B. G. Zhukov

Physics of solid armature launch transition into arc mode

The railgun launch of a solid armature is conventionally divided into two stages, namely, the initial stage, where a metallic contact is retained between the armature and the rails, and the second, starting at a velocity /spl sim/0.3-2 km/s, where the contact occurs through the arc plasma. While a variety of reasons for the transition were considered, no universally accepted physical picture supported by experiment has thus far been formulated. We showed experimentally that under usual launch parameters, 3D MHD instabilities form in the contact gap because of the absence of shear resistance, which result in the formation, collapse, and, eventually, explosion of pinch waists (see E.M. Drobyshevski et al. Tech. Phys. Letts., vol.25 no.3, p.245-7, 1999). The destruction of the pinch waists gives rise not only to termination of current flow from the armature to the rail through the metal and transformation of the metallic contact in the armature/rail interface into an arc gap, but to ejection of the liquid metal and dusty plasma from the gap forward and back into the railgun bore as well. Even in the absence of MHD instabilities, armature material can be shed due to very inhomogeneous distribution of fields of thermal, electric, and dynamic parameters in the armature body. Appearance of easily-ionized matter in the bore results in the formation of shunting arcs there, which naturally reduce the armature launching efficiency. The new physics revealed by the authors in the railgun solid-armature launch transition to the arc regime provides a basis for a search for ways of increasing the launch efficiency.

The importance of three dimensions in the study of solid armature transition in railguns

It is shown experimentally that the main processes first causing breakdown of the sliding solid contact (SSC) carrying large currents are not two-dimensional processes of the velocity skin-effect type, but pinch instabilities developing in the contact interface. Electromagnetic and electrothermal explosive ejection of ionized material of low-mass pinch waists ignite parasite-shunting arcs behind and ahead of the armature under acceleration. Thus one has to speak of transition into arcing mode not of the SSC alone, but of the launch process as a whole. It has been noted that during the transition into the arcing mode the solid armature begins to behave as a hybrid armature of pulling type. At this operation mode, two stable arcs appear, due to pinch cords moving forward along the contact surfaces, which are fixed to both contact surfaces of the armature near their leading edges. Consequently, the armature main body is accelerated under the action of internal tensile forces originating in its leading part where the I × B forces are applied.

Railgun launch of small bodies

Small body launching using gas or plasma faces the fundamental problem caused by excess energy loss due to great wall surface/volume of the barrel ratio. That is why the efficiency of the plasma armature (PA) railgun acceleration is maximum for 8-10 mm-size bodies and drops as their size decreases (see Drobyshevski et al., Sov. Phys. Tech. Phys. vol. 36, no.8, p.903, 1991). For the nuclear fusion applications, where #1-2 mm-size pellets at 5-10 km/s velocity are desirable, one is forced to search for compromise between the body size (3-4 mm) and its velocity (3 km/s). Under these conditions, EM launchers did not demonstrate an advantage over the light-gas guns. When elaborating the #1 mm railgun, we made use of our ideology of the body launching at constant acceleration close to the body strength or the electrode skin-layer explosion limits (see Drobyshevski, et al., Sov, Phys. Tech. Phys. vol.36, no.4, p.469, 1991). That shortened the barrel length sufficiently. The system becomes highly compact thus permitting rapid testing of new operation modes and different modifications of the design including the magnetic field augmentation. As a result of these refinements, the difficulties caused by the catastrophic supply of mass ablated from the electrodes were overcome and regimes of #1 mm body speed-up to 4.5 km/s were found. Potentialities of the small system created are far from being exhausted. >

Parameters of an Erosion Carbon Plasma in the Channel of a Railgun

The working current dependences of the thermodynamic and electrophysical parameters of a free plasma piston moving with a near-maximal velocity in the channel of an electromagnetic rail launcher with graphite electrodes are obtained. The composition and weight of the plasma depend on the degree of electrode erosion due to discharge current passage (i = 40–80 kA). It is shown that the mean temperature of the plasma piston only slightly depends on the plasma mean pressure and plasma piston weight and increases with current by a near-power law. The measured values of the maximal velocity of the plasma piston front are compared with the calculated value of the sound velocity inside the piston. With the working current and cross-sectional area of the channel fixed, the initial gas density in the channel is found to influence the ratio of the piston maximal velocity to the sound velocity in the plasma. If the initial gas density is low (lower than some critical value), the maximal velocity of the plasma piston front exceeds the sound velocity in the plasma.

