P. V. Sasorov

Studies of penetration of the magnetic field into electrically imploded loads in the Angara-5-1 facility

Results are presented from measurements of the distributions of the azimuthal magnetic field in aluminum, copper, molybdenum, tungsten and other wire arrays electrically imploded at currents of up to 3 MA in the Angara-5-1 facility. It is shown that the time during which the magnetic field of the current pulse reaches the array axis depends on the material of the wires or wire coating. The current of the precursor formed on the array axis before the implosion of the main load mass is measured. It is shown that the penetration of the load material with the frozen-in magnetic field into a polymer (agar-agar) foam liner is drastically different from that in the case of a wire array. It is found that the rate of current transfer to the array axis is maximum for tungsten wire arrays. The rates of plasma production during implosion of loads made of different materials are compared.

Relation between the electric parameters of a Z-Pinch discharge and plasma production in the load during the implosion of a cylindrical wire array

A study of the process of implosion of a cylindrical tungsten wire array by electrical and optical methods shows that it involves two phases. In the first phase, the plasma is produced from the dense wire cores under the action of the heat flux from the current-carrying plasma. This plasma then fills the internal space of the liner array. The measured inductance of the liner and its visible diameter vary only slightly in this phase. During the second phase, the total material of the liner is compressed toward the axis and the inductance of the discharge gap increases. The process of the implosion of wire arrays is studied by analyzing the electric parameters (current and voltage) of the load in the Angara-5-1 facility. The time behavior of the load inductance, the average current radius, and the start time of the liner compression are determined. The compression start time determined from the visible size of the liner is found to coincide with that determined from electric measurements. The compression ratio of the liner in terms of the average current radius turns out to be lower than that measured by optical and X-ray diagnostics. The reason is that, by the instant of maximum compression, only a portion of the current flows at the periphery of the initial wire array.

Transportation of an electromagnetic pulse to the load in the Angara-5-1 facility

One of the main problems in Z-pinch experiments is to transport power and energy from the generator to the load. As the pulse produced in a double forming line propagates to the load along a water-vacuum insulator, its power and energy decrease due to current leakage in the plasma shortening the gap and during the establishment of magnetic self-insulation in regions with a zero magnetic field. Only a fraction of the delivered energy is spent on the load implosion, whereas the rest of the energy goes on creating the magnetic field around the load. In this work, an analysis is made of what is the fraction of the generator energy that reaches the liner, what fraction is radiated, and what are losses of energy and current in different stages of transporting the electromagnetic pulse to the load of the Angara-5-1 facility.

Formation and dynamics of plasma layers formed on the foil surface under the action of a high-current pulse

Results are presented from studies of the possibility of using a thin metal foil for recyclable vacuum transmission lines with magnetic insulation in a conceptual fusion reactor based on high-voltage high-current electromagnetic generators. Numerical simulations and experiments in the Angara-5-1 facility were carried out to determine both the threshold for the explosion of a foil heated by a current pulse and the parameters of the plasma layer formed at the foil surface. It was found experimentally that an additional plasma current channel forms on the surface of a 120-μm stainless-steel foil at a linear current density of 0.25–0.5 MA/cm, which corresponds to a magnetic field of 0.3–0.6 MG. For the same conditions, one-dimensional computer simulations of the foil heating were performed in an MHD model by using a wide-range semiempirical equation of state for stainless steel. The calculated threshold for plasma generation on the foil surface is compared with the experimental data. The main parameters of the plasma layer are also calculated at linear current densities of 2–10 MA/cm, which far exceed the threshold current density. The plasma layer parameters as functions of the linear current density are determined for the case of an iron foil.

Cavitation and formation of foam-like structures inside exploding wires

Large-scale molecular dynamics (MD) simulations are used to study explosions of aluminum wires heated by electric current pulses. It is shown that the observed nonuniform radial expansion of the heated wire is associated with a liquid-vapor phase transition, which is caused by convergence of a radial tensile wave towards the center of the wire. Tension within the wave leads to cavitation in stretched melt that subsequently forms into a low-density foam-like material surrounded by a dense liquid shell. The foam decays into liquid droplets before the outer shell breaks apart. Simulated density profiles demonstrate good qualitative agreement with experimental high-resolution X-ray images showing the complex hollow structures within the long-living dense core.

Peculiarities of wire resistance behavior on initial stage of explosion

Large achievements in soft X-ray production were obtained by Sandia with multiwire array implosion. The efficiency of the generator energy conversion to radiation strongly depends upon the quality of the plasma shell. Thus the investigation of the initial stage of wire explosion and the method of plasma shell creation are very important. To study the different processes of plasma creation at the initial stage of wire explosion the precision voltage on the wire array axis and current measurements have been carried out.

The measurement of plasma density at the periphery of Z-pinch on Angara-5-1 facility

In the physics of the wire array implosion the amount of the substance left on the periphery of the initial wire array at the moment of final liner implosion is an essential factor. Estimates show that plasma with a small mass fraction (a few percent of the initial wire array mass) may shunt an appreciable current fraction which can result in changing output characteristics of implosion. The given paper reports the investigation into the plasma density distribution on the periphery of the initial wire array near the moment of final implosion.Knowledge of spatial mass distribution is important for understanding the physics of implosion of megaampere-current wire arrays. The paper presents results from studying the electron density distribution at the periphery of a tungsten wire array near the instant of maximum compression by using laser interferometry at λ=0.69 µm. It is found that, at the instant of maximum compression (∼100 ns after the beginning of the discharge), the estimated maximum local electron density inside the wire array reaches ∼1018 cm−3 at a distance of 0.3–3 mm from the initial wire positions. Assuming the average tungsten ion charge to be 10, the local linear mass density in this region turns out to be 3 µg/cm, which amounts to about 10% of the total linear mass density of the liner. A fraction of the generator current flows through this plasma. The duration of the soft X-ray pulse is 5–8 ns, which indicates the achievement of a fairly high compression ratio.

Dynamics of plasma jets in multiwire arrays

Dynamics of plasma jets ablated from separate wires in multiwire array is investigated theoretically under assumption that width of the jets in azimuth direction is much less than the interwire gap. Using these results, an updated estimation of plasma production rate, which takes into account the strong azimuth structure of the plasma flow, is obtained.

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.