Railgun Operation Research Articles

Power Supply Options for a Naval Railgun

Large railguns require powerful power supply units. At the French-German Research Institute of Saint-Louis (ISL), most experimental railguns are driven by power supply units based on capacitors. Recent investigations at ISL explore the possibility to use coil-based systems to increase the energy density and reduce the footprint of the power supply. In this paper, an electrical circuit simulation is used to investigate the difference for railgun operation in between a capacitor and a coil-based power supply with respect to the current amplitude behavior, projectile velocity, and launch efficiency. For this, a scenario of a 25-MJ muzzle energy railgun is simulated with two different power supply options; one is based on capacitors, and the other is on coils. The circuit resistance determines a large part of the losses of the system and therefore defines the efficiency of the launch and the size of the power supply. Therefore, a range of different resistances were simulated as well. The interpretation of the results of the performed simulations leads to the conclusion that the capacitor-based system “naturally” produces a favorable current pulse trace with respect to launching a mechanical delicate payload. Further simulations show that the disadvantage of the inductor-based supply can be mitigated by increasing the power supply unit subdivision into smaller units.

Time- and Frequency-Domain Characterization of Railgun Sliding Contact Noise

The noise in a railgun sliding contact is characterized in both the time and frequency domain in this investigation. Railgun operating parameters are typically 4 g projectiles traveling up to 100 m/s. The railgun is powered by a sequentially fired pulse forming network (PFN) consisting of eight electrolytic capacitor banks. Typical V–I characteristics of the PFN are approximately 20-kA peak current and 400-V peak voltage. The total pulsewidth is on the order of 6 ms. The noise is extracted from the muzzle voltage measurements using an improved method that can be applied to any electromagnetic launcher with a sliding contact. A variety of rail materials and rail coatings were investigated in regard to their effect on sliding contact noise. The noise is characterized in the time domain using statistical terms such as rms quantities. The Fourier transform is used to characterize the noise in the frequency domain. The results show that good and poor sliding contacts have particular average rms noise voltages and spectral signatures. The spectral signatures are used to quantitatively identify the point the difference between a good and a poor performing contact and to gain insight into the nature of the sliding contact’s current conduction mechanisms.

Structural Mechanics of Railguns With Open Barrels and Elastic Supports: The Influence of Multishot Operation

The dynamic behavior of the railgun housing including the rails is of great importance for the system performance. The transient loading (magnetic pressure) may have a negative influence on the sliding electrical contact required for this acceleration technology. Therefore, the analysis of the displacement of the inner rail surfaces due to elastic waves is of great importance. The French-German Research Institute of Saint Louis (ISL) is operating several railguns with housings characterized by an open barrel design allowing for innovative diagnostic techniques. The repelling forces between the rails are countered using discrete steel bolts. The rails are mounted to bars made from composite material. In the past, we presented the experimental and numerical findings concerning the dynamic behavior of such railguns in single shot mode. In this paper, we present for the first time 3-D finite element calculations concerning the dynamical behavior of the ISL RApid FIre RAilgun operating in multishot mode. Multishot scenarios with fire rates of 50 Hz are considered and particular attention is paid to the question if the dynamical behavior of the railgun is influenced by preceding shots.

EM Gun Bore Life Experiments at Naval Research Laboratory

The Naval Research Laboratory (NRL) performs basic and applied research on high power railguns as part of the US Navy EM Launcher program. The understanding of damage mechanisms as a function of armature and barrel materials, launch parameters, and bore geometry is of primary interest to the development of a viable high power railgun. Research is performed on a 6-m, 1.5-MJ railgun located at NRL. Barrel studies utilize in situ diagnostics coupled with detailed ex situ analysis of rail materials to provide clues to the conditions present during launch. Results are compared with coupled 3-D electromagnetic and mechanical finite element analysis models of railgun operation. Results of several experiments on rail wear will be discussed.

