Railgun Plasma Research Articles

Spectroscopic diagnostics of railgun plasma armatures

The Railgun Diagnostics Group of the Applied Physical Electronics Research Center at the University of Texas at Arlington has been investigating railgun plasma armatures in a variety of operating conditions and developing new techniques and a database for such studies. The authors discuss the basic assumptions and their limitations in using plasma spectroscopy to study the conditions in railgun plasma armatures and describe their database for such spectroscopic studies. They also described a multi-input space-resolved spectroscopy system developed for the efficient study of railgun plasma armatures. In addition, they present the preliminary results for RGS-II. By using a novel spectroscopic technique, they have measured a plasma temperature of 1.2*10/sup 4/ K and an electron density of 3.7*10/sup 16/ cm/sup -3/ in the railgun plasma armature.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

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. >

Effect of a transient current profile on the dynamics of railgun arcs

A one-dimensional, time-dependent model for describing transient behavior in railgun plasma armatures is used to investigate the effects produced by a time-varying current profile. Ablation from the rail surface is also accounted for. The governing equations are solved numerically to produce as a function of time and space the electrodynamic and thermodynamic properties of the arc. These quantities are shown to be considerably different from those predicted by commonly used quasi-static models. The importance of these transient effects and how they can affect the design of projectiles and the analysis of diagnostic data are discussed. >

Effect of current variation on the dynamics of railgun plasma armatures

The effect of current variations on the behavior of railgun plasma armatures is investigated with a one‐dimensional, time‐dependent model. Three typical current profiles are considered: two inductive profiles, one with a time constant of 0.5 msec and the other with a time constant of 2.0 msec, and a capacitive profile. All three calculations are for an armature in a 5 cm square‐bore railgun. It is observed that current variations create oscillations in the armature length, projectile acceleration, and projectile jerk. In addition, the current profile significantly affects the axial distribution of the plasma velocity, current density, and total pressure. The significance of these transients on the design of projectiles and the interpretation of diagnostic data is discussed, and suggestions for future calculations are outlined.

Determining railgun plasma current distribution using Jansson's method to deconvolve B-dot probe signals

The authors present a method of determining the plasma armature current distribution from B-dot probe signals. This method utilizes the knowledge that the B-dot probe signal can be shown to be a convolution of the plasma armature current distribution with the impulse response function of the B-dot probe. Jansson's (1984) relaxation-based nonlinear deconvolution technique is developed for use with recorded B-dot probe data. Results obtained with this technique indicate that significant improvement in estimations can be obtained over previously used linear deconvolution methods. >

The internal structure and dynamics of the railgun plasma armature between infinitely wide ablating rails

Computer simulations of the plasma flow in two-dimensionally symmetric railgun plasma arcs were performed. The direction of symmetry is normal to the insulator surface, so that the rails are effectively infinite in width. The rail surface ablates according to one of two ablation models, in which either all absorbed energy flux, or only the excess over that which the rail material can conduct away, ablates mass. A number of combinations of initial conditions, boundary conditions, and resistivity models were explored. The full ablation model produces an arc of continuously growing mass and length in which the current distributions reaches from the projectile halfway to the breech. The conduction-limited ablation model produces a compact arc approximately eight times the bore height in length, which ceases to ablate material from the rails before the projectile reaches a velocity of 1 km/s. There is need for further study in several areas. These include the arc initiation process, the ablation of the insulators, and three-dimensional effects.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Stiff railguns

Stiff guns have been operated with both plasma and solid armatures. A performance gain was seen in the plasma railgun as stiffness was increased. A stiff gun will help to maintain the bore shape and preserve the integrity of the seam between rail and insulator under the extreme asymmetric loads sustained during high-pressure operation. The hydraulically preloaded moly and ceramic gun has been fired six times at pressures as high as 87 ksi, and the bore still holds roughing vacuum up to two hours after the test. The elimination of seam leakage helps control bore erosion associated with plasma reconstitution from the rail and plasma perturbation that might result in loss-initiating instabilities. Reduced rail deflection allows solid and transitioning armatures to track the bore surface. Similar to all plasma armature operation, the stiff gun seals the plasma associated with a transitioning solid armature. The efficiency of transitioning solid armatures increases in a stiff railgun. An analysis of the strain energy associated with the deflection of the railgun structure is presented, and this mechanism is found to be a small fraction of the energy associated with armature loss and the rail resistive loss. The design and scale testing of the next generation of stiff railguns is underway, with the emphasis on reduced weight for field-portability and space-traceability requirements.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

