B. I. Reznikov

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).

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.

Using anisotropic heat flux sensors in aerodynamic experiments

We present the results of comparative measurements of the heat flux to a flat plate in a supersonic flow at a Mach number of M = 6, which were performed using the two following anisotropic heat sensors with different thicknesses of sensor elements: (i) Atomic Layer Thermo Pile (ALTP, Fortech GmbH, Germany) with a thickness of ∼0.5 × 10−6 m and (ii) gradient heat flux sensor (GHFS, St. Petersburg State Polytechnic University, Russia) with a thickness of ∼2 × 10−4 m. The ALTP sensor can be used for directly measuring heat fluxes in processes with a characteristic time above 10−6 s. A method for mathematically processing the GHFS response signal is proposed that allows heat flux oscillations to be revealed in gasdynamic process with a characteristic time on the order of 10−4 s.

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.

The effect of erosion mass capture on plasma acceleration in electromagnetic railgun accelerators

Results of a series of experiments on plasma acceleration in an electromagnetic railgun accelerator with copper electrodes are presented for various discharge currents and initial pressures in a channel filled with argon or helium. A notion of an effective erosion coefficient is introduced that takes into account that only a particular amount of the erosion mass lost by railgun electrodes during discharge current passage is captured and involved in the motion of accelerated plasma. This coefficient has been calculated in the frame- work of a dynamic model of plasma motion, which includes the force of shock-compressed gas and the drag force that arises when the captured part of erosion mass is entrained by the accelerated plasma piston. It is established that the effective erosion coefficient and the rate of plasma mass variation are weakly nonlinear functions of the discharge current, which also depend on the initial gas density in the railgun channel.

Synchronous acceleration of two millimeter-sized bodies up to hypersonic velocities in a multichannel railgun

A two-channel electromagnetic railgun accelerator of bodies of original design is described. Using this device, synchronous group flight in the atmosphere has been realized for two plastic cubes with a 2-mm edge length and ∼0.01-g mass each at a velocity exceeding 4 km/s. The flight and related gas-flow patterns have been monitored by high-speed photography using a double-frame schlieren system.