Analytical Study of the Injection of a Moving Projectile Into a Railgun

Calculations are performed for an initially moving projectile into a railgun. The injection velocity is provided by a 26-mm-diameter conventional propellant gun. A plasma armature is assumed for the railgun. A capacitor-based pulsed-power supply, located at Barricade C, Aberdeen Proving Ground, MD, is assumed to provide the electrical energy for boosting the velocity to 2.5 km/s. Various scenarios are examined with respect to electrical pulse shape, the effect on stored electrical energy, and its distribution in the railgun. Three types of comparisons are used to illustrate the effect of injection velocity on stored electrical energy: (1) efficiency; (2) peak loads; and (3) energy storage. Examples for each category are discussed, illustrating complementary areas for propellant gun and railgun operation. Results are promising; however, the initial velocity must be considered in detailed simulations in order for any advantages to be realized.

Vilnius High Magnetic Field Centre Facilities

A brief overview of the high-field facilities at the Vilnius High Magnetic Field Centre (VMC) is presented in this paper. We highlight the main activity of the VMC, which lies in studying the effects of pulsed magnetic and electrical fields on material properties, and give a short description of the pulsed magnets. A new method for generating nanosecond duration strong electric field pulse in conjunction with a magnetic field pulse is presented. The possibility of modulating electric field pulses by pulsed magnetic fields is discussed. Here, we also report on our investigations of the colossal magnetoresistance (CMR) effect in thin polycrystalline La-Sr-MnO films, which are of special interest for the design of new B-scalar magnetic field sensors used for magnetic field distribution measurements during railgun operation and other applications.

Magnetic Diffusion in Railguns: Measurements Using CMR-Based Sensors

Magnetic diffusion is of great importance in the domain of electromechanical energy conversion since it can be a performance-limiting mechanism. This paper deals with magnetic diffusion in the case of the rails of a railgun. The experiments benefit from the use of a new type of high magnetic-field sensor based on thin ( < 1 mum) manganite films, which exhibits a colossal magnetoresistance (CMR) effect and has very small sensitive areas (e.g., 0.5 mm times 50 mum). Some basics about CMR and the design of the sensor are given. Several sensors were used and calibrated in the course of static transient railgun experiments. Here, varying spatial field distributions due to the use of different rail materials were recorded. Magnetic-field measurements during railgun operation with projectile velocities exceeding 1000 m/s have been successfully performed. By comparison with 3-D finite-element calculations, interesting results concerning magnetic diffusion have been worked out.

Shielding of High Magnetic Fields

This paper revisits the problem of shielding low- frequency high magnetic fields. Such fields are encountered in the domain of electromechanical energy conversion in particular during railgun operation. Using finite-element (FEM) analysis, we investigate, separately, the mechanism of redirection of the magnetic flux due to induced eddy-currents and the magnetic flux shunting due to highly permeable materials. In a first phase, various shapes of highly conductive shields are analyzed in the frequency domain. In a second phase, we study the shunt effect of different shields being made of magnetic materials. In a magnetostatic model afterwards, for a shield that combines both highly conductive and magnetically highly permeable materials, the case of a double-exponential shaped magnetic field is treated in the time domain. Finally, we make some remarks on the concept of shielding effectiveness.

Effects of Pressure-Dependent Contact Resistivity on Contact Interfacial Conditions

Adequate contact pressure needs to be applied at electrical contacts to overcome contact separation force due to current constriction effects. In the railgun operation, the initial contact pressure is provided by the interference pressure between the armature and the rail. As the magnetic field is established in the bore, the electromagnetic force provides the contact pressure, as the initial contact pressure reduces due to wear or contact melting. Excessive initial contact pressure may delay the motion of the armature and cause overheating of the armature. On the other hand, insufficient initial pressure would result in contact separation of armature and rail. The contact interface is never perfectly smooth. It only contacts at discrete asperities at micro scale and at discrete points due to deformation of the contact interface at macro scale. The interface characteristics are different from the bulk behavior. A detailed model of contact resistivity is essential to compute the interfacial current density and temperature distributions accurately. A contact resistivity model was developed that is a function of contact pressure, the hardness of the softer contact member, average bulk resistivity, and contact constants. The model was implemented into the code EMAP3D. This paper presents the simulation results of the start-up of a sample railgun with and without contact resistivity.