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.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Two-dimensional heat conducting simulation of plasma armatures (for railguns)

Development of a two-dimensional magnetohydrodynamic (MHD) code to simulate internal motions in a railgun plasma armature is discussed. The authors use the equations of resistive MHD, with ohmic heating, and radiation heat transport. A flux corrected transport code is used to advance all quantities in time. The runs show the development of complex flow, subsequent shedding of secondary arcs, and a drop in the acceleration of the armature. >

A more general model for determination of plasma armature current distribution from B-dot voltage by method of least squares

A more general and precise model for the B-dot coil voltage induced by the railgun plasma armature current has been derived and used to obtain armature current distributions. Previously, the voltage model assumed the current elements in the armature were no closer than a bore diameter from the coil. A solution for the coil voltage has been found which eliminates this approximation. In addition, the model has been generalized to include multiple armature currents in any direction perpendicular to the rails, and nonuniform current across the bore. The model is used with least squares reduction, as in previous work. Using this model, current distributions were determined for the square-bore and the MARC IV (four perpendicular armatures) railguns. The results of these reductions are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Cross-correlation-based method for determining the position and velocity of a railgun plasma armature from B-dot probe signals

A method of determining the position and velocity of a plasma armature from B-dot probe signals is presented. It utilizes the physical processes governing the plasma armature current density to model the leading edge of the armature as a unit step function. This model of the current profile is convolved with the impulse response of the probe and compared with B-dot probe data via a local-area cross-correlation (LACC) coefficient. Two-parameter iteration determines the time of passage and velocity of the leading edge of the armature as the values for which the correlation is maximized. Results obtained using this technique indicate that significant improvement in accuracy for time of passage estimates can be obtained over previous ad hoc methods. Velocity estimates from individual B-dot probe signals can also be obtained.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Experimental investigation of a hydrogen plasma railgun

The plasma velocity and temperature and composition distributions in a hydrogen plasma railgun were measured. Typical velocities near the muzzle were approximately 95 km/s at an initial capacitor bank stored energy of 4.1 kJ. Temperatures ranged from a peak of approximately 24000 K in the current-carrying plasma to approximately 8500 K in the tail. The current-carrying plasma was composed of roughly equal parts of hydrogen and copper on a mass basis, while the tail was primarily copper. Computer modeling of armature B-dot probe signals has yielded a simple interpretation of the signal. >

Experimental testing of least-square procedure for determining railgun plasma current distribution

The plasma current distribution (initial length, velocity, amplitude versus x, and expansion rate) was determined by simultaneously fitting (using the method of least squares) the measured voltages of multiple B-dot probes to a function model. The model for the probe voltage was derived using the Biot-Savart equation and some simplifying assumptions, one of which was to divide the current into segments and then sum for the total voltage. The computer model contains selectable form functions which give the amplitude of the current for each segment. The procedure was tested by comparing multiple fits of symmetrically positioned probes on opposite sides of the bore, comparing fits using different form functions and different numbers of segments, comparing fits of different-size probes located at different distances from the bore, and comparing the fitted parameters of the plasma current distribution with other sensors. As far as could be determined, the current distribution determination gives meaningful data which correlate well with other measurements. >

Arc motion in railgun plasma armatures

H.H. Maecker's 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.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Two-dimensional MHD simulation of isothermal plasma armatures

