R. O. Kurakin

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.

Detection of several daemon populations in earth-crossing orbits

Detection of negative daemons, DArk Electric Matter Objects, viz. Planckian supermassive (~2*10^-5 g) particles, whose population has been detected in March 2000 to populate near-Earth, almost circular, heliocentric orbits (NEACHOs), is being continued. The NEACHO objects hit the Earth with a velocity ~10-15 km/s. The results of these and new experiments (April-June, 2001) are now processed taking into account the scintillation shape depending on the magnitude and sign of the velocity of the daemons crossing our detector. The data accumulated during the time of the experiment and processed in this way reveal also the presence of (1) a high-velocity (~35-50 km/s) daemon population whose objects can be related to a population in the Galactic disk and/or that in strongly elongated, Earth-crossing heliocentric orbits (SEECHOs), as well as (2) a low-velocity (~3-10 km/s) population in geocentric Earth-surface-crossing orbits (GESCOs), whose objects traverse repeatedly the Earth to suffer a decrease in velocity by ~30-40% in a month in the process. An evolutionary relation between all these three (four?) populations is discussed. Assumptions concerning their manifestations in further observations are put forward. An analysis of possible interaction processes of daemons, which may have different velocities and directions of motion, with the detector components [ZnS(Ag) layers, 0.3-mm thick tinned-iron sheets etc.] on the atomic (emission of Auger electrons) and nuclear (nucleon evaporation from a nucleus excited in the capture and, subsequently, the decay of its protons) levels has permitted estimation of some characteristic times. In particular, the decay time of a daemon-containing proton is ~10^-6 s.Experiments on scintillator-based detection of negative dark electric matter objects, daemons, representing Planckian supermassive (about 2 ‐ 10 m 5 v g) particles that were detected in March 2000 as populating near-Earth, almost circular heliocentric orbits (NEACHOs), are being continued. The NEACHO objects hit the Earth with a velocity of about 10-15 v km v s m 1 . The results of these and new experiments (April-June 2001) are now being processed, taking into account the difference in scintillation signal shape depending on the magnitude and sign of the velocity of the daemons crossing our detector, which was purposefully made asymmetric with respect to the up-down direction of flight. The data accumulated during the experiment and processed in this way also reveal the presence of, firstly, a high-velocity (about 35-50 v km v s m 1 ) daemon population whose objects can be related to a population in the Galactic disc and/or that in strongly elongated Earth-crossing heliocentric orbits and, secondly, a lo...

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.

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).

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.

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.

Role of the pinch effect in a high-velocity metallic contact with a high current

Experimental evidence is presented in support of the hypothesis that the main factor determining the flow of physical processes on the interface of a sliding solid-state contact carrying a ∼0.1–1 MA/cm2 current is the sausage-type MHD pinch instability. It leads to the appearance and explosive destruction of tightening (quasi) liquid constrictions connecting the contact surfaces, so that under magnetic-suspension conditions the sliding of the latter relative to one another is virtually contact-free on the greater portion of their area.

Intense shock waves and shock-compressed gas flows in the channels of rail accelerators

Shock wave generation and shock-compressed gas flows attendant on the acceleration of an striker-free plasma piston in the channels of electromagnetic rail accelerators (railguns) are studied. Experiments are carried out in channels filled with helium or argon to an initial pressure of 25–500 Torr. At a pressure of 25 Torr, Mach numbers equal 32 in argon and 16 in helium. It is found that with the initial currents and gas initial densities in the channels being the same, the shock wave velocities in both gases almost coincide. Unlike standard shock tubes, a high electric field (up to 300 V/cm) present in the channel governs the motion of a shock-compressed layer. Once the charged particle concentration behind the shock wave becomes sufficiently high, the field causes part of the discharge current to pass through the shock-compressed layer. As a result, the glow of the layer becomes much more intense.

Effective erosion coefficient and plasma velocity limitation in the channel of an electromagnetic rail accelerator

The concept of an effective erosion coefficient, which takes into account the capture and entrainment in motion (by accelerated plasma) of only part of the erosion mass lost by rail accelerator electrodes, is introduced to describe the plasma acceleration dynamics in the channel of an electromagnetic rail accelerator. This parameter is determined from a comparison of the experimental and calculated plasma velocities at the stage of velocity saturation. The plasma velocity is calculated using a model that takes into account the pressure force of a shock-compressed gas and the deceleration force that appears during the capture of the erosion mass by a plasma piston. The ratio of the captured mass to the mass lost by the electrodes is found to depend on the current; for copper, this ratio is 1/4–2/3. The effective erosion coefficient is 0.6–0.7 mg/C at a current of ∼40 kA.