Influence of the gas density on the motion of a free plasma piston in the railgun channel

The influence of the gas density on the acceleration of a plasma armature inside the railgun channel filled with various gases (xenon, air, or helium) under atmospheric pressure is investigated experimentally and theoretically. It is shown that, after the discharge current has reached a steady value, the velocity of the glowing plasma front ceases to grow and remains constant as long as so does the current. The length over which the velocity saturates is equal to a few centimeters, i.e., is much shorter than the railgun channel length. The maximum velocity of the plasma piston meets a predicted limit, which is determined by the drag of the medium and a decrease in the acceleration of the plasma armature when a fraction of the material evaporated from the rails is involved into motion. The plasma composition depends on the electrode material. The velocities measured when the channel is filled with helium (V = 17.5 km/s) or air (V = 9.8 km/s) noticeably exceed the sound speed inside the plasma piston (5–7 km/s).

Direct observation of isolated ultrananodimensional diamond clusters using atomic force microscopy

Isolated ultrananodimensional diamond (UND) particles obtained by means of detonation synthesis have been studied using atomic force microscopy (AFM). The UND particles were deposited onto the surface of highly oriented pyrolytic graphite from a suspension based on organic compounds. The deposited UND particles were deaggregated using a two-stage treatment with ultrasound and high-dynamic-pressure pulses. The isolated UND particles were stabilized in suspension by a benzene additive. AFM images of individual UND particles have been obtained, and the phenomenon of their alignment along atomic steps on the substrate surface has been observed.

Head-on collision opens 15–20 km/s opportunities

Abstract Velocity doubling in head-on collision of solid projectiles can practically be achieved with the use of electromagnetic launchers that permit precise synchronization of shots. Most promising for this purpose are so-called “fast” railguns operating in the regime of maximum constant acceleration limited only by strength of the projectile or electrode skin-layer explosion. No light-gas preaccelerator is used. This minimizes the acceleration path and the scatter of shot parameters. Our experimental system uses an augmented magnetic field and compacted plasma armature (PA); it launches two lexan cubes of 2 mm size (∼ 10 mg), at 5 km/s each, to head-on collision at relative velocity ∼ 10 km/s. The collision process in air is registered using the shadowgraph Toepler method on static film with high-speed camera with rotating mirror and a multipulse laser. Previously reported velocities of > 7 km/s for “fast” railgun launching 1 g projectiles can be further increased with making use of the compacted PA, which opens new prospects of the head-on-collision method for studying hypervelocity impacts and their applications.

Generation of high-velocity plasma flows in railgun channels filled with gases of various density

We have studied the motion of a plasma piston (PP) in the channel of an electromagnetic railgun accelerator with a cross-sectional area of 36 and 86 mm2. The accelerator was placed in a chamber that could be evacuated and then filled with helium or argon at a pressure within 25–250 Torr. It is established that the stationary PP velocity in the channel depends on the gas type and pressure. The results are satisfactorily described by the proposed model. Experimental data can be used for evaluating the effective running inductance of the accelerator and the coefficient of erosion of the electrode material.

An electromagnetic railgun accelerator: a generator of strong shock waves in channels

Processes that accompany the generation of strong shock waves during the acceleration of a free plasma piston (PP) in the electromagnetic railgun channel have been experimentally studied. The formation of shock waves in the railgun channel and the motion of a shock-wave-compressed layer proceed (in contrast to the case of a classical shock tube) in a rather strong electric field (up to 300 V/cm). The experiments were performed at the initial gas pressures in the channel ranging from 25 to 500 Torr. At 25 Torr, the shock-wave Mach numbers reached 32 in argon and 16 in helium. At high concentrations of charged particles behind the shock wave, the electric field causes the passage of a part of the discharge current through the volume of the shock-wave-compressed layer, which induces intense glow comparable with that of the PP glow.

A compact railgun accelerator for millimeter-sized dielectric solid armatures

Millimeter-sized dielectric solid armatures have been accelerated in a compact railgun system. It is shown that application of an external pulsed magnetic field can solve the problem of catastrophic erosion of electrodes at the initial stage and accelerate small armatures up to a velocity of about 6 km/s.