The authors report results on the use of a two-dimensional resistive MHD code to simulate the internal dynamics in a railgun plasma armature, starting from a slightly perturbed equilibrium initial state. The plasma temperature, conductivity, and ionization fractions are treated as uniform in space and constant in time. The rear of the plasma is in contact with a low-pressure nonconducting gas. A finite-difference, explicit, Eulerian, flux-corrected transport code is used to advance all quantities in time. The computation grid has 200 cells parallel to the rails, and 20 across the rails. The results show the growth and subsequent shedding of the plasma mass toward the nonconducting region at the rear. The mass lost is not replenished and the armature becomes shorter, with steeper pressure and magnetic field profiles. The bulk of the profiles is not thoroughly disrupted, but behaves rather sturdily and seems to maintain a shape fairly close to the equilibrium profiles.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Equilibrium lengths of railgun plasma armatures

Recent data have shown that the length of railgun plasma armatures increases with bore size. The mechanisms for plasma growth and mass loss are treated analytically through a simple thermodynamic model in which mass is added through ablation of bore materials and lost through friction. It is assumed that at the bore wall, a thin boundary layer is formed in the plasma in which the velocity is sufficiently slow that the boundary layer is left behind the bulk of the plasma. Using the scaling relations of J.D. Powell and J. Batteh (1983), a functional relationship between the plasma length l/sub p/ and bore height h of the form l/sub p/=kh/sup alpha /, where alpha approximately=0.68. Using the model developed in the present work, a linear relationship between l/sub p/ and h is derived, but the proportionality factor depends on temperature, plasma conductivity, velocity, and boundary-layer thickness. Predictions of the model are interpreted in the light of available data. The model is shown to predict lengths which are in reasonable agreement with the observed data. >

Thermodynamic and electrical properties of railgun plasma armatures

A model including the nonideal effects is presented that describes the plasma composition, equation of state, internal energy, and electrical conductivity for a very dense and relatively cool plasma like that which exists in a railgun plasma armature. Results are presented for four elements of specific interest for plasma armatures-hydrogen, lithium, aluminum, and copper-for pressures ranging from 1 to 1000 atm and temperatures ranging from 10000 to 50000 K. The results are presented in a graphical format which should prove useful to modelers and experimentalists investigating plasma armatures.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Study of a railgun plasma armature

An experimental and theoretical study of a railgun plasma is presented. The experimental results have been obtained by recording the voltage and current variations as well as the spectral emission of the plasma. The gas pressure, helium, or air in contact with the exploding foil has been varied from 0.1 to 2 MPa. The apparatus consisted of a test chamber simulating a railgun facility. To complete the study, some measurements of the plasma characteristics in the 500-kJ EMA 1 railgun system have been recorded. It is shown that the resistance of the plasma, which is 15 m Omega at 10 kA, decreases to 0.2 m Omega at 500 kA. These results indicate the effect of the energy flow to the walls and that of the ionization potential of the gas in contact with the exploding foil at lower currents.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Transient modelling of railgun plasma armatures

Simulations are performed with a one-dimensional, time-dependent model to investigate the dynamic behavior of railgun plasma armatures. Specifically, the model is applied to the analysis of three problems: (1) the simulation of the release of a projectile; (2) the simulation of the latter stages of the initiation of an armature, with the initial arc length 50% of its final equilibrium length; and (3) a second initiation simulation where the initial arc length is 10% of its final equilibrium length. For all three simulations, the armature length and projectile acceleration exhibit damped oscillations. After the first armature expansion, the period of oscillation of the armature length can be approximated by 3l/sub eq//a/sub 0/ where l/sub eq/ and a/sub 0/ are the armature length and acoustic propagation velocity, respectively, calculated for an equilibrium, isothermal plasma whose thermodynamic characteristics are obtained from a mass average of the properties in the nonisothermal simulation. This characteristic period of oscillation is consistent with the period observed in isothermal armature simulations. >

Two dimensional time dependent MHD simulation of plasma armatures

The authors report on the development of a two-dimensional MHD (magnetohydrodynamic) code to simulate internal motions in a railgun plasma armature. The equations of resistive MHD with ohmic heating and radiation heat transport are used. A flux-corrected transport code is utilized to advance all quantities in time. Preliminary runs show the growth and subsequent shedding of vortex structures in response to a small perturbation on an initial equilibrium. >