In-Flight Armature Diagnostics

A feasibility demonstration is reported for a method of determining instantaneous temperature and velocity of an armature in flight. Instantaneous diagnostics such as this could be critical for achieving further improvements in railgun operation. Such activity has the potential to enable design enhancements by providing information on the state of the armature and its relationship to the rail as it proceeds down the bore. The method exploits the temperature dependence of fluorescence from a phosphor coating applied to the armature. The demonstration used both a very small-scale portable railgun and a small-scale benchtop railgun. For these tests, the output of a pulsed ultraviolet (UV) laser is delivered by optical fiber through an access port drilled into the insulator between the rails. As the armature passes, the UV light illuminates a small area of phosphor on the armature. The phosphor fluoresces and decays at a rate dependent on the temperature of the phosphor. A second optical fiber in close proximity collects the fluorescence and conveys it to a detector and associated data acquisition system. Temperature is determined from a measurement of the decay time. To provide for velocity measurement on the small-scale railgun, light from a red diode laser, delivered by fiber probe inserted into the bore, produced distinctive reflections at the leading and trailing edges of the armature as it passed. Also, two grooves cut into the armature produced fiducial pulses that enabled velocity measurement

Measurement of High-Strain-Rate Adiabatic Strength of Conductors

The current-carrying conductors in electromagnetic launchers and rotating machines are exposed to high stress and thermal loads that last for a few milliseconds, with temperatures ranging up to the melting point of the materials. Even though equilibrium behavior of materials at high temperature is well characterized, nonequilibrium, short-duration behavior is not well understood. Properties for short-duration exposure to high temperatures (a quasi-adiabatic process) are crucial to understanding the transient physics of railgun components, since use of equilibrium (isothermal) properties would entail compromise in evaluation of stress and strain fields. These errors can grow when considering multiple applied pulses, as in cycle life evaluation . In this paper, we describe an experiment that will help characterize such properties near railgun operating conditions. We present preliminary data obtained for copper

Radiated fields in rail launchers operation

In this paper electromagnetic fields radiated from a railgun operating system with a plasma armature are investigated. A small-bore railgun has been fabricated in their laboratory, a capacitor bank (0.12 F-160 V) has been used as energy supply and an experimental setup has been implemented. Experiments have been performed in a remotely operated shielded semi-anechoic chamber in conformity with current standards. The analysis has been conducted in the time domain, using suitable current probes and antennas and an oscilloscope with high bandwidth and high sampling rate. Results were reproducible and a comparison and discussion with previous work are reported.

Dependence of railgun operation on current rise time

Railguns are facilities to accelerate projectiles up to several kilometers per second by electromagnetic force generated by driving current. Worldwide, a variety of current waveforms are used in railgun systems. This paper describes the dependence of railgun operation on the rise rate of the driving current. The use of the exploding wire as an opening switch makes it possible to change the rise time of the driving current. A secondary arc is likely to appear and the erosion on a rail surface is more severe when current with a short rise time is used. According to the nondimensional railgun simulation, current with a short rise time heats up the plasma so fast that many particles ablated from the bore surface, which seem to be the cause of the secondary arc, increase rapidly in the early phase of acceleration. © 1999 Scripta Technica, Electr Eng Jpn, 129(3): 17–28, 1999

Dependence of railgun operation on current rise time

Railguns are facilities to accelerate projectiles up to several kilometers per second by electromagnetic force generated by driving current. Worldwide, a variety of current waveforms are used in railgun systems. This paper describes the dependence of railgun operation on the rise rate of the driving current. The use of the exploding wire as an opening switch makes it possible to change the rise time of the driving current. A secondary arc is likely to appear and the erosion on a rail surface is more severe when current with a short rise time is used. According to the nondimensional railgun simulation, current with a short rise time heats up the plasma so fast that many particles ablated from the bore surface, which seem to be the cause of the secondary arc, increase rapidly in the early phase of acceleration. © 1999 Scripta Technica, Electr Eng Jpn, 129(3): 17–28, 1999

