Dennis Keefer

Experimental comparison of conventional and trans-augmented railguns

A 2.4-m-long, 1-cm-diameter round bore railgun was constructed to perform careful experiments which compared the performance of conventional and transaugmented plasma armature railguns. Conventional railgun experiments were performed over a range of currents to characterize the base performance of the railgun and to provide data for the development of ablation drag and thermal propulsive models. Experiments were then performed for a range of (separately powered) augmentation currents but with railgun currents equal to those used in the conventional experiments. The enhanced performance provided by augmentation was found to be significantly less than predicted by theory, even though the railgun operated without the formation of secondary armatures or restrikes. >

Optical diagnostics of railgun plasma armatures

A 1-cm square-bore railgun facility, dedicated to the development of diagnostic techniques and the study of plasma armatures, has been put into operation. Time-resolved, in-bore optical measurements of laser transmission and plasma spectral emission have been obtained, together with B-dot probe and piezoelectric pressure measurements. These measurements provide a means to determine the spatial profiles of armature current, pressure, and plasma emission relative to the measured projectile position and velocity. A novel method was used to deconvolve the signal from B-dot probes to obtain the current density in the arc without recourse to a priori assumptions for the mathematical form of current profiles. It was observed that plasma emission passes through a minimum near the armature current maximum, and it is suggested that insulator ablation products in the boundary layer may play a significant role in the wall radiative transfer. High-resolution, time-integrated spectra from the bore interior revealed that resonant absorption lines constitute the only significant spectral features, and that copper and aluminium each constitute approximately 10% of the armature material. >

Particle simulation of ion optics and grid erosion for two‐grid and three‐grid systemsa)

A particle simulation code has been developed to study ion optics and the effect of charge‐exchange‐induced grid erosion in electron bombardment ion thrusters. The code is based on the particle‐in‐cell method to simulate the beam extraction and ion optics and a Monte Carlo method to calculate the charge exchange collisions between fast beam ions and neutral particles. The space charge effects for both fast beam ions and slow charge exchange ions are included. Both two‐ and three‐dimensional versions of this code have been developed for both two‐grid and three‐grid systems. The two‐dimensional model provides a qualitative description of the details of the ion flow and the charge exchange collisions. The three‐dimensional version has successfully predicted the experimentally observed erosion pattern in which the maximum erosion occurs at the geometric centers of accelerator grid apertures.

3-D plasma armature railgun simulations

A three-dimensional Navier-Stokes code has been extended to provide a simulation for the railgun plasma armature. This code was previously used with an approximate electromagnetic model to provide a two-dimensional simulation of a railgun plasma armature flow in the plane containing the insulators. A new three-dimensional electromagnetic solver has been incorporated into the code to permit full 3-D nonsteady MHD simulations of the plasma armature flow in a railgun. The finite-difference equations for magnetic vector potential and electric potential are solved using the ICCG method. The new electromagnetic solver was validated using 3-D solutions obtained from the finite element electromagnetic code MEGA. A full bore, 3-D simulation of a plasma armature reveals flow patterns significantly different than those of rail or insulator plane 2-D simulations. In particular, the maximum J/spl times/B force occurs off-axis and results in a plasma flow away from the projectile along the axis of symmetry. A zone of high shear flow forms near the rail surfaces which increases the viscous losses. Zones of low current, nearly stagnant flow form near the base of the projectile, consistent with experimental observations of a buffer between the armature and the projectile. >

In-bore spectral measurements in a plasma-armature railgun

Quartz optical fibers have been used to obtain quantitative, time-integrated, high-resolution, in-bore spectra from a plasma-armature railgun. The observed spectrum consists primarily of continuum radiation, and the only significant spectral features are resonant absorption lines. A detailed plasma-radiation model was used to predict the spectrum in the region of the absorption lines, providing an estimate for the concentration of aluminum and copper in the plasma. This model was also used to estimate the temperature profile of the armature. A reasonable agreement between the model and the observed spectrum yielded a concentration of 10% aluminum and 10% copper in the armature, with a centerline plasma temperature of 20000 K. >

Railgun hybrid armatures, experimental results and performance characteristics

Six hybrid armature designs were evaluated in the UTSI one-centimeter square-bore railgun. Advanced diagnostic instrumentation was used to determine the characteristics of hybrid armatures which include a compound armature (current in plasma brushes to the metal armature followed by a plasma armature). The hybrid armatures were compact, with shorter current distributions than plasma armatures as measured with both optical and electromagnetic probes. Although the results are preliminary, due to the limited geometries investigated, the results are very encouraging. The short (less than one-bore-diameter) hybrids demonstrated efficiencies equal to or greater than plasma armatures and good armature stability. >

Solid ring armature experiments in a transaugmented railgun

The UTSI 2.4 m long, 11 mm diameter, transaugmented (separately augmented) railgun was used to accelerate solid aluminum ring armatures to velocities up to 3 km/s. The armature consisted of a 15 mm long aluminum tube with a tapered nose and wall thickness of 1 mm. The 1.2 to 1.3 g armatures were tested in a series of both conventional and augmented experiments to evaluate performance, transition velocity and transition action. Precise transition location was determined by correlation of muzzle voltage and the armature light emission measured by a PIN diode looking down the muzzle. The electromagnetic forces in this armature were determined using the MEGA 3-D finite element code. The ring armature produces a magnetic squeeze in the insulator plane which, because of the high compliance of the thin ring, loads the ring against the rails to increase the contact force. Augmentation increases the contact force and increases the transition velocity. This armature configuration may have application in all velocity ranges since it appears to operate as a true hybrid after transition with no plasma armature formation. >

Arc motion in railgun plasma armatures

H.H. Maeckers theory (1971) for arc motion and displacement is extended to the pressure-dependent arc conditions found in railgun armatures. It is shown that for the railgun plasma armature there are thermal mechanisms, which do not exist for solid armatures, that tend to move the peak of the current density forward toward the base of the projectile, thereby mitigating the tendency of the velocity skin effect to concentrate the current density toward the rear. This effect may explain the variance between some theoretically predicted current density profiles in plasma armatures and those measured by B-dot probes. >

Simultaneous in-bore rail and insulator spectra from a railgun plasma armature

Absolute spectral radiance measurements have been obtained simultaneously at the rail and insulator surfaces of a 1 cm square-bore railgun. The emission spectra were obtained through the use of quartz optical fibers which penetrated both the rail and the insulator walls. The spectral characteristics of the rail and insulator emission are quite similar, but differ significantly in magnitude. A detailed plasma radiation model was used to analyze these spectra. In order to obtain reasonable agreement between the model predictions and the experimental spectra, it was necessary to assume that there existed a broadband absorbing layer at the insulator surface. This result suggests a new physical model of the plasma armature in which insulator ablation leads to significant three-dimensional flow and affects the shape of the current emission pattern on the rail surfaces. >

Velocity limiting magnetohydrodynamic effects in railgun plasma armatures

The results of a four-year theoretical and experimental research program in railgun armature physics at the University of Tennessee Space Institute are reviewed with an emphasis on understanding how the magnetohydrodynamic (MHD) flowfield and electromagnetic interactions limit the velocity in plasma and hybrid armature railguns. The understanding of the 3D plasma armature flowfield has been developed through a combination of highly instrumented railgun experiments, secondary arc performance models, and 2D MHD calculations. These 2D calculations are only qualitative in nature, but they provide insight into the complex armature flow and indicate that the formation of secondary armatures, ablation drag, and primary armature preparation are interrelated. Experimental evidence suggests that 3D MHD flow effects are very important in controlling these phenomena. >