness, within the ranges of velocity and thicknesses, investigated here. A detailed simulation study, using the Eulerian processor of the PISCES 2-D ELK code, was performed in order to better understand the interaction between long-rods and ceramic tiles, and particularly, to adjust a proper failure criterion to the tiles. We found that a simple version of the Johnson-Holmquist model, with a single parameter, is fairly adequate to account for most of the data. These include: lateral confinement, tile thickness and impact velocity influence on the penetration depth. We used the code to further investigate the influence of lateral dimensions on tile performance.     

Hypervelocity impacts on solar cells — observations, experiments, and empirical scaling laws

The recent retrieval of the EURECA satellite and one solar array from the Hubble Space Telescope initiated meteoroid and debris impact investigations on these surfaces. This paper gives an overview of observations of impact sites found on retrieved spacecraft surfaces, hypervelocity test programs performed as part of the European Space Agency (ESA) post-flight investigation programs (PFIP), and develops empirical scaling laws for various geometric parameters. These empirical scaling laws are intended to relate particle parameters to crater dimensions to support the modeling of the microparticle environment in near-Earth space.     

Hypervelocity launch dynamics

The launch dynamics from a generic hypervelocity cannon are considered over the velocity range from 1500 through 3500 m/s. Both fin and flare stabilized projectiles are examined for the influence of in-bore dynamics, muzzle blast, sabot discard, and free flight aerodynamics upon their trajectory. Computations are performed using codes developed and validated for conventional cannon with ordnance muzzle velocities (approximately 1700 m/s). While the extension into the hypervelocity regime is not supported by experimental data, this initial study is intended to aid in defining potential problem areas.     

The CEM-UT rapid-fire compulsator railgun system-recent performanceand development milestones

Twenty-seven compulsator-powered railgun experiments have been performed, including a 1.0 MJ discharge at 3510 r/min. In this test, a 724 kA current pulse accelerated an 80 g, aluminum armature to 2.05 km/s, thus exceeding the projectile velocity goal at 73%-rated machine speed. Furthermore, operation with a single gun barrel has been achieved using a parallel path, solid-state closing switch to deliver 132 kA to the railgun injector. The latest data are presented from the rapid-fire compulsator railgun facility. Included is a discussion of the energy transfer, power output, and system efficiency during a 1.0 MJ discharge. Also shown are the injector current, voltage, and di/dt curves for this test which were used in the design of the solid-state closing switch. Results of railgun experiments using the solid-state switch are analyzed     

Microparticle acceleration for hypervelocity experiments by A 3.75MV van de Graaff accelerator and a 100KV electrostatic accelerator in Japan

In-situ dust detectors have been calibrated by dust electrostatic accelerators that can accelerate projectiles to expected mass and velocity ranges of space debris and micrometeoroids. Unfortunately, In Japan, there was no such a facility dedicated to space science research until our research group was established a few years ago. Therefore, we have developed two high voltage accelerators. One is a modified 3.75MV Van de Graaff accelerator operated by High Fluence Irradiation Facility, Research Center for Nuclear Science and Technology, the University of Tokyo (HIT), and the other is a 100kV accelerator dedicated to dust experiment at the Institute of Space and Astronautical Science (ISAS). The particle velocity using the HIT Van de Graaff accelerator is higher than those reported in other accelerator facilities under the same particle mass conditions and encompasses the desired velocity range of micro-meteoroid. Time-Of-Flight dust mass spectrometer and Hybrid dust detector which are under development in Japan have been investigated using HIT dust accelerators. We have also constructed a 100kV electrostatic accelerator designed for easier handling and lower cost operation which is dedicated to dust acceleration, because the HIT Van de Graaff accelerator is being used for ion beam experiments mainly.     

Hypervelocity jacketed penetrators

The use of steel jackets was found to significantly improve the penetration efficiency of tungsten alloy rods. Experiments and analyses were conducted with L/D=10 projectiles of constant exterior dimensions at a nominal impact velocity of 2.2 km/s. The fraction of jacket material was varied to see which geometry would have the best performance. For a core-to-jacket diameter ratio (μ) of 0.6, the experiments showed the penetration efficiency (P/KE^1/3) increased by 21% relative to an all-tungsten baseline rod of the same exterior dimensions. Experiments and AUTODYN simulations showed the same penetration efficiency trends. The simulations, however, did not show that the tungsten core outran the jacket, contrary to what was observed in the experiments.     

Hypervelocity penetration of concrete

Experiments have been conducted with 6.25 mm diameter tungsten rods striking concrete at 2.2 km/s. Three concretes were used—one was 2.35 g/cm³ and the other two were 2.27 g/cm³. The erosion rates were measured to be T/ΔL = 2.4–3.1 depending on the density of the concrete. This is greater than the hydrodynamic value, which shows that the strength of the penetrator is affecting the penetration. The cratering efficiency was computed (which included surface spall) and was found to be commensurate with the strength of the concrete, 28–34 MPa. CTH calculations were conducted using the brittle fracture kinetics (BFK) and Holmquist–Johnson–Cook (HJC) material models for concrete. Density in the calculations was 2.25 g/cm³. It was not possible to match erosion rates at 2.2 km/s, which were too high in the calculations. Also, computed crater volumes were much too small, mainly due to spall in the experiments that was not shown in the computations. Another significant inaccuracy of the calculations was the damage extent, which became unrealistically widespread as time increased in the BFK model.     

Hypervelocity impact tests on coated thermoplastic films at cryogenic and elevated temperatures

The hypervelocity impact facility at Space Research Institute (SRI), Auburn University has recently completed a series of tests on coated thermoplastic films at cryogenic (40 K) and elevated temperatures (420 K). With a 1-in gap, two films were mounted in a frame that applied biaxial tension to each film. The materials were impacted with 40–100 μm soda lime spheres utilizing a plasma drag gun to accelerate the particles to velocities between 5 and 12 km/s. The facility diagnostics allow for the determination each particle's, size, velocity and impact location along with micrographs of the nature of the impact damage. This summary of the test includes a general overview of the nature of damage on the films along with representative impact micrographs of the impact sites.     

Formation of macroparticle charge in a classical Coulomb plasma

An expression is derived for the average macroparticle charge and the electron temperature in a heated dense gas. Pis’ma Zh. Tekh. Fiz. 25, 52–55 (January 12, 1999)     

Formation of macroparticle structures in an rf induction discharge plasma

It was demonstrated experimentally that macroparticles may undergo levitation and form ordered structures in an rf induction discharge plasma. The experiments were carried out using 1.87 μm melamine formaldehyde particles in neon at a pressure of 25–500 Pa. The generator frequency was 100 MHz. Pis’ma Zh. Tekh. Fiz. 24, 62–68 (October 12, 1998)     

Individual monitoring for external radiation at accelerator facilities

Individual monitoring at accelerator facilities is discussed, within the framework set out by the International Commission on Radiological Protection and with reference to the implementation of the recommendations of that body within the European Basic Safety Standards. Legislation in other parts of the world may differ, but a worldwide perspective on this subject would be too exhaustive. The fields at accelerator facilities are contrasted in terms of particle type and energy with those encountered at more conventional sites within the nuclear fuel cycle, medical applications and general industry. The implications for individual monitoring are discussed in relation to the dose quantities for these accelerator fields and also with respect to the personal dosemeters options.