レールガン動作の駆動電流立ち上がり依存性

Railguns are facilities to accelerate projectiles up to several km/s by the electromagnetic force due to a driving current. A variety of current waveforms are used for railguns developed in the world This paper describes the dependence of a railgun operation on a rising rate of the driving current. The use of the exploding wire as an opening switch enables to change the rise time of the driving current. A secondary arc is likely to appear and the erosion on a rail surface is more severe when the current with a short rise time is used. According to the non-dimensional railgun plasma simulation, the current with a short rise time heats up the plasma so fast that plenty of particles ablated from the bore surface, which seem to be the cause of the secondary arc, increase rapidly in the early phase of acceleration.

Spatial effects in a railgun plasma armature

The results of mathematical simulation of thermal, mechanical and electromagnetic interaction of railgun plasma armature and channel walls are presented. A generalized 2D-approach taking into account turbulent friction, radiative and turbulent heat transfer and spatial inhomogeneity of the magnetic field was used. The heat regime of electrodes was considered in the 2D-approach. The momentum and energy transfer from the flow core to the electrodes and the process of involving the erosion mass into acceleration were investigated in detail. It has been shown that magnetic field inhomogeneity causing the appearance of spatial induction currents influences essentially the momentum, energy and mass balance in the near-electrode region and the armature characteristics as a whole. It leads to an additional drag force and the release of Joule heat near the rail surface and to the increase in erosion. Under some conditions this can cause instability of plasma armature and its sharp extension. Calculations have shown that the electrode melting, destruction of the liquid film, involving of drops into acceleration and their vaporization in the flow seem to be the most probable mechanism of plasma mass increase for typical regimes of railgun operation.

MHD-aspects of EML efficiency increase due to channel segmentation

It is known that one of the possible ways to increase the electromagnetic launcher (EML) efficiency is to segment the working channel. This paper presents the results of the theoretical investigation of the MHD-processes in a railgun channel for two types of segmented launchers: (A) segmented configuration where all segments are the independent railguns divided by inter-stage dielectric inserts, and (B) distributed configuration which has the nonsegmented rails and distributed energy sources. The operation of each stage of a segmented railgun is determined by intricate MHD-flows. The plasma flow interaction with the magnetic field is of complex character and has an ambiguous influence on the muzzle velocity. The character of the discharge behaviour was investigated for both schemes. The influence of the current shape, of augmentation, of section length and the optimization of the multistage railgun operation for these parameters were studied. It is shown that the energy input to the plasma armature by means of short fast current pulses reduces momentum losses and provides the considerable velocity increase.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Diamond Materials for Electromagnetic Railguns

Abstract The use of diamond film insulator in electromagnetic railguns is currently being seriously considered since the state of the art in synthetic: diamond exceeds the requirements of electromagnetic railguns. The reasons for not using diamond, however, include the major difficulty in producing diamond insulators in the required shape and size that can survive the harsh railgun environment. This paper reviews railgun operation dynamics and potential materials including diamond for high performance of railguns. It further reviews the present status of scientific, technological and commercial developments; of diamond coatings. Alternate coatings and the properties that make them amenable for railgun applications are also discussed.

Controls and diagnostics on a fuseless railgun for solid hydrogen pellet acceleration

A two-stage railgun system has been built, incorporating several controls and diagnostics, some including unique features to account for the fact that the projectile is a frozen hydrogen pellet for fusion reactor refueling. A timing circuit has been developed to monitor projectile breech and muzzle velocities and to automatically trigger a sequence of events critical for effective plasma armature railgun operation. This circuit can initiate electrical breakdown of the propellant gas directly behind an incoming projectile, thus enabling fuseless operation. It also triggers a streak camera and a flashlamp for photographing the arc and the outgoing projectile, respectively. The automatic timing circuit is expandable and has been extended to incorporate a trigger for transaugmentation. The timing circuit is immune to mistriggering due to electromagnetic interference or fragmentation of the fragile hydrogen pellets. >

Theoretical and experimental modeling of railgun operation

Theoretical and experimental modeling of railgun operation