S. I. Tkachenko

Study of the prebreakdown stage of a gas discharge in a diode with point cathode by laser probing

The prebreakdown stage of a gas discharge in a diode with strongly overloaded cathode is studied by laser methods (by simultaneous use of multiframe interferometry and shadow and schlieren photographing) at atmospheric pressure. The spatial resolution of the methods is about 20 μm. A probing pulse of a laser (LS-2151 Nd: YAG laser with a half amplitude duration of 70 ps and a pulse energy of up to 40 mJ) is synchronized with a voltage pulse with accuracy of about 1 ns. High field strength at the cathode is achieved due to the use of thin individual metal tips on the electrodes. It is shown that the initial stage of breakdown of a discharge gap is accompanied by the emergence of a dense plasma cloud at the end of a tip with electron density of about 5 × 1019 cm–3 with a size of tens of microns, as well as by a sharp increase in the total current through the diode. After the emergence of a dense plasma cloud at the end of a cathode tip, a similar cloud is formed on the surface of the anode; sometime later, these clouds join together and form a tubular current channel. The dynamics of the breakdown, as well as the parameters of the plasma are studied by the abovementioned techniques in three independent optical channels.

Anode Plasma Formation at the Initial Stage of a Nanosecond Air Discharge

The initial stage of a nanosecond discharge in gaps with a high electric field at a cathode is studied by laser methods (interferometric, shadow, schlieren methods). The studies are performed in air at atmospheric pressure. Prominence is given to studying the evolution (appearance and growth) of the plasma channels at an anode and to estimating their parameters.

Features of interferometry of small-sized plasma channels obtained during nanosecond discharge

The applicability of the method of smooth perturbations for processing interferograms of small (5–100u2009μm) objects by taking into account the diffraction is investigated. The test problems showed the consistency of the method to the range of parameters that are interesting for us. The requirements for preliminary processing of experimental data are determined, adhering to which it is possible to improve the accuracy of the final results. The analysis of experimental interferograms by assuming the cylindrical symmetry of the plasma object is carried out: the two-dimensional map of the electron density for the plasma object formed at the initial stage of nanosecond discharge was obtained from the phase map.The applicability of the method of smooth perturbations for processing interferograms of small (5–100u2009μm) objects by taking into account the diffraction is investigated. The test problems showed the consistency of the method to the range of parameters that are interesting for us. The requirements for preliminary processing of experimental data are determined, adhering to which it is possible to improve the accuracy of the final results. The analysis of experimental interferograms by assuming the cylindrical symmetry of the plasma object is carried out: the two-dimensional map of the electron density for the plasma object formed at the initial stage of nanosecond discharge was obtained from the phase map.

Studying the Parameters of Aluminum Flyer with a Linear Current Density of 4.5 MA cm−1 on the Angara 5-1 Installation

Preliminary results are presented for the parameters of flyer used in experimental studies of material properties in the shock-wave compression of a sample by a magnetic field.

Reconstruction of the current pulse passing through a thick-walled tube

A method is proposed for reconstructing a current pulse flowing through a thick-walled tube, using the electric field intensity measured on its inner surface. The results are presented for the experimental data obtained on the Angara-5-1 installation. The reconstructed current pulse was used for magnetic hydrodynamic simulation; as a result, we obtained the distribution of the matter parameters over tube thickness. The electric field intensity obtained during numerical simulation is in good agreement with the experimental results.

Result of curretn flow with a linear density of 1–3 MA/CM and duration of 100 NS across stainless steel electrodes

Summary form only given. When the current has a linear density of 1-3 MA/cm and duration of 100 ns the diffusion of the magnetic field deep into the electrode is complicated by heating the metal and the formation of the plasma on the surface. A series of experiments on the transmission of current with a rise time of 100 ns and amplitude of about 3 MA across stainless steel electrodes was conducted on the Angara-5-1 facility. The electrodes were hollow tubes with a wall thickness of 1 mm. The diameters of the tubes were 3 mm and 12 mm.Rate of expansion of the outer boundary of the electrode by passing through it a current pulse was measured by laser shadow picture. The laser pulse duration was of 0.1 ns. Laser shadow pictures were made in time moment, when current arrived at maximum. It is found that the average rate of expansion of the plasma on the outer surface of the tube at a time close to the peak of the discharge current with a linear density of 2.5 MA/cm, is about 4.6 km/s. The electric field arising on an internal surface of a tube, was measured by means of a resistive divider. It was found that at mid-height profile of the electric field on the inner surface of the tube behind the current profile of 200 ns. However, the maximum electric field intensity on the inner surface of the tube is behind the current profile for 300 ns. This is apparently due to nonlinear processes of heating, melting, evaporation, ionization and plasma formation. Comparison of the registered electric field temporal profile of the inner surface of the tube with the results of its calculation allows for verification of the computational model.

