Railgun Plasma Research Articles

A time-dependent model for railgun plasma armatures

In this paper a one-dimensional, time-dependent model is described for the analysis of railgun plasma armatures. The model assumes that the armature is isothermal, and that the electrical conductivity and degree of ionization in the plasma are uniform and constant in time. The model is applied to the analysis of three problems—armature initiation, armature response to a change in current, and armature response to a change in mass. In each case, the perturbation induces damped oscillations in the armature length and projectile acceleration about the corresponding steady-state values. The period of the oscillations is, for early times, approximately equal to 3leq/a0, where leq is the equilibrium armature length corresponding to the steady-state solution and a0 is the acoustic propagation velocity in the plasma. The exponential decay time for the cases studied ranges from 140 to 200 μsec. Although most calculations are completely numerical, a partly analytic, limiting-case perturbation solution of the governing equations is also carried out. This analysis suggests why the same oscillation period is obtained in all three cases. The impact of transient phenomena on gun performance and diagnostics, as well as plans for additional analyses, are discussed.

Determination of current distribution in EM gun armature by least squares fitting of B/sub Si/ (small-induction) coil voltage

The electromagnetic (EM) plasma rail-gun current distribution was determined by fitting (using the method of least squares) the voltage induced on small induction (B/sub Si/) coils to a derived function model. The voltage function model was derived using the Biot-Savart equation. The model was derived assuming that the current flowed in sheets perpendicular to the rails. The sheets of currents varied in time and in the rail direction but were assumed to be constant (at a given time) perpendicular to the bore direction. The plasma current distribution in the bore direction, the initial length, and expansion characteristics of the plasma were determined from B/sub Si/ coil voltage measurements taken at short time increments. Fitted parameters correlate well with measurements taken by other sensors.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Soft acceleration of a metal plate by a rail gun plasma

The following situation is considered: A dense plasma armature which has been accelerated to a high velocity in a rail gun is allowed to impact a stationary metal plate. At impact, a shock wave is transmitted into the plate and a shock wave is reflected back into the plasma. For very high plasma velocities, the subsequent behavior depends primarily on the plasma density profile just prior to impact. The appropriate partial differential equations are analyzed in plane geometry and the conditions required to prevent spallation of the plate are determined. These results are compared with the "plasma only" armature plasma profiles predicted by Sloan, and it is shown that the conditions for not spalling the metal are well satisfied. The typical rail gun makes use of a low mass plasma armature to continuously accelerate a more massive solid projectile down the length of the rails. It has been suggested that substantially higher velocities and energies may be achievable with a "plasma only" rail gun-which has no solid projectile, but which has an armature mass comparable to the mass of a solid projectile. Subsequent to the acceleration, the momentum of this high velocity plasma must be transferred to a solid projectile. One way to do this is to simply allow the plasma to collide with the projectile. For this to be of interest, the projectile should not shatter and the energy transfer should be reasonably efficient. The purpose of the paper is to analyze an idealized model of such a collision.

Rail gun performance and plasma characteristics due to wall ablation

The experiment of Bauer et al. is analyzed by considering wall ablation and viscous drag in the plasma. Plasma characteristics are evaluated through a simple fluid-mechanical analysis considering only wall ablation. By equating the energy dissipated in the plasma with the radiation heat loss, the average properties of the plasma are determined as a function of time.

Physics of rail gun plasma armatures

High current rail gun plasma armatures are appropriately described by the MHD equations of motion, supplemented by radiation transport and Saha equilibrium. Procedures which allow accurate 3-D solutions to the governing equations are presented, as well as investigations of plasma-only projectile characteristics. Current research areas of concern, including turbulence effects, wall thermal loading, and magnetic diffusion processes are also discussed.

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.

Non-ideal plasma behavior of railgun arcs

A theoretical framework for modeling the characteristics of railgun plasma armatures is developed taking into account non-ideal transport properties and equation of state. The electrical resistivity of the plasma is calculated as a sum of contributions from electron-ion and electron-neutral collisions. The degree of ionization is calculated from the Saha equation with the appropriate lowering of the ionization potential and pressure calculated using non-ideal plasma theory. For electron-ion resistivity, we use the basic formulation of Khalfaoui for highly non-ideal plasma, smoothed and calibrated with respect to the results of Kurilenko & Valuev, Vorobiov et al and Kovitya. For electron-neutral resistivity, we use a formulation which allows the fitting of an infinite half-power series to experimentally determined energy spectrum of electron-neutral collision frequencies for a given atom. Finally, a steady-state magneto-gasdynamic model improved over earlier works is developed. In this model, the net Lorentz force exerting on the plasma armature is exactly accounted for.

Non-steady phenomena in railgun plasma armatures

We report some recent diagnostic measurements of the early-time behavior of a railgun plasma armature, and the development of a one-dimensional, fully time-dependent model to account for this behavior. The experimental measurments describe both the initiation of the arc and its subsequent collision with a projectile located down bore. The intent of the experimental work is to provide the appropriate initial conditions for the theoretical treatment, as well as to provide a basis for comparison with the theoretical calculations. The formalism included in the model is discussed in some detail and the results of some preliminary calculations are presented. The extent to which they support the experimental observation is discussed.

Application of Railgun Principle to High-Velocity Hydrogen Pellet Injection for Magnetic Fusion Reactor Refuelinga

This report contains three documents describing the progress made by the University of Illinois electromagnetic railgun program sponsored by the Office of Fusion Energy of the United States Department of Energy during the period from July 16, 1990 to August 16, 1991. The first document contains a brief summary of the tasks initiated, continued, or completed, the status of major tasks, and the research effort distribution, estimated and actual, during the period. The second document contains a description of the work performed on time resolved laser interferometric density measurement of the railgun plasma-arc armature. The third document is an account of research on the spectroscopic measurement of the electron density and temperature of the railgun plasma arc.

Some diagnostic interpretations from railgun plasma profile experiments

Some aspects of a railgun experimental series to investigate plasma profiles are reviewed. Certain diagnostic records clearly show plasma leakage past the projectile, and correspondence between various in-bore events and muzzle voltage. A muzzle flash detector is shown to have a useful role as a plasma diagnostic tool.

Comments on “Snowplowing in a Plasma Rail Gun”

First Page

Snowplowing in a Plasma Rail Gun

Agreement between data from a plasma rail gun as analyzed by a time-of-flight technique and predicted by snowplow theory indicates that there is no necessity for any theory of drag force in the gun and seems to indicate that one theory for drag force which also accounts for the conduction of electrical current into a moving plasma should be re-examined.

Hypervelocity impact in thin sheets and semi- infinite targets at 15km/sec

Pyrex spheres (0.15-mm diam) accelerated to a velocity of 15 km/sec by a plasma rail gun were used to produce holes in thin stainless steel and aluminum targets. The velocity was measured by time of flight, using a photomultiplier to detect the time at which a thin target was penetrated. The sphere diameter was measured by extrapolating the hole size in thin targets to a zero thickness target and arguing that that is the projectile diameter. The experimental hole diameters in thin targets were proportional to the velocity to the 0.2 power, and target thickness to the f power. Also, the 15 km/sec spheres were used against 2014 T6 and 1100 aluminum semi-infinite targets to measure the crater size and obtain an indication of the effect of strength at the above velocity.