Shyke A. Goldstein

EMET technology for rail launchers

The EMET concept is a marriage of electrothermal plasma jet technology with rail accelerators using plasma armatures. By injecting a structured plasma immediately behind a moving projectile prior to the current pulse, the plasma armature properties can be highly controlled. Parameters of interest are the armature mass and length, molecular weight, specific heat ratio gamma, and temperature. Proper control of these parameters leads to control of problems facing rail launchers such as wall ablation and viscous wall drag. In support of EMET, a Material Test Facility (MTF) has been developed for performing basic physics and materials research on hypervelocity launchers, by making direct measurements of the plasma pressure and jet velocity in a 1 cm bore. These measurements are then compared with theoretical models for various types of plasmas, in order to understand and eliminate barrel ablation. The paper discusses measurement techniques used on MTF, and the approaches being taken to develop EMET in the laboratory.

Experiments on a repetitively pulsed electrothermal thruster

This paper presents experimental results for a pulsed electrothermal (PET) thruster using water propellant. The PET thruster produces several hundred atmospheres pressure in a capillary-confined discharge, coupled to a supersonic, equilibrium flow nozzle. The thruster head has a ceramic capillary insulator and tungsten alloy electrodes, with water propellant injected axially from a central orifice. The thruster is repetitively driven at 2-10 pps by a capacitive pulse-forming network (PFN), with a discharge power of 3-6 MW and an average power of 80-630 W. The measured thrust-to-power ratio is T/P = 0.07 ±0.01 N/kW. Discharge conditions are inferred from a numerical model that predicts pressure and temperature levels of 300-500 atm and 20,000 K, respectively. The estimated exhaust velocity, with a test tank background pressure of 10-20 Torr, is 14 km/s, corresponding to a thrust efficiency of 54 ±7%.

Railgun experiments with Lexan insulators

A series of railgun experiments has been performed using Lexan insulators in both round and square bores, and with closed-breech and open-breech/injected configurations. Measured armature lengths have been roughly constant at 5-10 cm in a 1-cm bore for all Lexan insulator shots, indicating that the ablated Lexan is not swept up. Projectiles have been observed to reach peak velocity of 5.65 km/s with clean armature structures: i.e. no separated secondary arc or restrike. However, in most cases a secondary arc does occur with Lexan and limits the achievable velocity. Occasionally, stationary secondary arcs have also been observed for a particularly leaky gun assembly. The effect of insulator ablation on performance is discussed, indicating that Lexan may be useful at up to 8-10 km/s for well-sealed railguns. >

A second generation EMET railgun for secondary arc studies

Since 1985 GT-Devices has been operating a pair of railguns with lengths of 0.9 m and 3.6 m, respectively. A new second-generation railgun is now being constructed to improve straightness, stiffness, sealing, and diagnostic access. The basic design consists of a steel tube with a thin lengthwise slit forming two halves in cross section with bolt preloading. The internal structure consists of split tubular G-10 compression blocks with Glidcop AL-15 rails and polycarbonate insulators formed from 90 degree tube sections. A new 0.9 m launcher of the same design is now under construction, with a 3.6 m version to follow. An upgraded electrothermal injector has been developed using modified armature injection module (AIM) hardware. Injection velocities of 2500 m/s have been attained with 1.1 gram polycarbonate projectiles for stored bank energies of 65 kJ. Injection velocities of 3000 m/s may be possible. The design details of the new railgun, injector, and diagnostics are discussed, and some initial experimental results are presented. >

A rail gun plasma armature model

A simple model is constructed for the plasma armature of a rail gun. We make the assumption that no gun tube ablation occurs, and then calculate the barrel inner wall heating when swept by the armature. The model thus provides operating parameters for which rail guns can be fired without melting or ablation. In this ablation-free domain, formulas for turbulent hydrodynamic flow in pipes can be used as a guideline for the heat flow from the turbulent plasma armature. This model is complementary to the Los Alamos ablative model of Parker et al. in which it was assumed that the energy dissipated in the armature arc is converted to ablated wall material which enters the arc.

Physics experiments on the GEDI EMET facility

The GEDI railgun program is a broadly based physics and engineering research and development effort for high-velocity, low-projectile mass railguns. A description of the facilities and some of the experiments is given. Areas of experimental investigation include (a) lexan and ceramic insulators, (b) plasma, hybrid and transitional armatures, (c) projectiles, (d) current pulse shape, and (e) bore shape. The objective is to achieve ablation-free acceleration to high velocity. Experiments have been performed on lexan insulator guns of 3.6 m length and on ceramic insulator guns of 0.9 m length. Experiments on the 3.6 m gun have demonstrated that armatures can be successfully formed from injected plasmas and have achieved velocities of 5.1 km/s. Experiments with ceramic insulator guns have successfully demonstrated acceleration of projectiles to velocities above 2 km/s without breakage of the ceramic. >

Anode scattering effects on electron pinch radius in low‐impedance REB diodes

The effect of different anode material on the size of very intense pinches is studied from the radial structure of the bremsstrahlung from a tightly pinched relativistic electron beam. Time‐dependent data are obtained which point out the influence of scattering by the anode which electrons experience near the pinch axis. The scattering broadens the bremsstrahlung FWHM for pinches with high‐Z (80) anodes to 6 mm, compared to 4 mm for low‐Z (13) anodes. When anodes composed of low‐ and high‐Z elements in different annular regions are used, translation of backscattered electrons enhances the current density upon the low‐Z regions and decreases it upon the high‐Z regions.

Performance of a self-augmented railgun

The accelerating force of a railgun 1/2L’I2a can be increased by augmenting the self‐induced magnetic field created by the armature current. Augmentation fields can be produced by external current coils or, as is done here, by shorting the railgun muzzle, and using the gun rails as the augmentation coil. Experimental results are presented for a 3.6‐m railgun operated in this self‐augmented mode, and effective inductance gradients are achieved which are as much as 9.3 times that of the unaugmented gun. A circuit model is presented which explains features of the measured shunt current and voltage. It is concluded that self‐augmentation is an effective way to reduce ohmic heating in the armature of a railgun.

Magnetic field effects on the emission law of electron current from cathodes

The Child‐Langmuir emission law is corrected to include magnetic field effects. The treatment is performed in local Cartesian coordinates near the cathode surface and all the terms obtained include first‐order relativistic effects. The effect of space charge flowing back to the cathode is also considered. The importance of using the corrected emission laws in the simulation of relativistic diodes is discussed.

The EMET railgun projectile

The EMET projectile uses joule heating to accelerate the projectile in a railgun with a predominantly electrothermal driving force. The structure is designed to conduct armature current within a thin annular band around the shank of the large L/D dumbbell-shaped projectile. Current is initiated by a fuse located around the shank, and an impedance of 8 m Omega is achieved, compared to the 1-2 m Omega observed for electromagnetic (EM) guns. A supersonic nozzle in the projectile tail section expands and cools the armature plasma to raise its resistivity, prevent secondary arcs, and provide additional accelerating thrust. Experimental data are presented for 9.5 mm diameter, 5 gm projectiles, accelerated to nearly 600 m/s at 55 kA in a 0.9 m railgun. The armature remains confined in the projectile structure, and 75% of the acceleration is provided electrothermally. The calculated armature temperature is 3.0-3.5 eV, and significant rail burning is observed, consistent with the 5*10/sup 8/ A/m current density of the contained armature. It is concluded from a current density argument that the EMET projectile is best suited to small-bore gun applications. >