Inverse problem of the current pulse reconstruction according to the penetration rate of electric field induced inside the tubular electrode

Summary form only given. A series of numerical simulations was carried out to study the evolution of hollow tube matter during exposition by submicrosecond current pulse with linear density of 1-3 MA/cm. The experiments with the same linear density were conducted on the Angara-5-1 installation. In this way the behavior of the electrodes under extreme energy and strength loads was simulated in those experiments and calculations. To obtain reliable results, the experimental time dependence of the current ought to be specified as boundary conditions during all time the process is modeled. However, the reliable current measurement was carried out within the first ~ 100 ns in these experiments. The evolution of tube material is of interest for a longer time period. To solve this problem the time dependence of current ought to be restored according to the time dependence of the electric field intensity, measured on the inner tube surface during 500 ns. For this purpose the inverse problem was solved; the data obtained were used in the MHD-simulation.

Cylindrical and quasi-spherical wire arrays investigation on Angara-5-1 and Baikal project

In SRC RF TRINITI investigated implosion of cylindrical and quasi-spherical wire arrays. Such arrays are made of metal wires and plastic fibers both coated by metal. For three-dimensional quasi-spherical arrays implosion the technique was developed to produce wires or fiber with a given distribution of linear mass along the wires. That mass distribution needs for good implosion to the center of array. The wires and fibers was formed by spraying a metal on wire or fiber. We have also developed the technique of manufacture of only tungsten wire with a given distribution of mass per unit length. Flux of emission near the center quasi-spherical array several times greater than the flux of emission for implosion of the cylindrical array. For cylindrical arrays was measured the difference of radiation flux in the radial and axial direction. We investigate the cylindrical conductors through which current flows with linear density 1-3 MA/cm and a rise time of 100 ns. For a cylindrical conductor with a wall thickness of 1 mm electric field on the inner wall was measured. Comparison of the measured and calculated that electrical field developed to evaluate the current carried in the plasma produced on the current flow surface of the conductor. For “Baikal” project the stage of development and state examination of project documentation is completed.

Aluminum wire explosion in vacuum: Experimental and numerical study

Distribution of matter in the discharge channel formed upon a nanosecond electrical explosion of Al wire in vacuum was studied experimentally and theoretically. Simultaneous use of optical and UV diagnostics and numerical results made it possible to distinguish qualitatively different regions of the discharge channel, such as the current-carrying plasma layers and the region occupied by a weakly conducting cold matter. Several series of experiments with 25 urn diameter 12 mm long wires were performed; the charging voltage and the current amplitude were U<inf>0</inf> = 20 kV and I<inf>max</inf> ∼ 10 kA, respectivelyi. Shadow and schlieren images of the discharge channel were obtained using optical probing at the second harmonic of a YAG: Nd<inf>+3</inf> laser (λ= 0.532 μm, τ ∼ 10ns).

Wire explosion studies in arrays with small numbers of wires using a range of dischrge parameters

Summary form only given. The initial phase of a wire explosion is important in the entire process of plasma load development in wire array experiments. It is very difficult to understand the behavior of individual wires in multiwire arrays, especially if an array consists of wires made from different materials. Decreasing the number of wires to 2-4 may help to study processes associated with wire explosion and shunting plasma development. Energy deposition and, hence, conditions of wire cores and initial shunting plasma parameters, should strongly depend on circuit and discharge current parameters. There are two limits of interest for the discharge: the voltage regime, when the current through an individual wire is determined by Ohm law and is inversely proportional to its initial resistance; and the current regime, when the current through each wire is determined by the inductance distribution in the load. The current regime is realized if the inductive impedance of the wire exceeds its ohmic resistance. It is obvious that the transition from one regime to the other depends on many parameters, especially the current rise rate through each individual wire. The explosions of individual wires and wire arrays with 2-4 wires have been studied under different discharge conditions using several electrical, optical and X-ray diagnostics. A small generator based on a low inductance capacitor, with a current rise rate of 10-50 A/ns (8 kA maximum current) and the BIN generator with a current rise rate up to 3 kA/ns (270 kA maximum current) were used to drive the wire explosions. It was shown that under some conditions, the initial coronal plasma development does not depend strongly on the wire material