K. N. Mitrofanov

Prolonged plasma production at current-driven implosion of wire arrays on Angara-5-1 facility

Results of experimental investigation and modeling of prolonged plasma production during implosion of cylindrical wire arrays are presented. Results of the radiography of dense cores of imploding wire array and the measurements of internal azimuthal magnetic field in wire array give new experimental evidences of prolonged plasma production phenomenon. This phenomenon is an important property of current-driven implosion of the wire arrays at current rise rates /spl sim/(0.5-1)10/sup 14/ A/s. The prolonged plasma production can determine the current and the density profiles before final stage of a Z pinch compression, and also in the moment of Z pinch stagnation. From this point of view, the requirement that residual uncompressed plasma should not shunt the current at the discharge periphery becomes of the greatest importance. The conditions exist when the prolonged plasma production isnt an obstacle for the achievement of high-power X-ray emission from Z pinch. Presented experimental results on multiwire array implosion can be explained on the basis of prolonged plasma production without referring to multiwire array azimuthal structure.

X-ray backlighting of the periphery of an imploding multiwire array in the Angara-5-1 facility

Backlighting diagnostics for studying the peripheral region of an imploding liner in the Angara-5-1 facility by using X-ray emission from an X-pinch is described. The spatial resolution of the diagnostics was no worse than 4 µm. The X-pinch emission passed through the plasma was recorded with a photofilm. The plasma density was reconstructed from the photofilm blackening density with the help of a step attenuator made of the same material as the liner. Results are presented from experiments on X-ray backlighting of the peripheral region of a multiwire liner at the 70th ns after the beginning of the discharge. It was found that, by this time, the wire cores were depleted to different extent, their masses totalled 70% of the original wire mass, and their diameters had increased approximately threefold. The plasma ejected from the wire cores was found to be axially stratified with a spatial period of 200 µm. Sometimes the axial nonuniformity of the core material with a characteristic scale length of 20 µm was observed.

Current-induced implosion of a multiwire array as a radial plasma rainstorm

We present our experimental results of the X-ray radiography of fast radiating Z-pinches based on cylindrical multiwire tungsten arrays. The experiments were carried out at the Angara-5-1 facility at an electrical power of up to 4 TW with a discharge current of up to 4 MA rising at a rate on the order of 5×1013 A s−1. The linear mass of single and composite arrays reached 500 µg cm−1, the initial radius was 4–10 mm, and the wire diameter was 5–8 µm. We have experimentally shown that for the current-induced implosion of multiwire tungsten arrays, significant azimuthal and axial plasma inhomogeneities result from discharge cold start and prolonged plasma production, which determine the subsequent course of the implosion. The Z-pinch structure also remains spatially inhomogeneous at the time of intense X-ray radiation. The generated inhomogeneous plasma collapses toward the array axis in the form of numerous radially elongated plasmoids with relatively small diameters. The stream of plasmoids is called a radial plasma rainstorm. As the plasmoids contract toward the array axis, they decrease in radial size and merge into isolated plasma current filaments, which are elongated mainly along the discharge axis. We critically discuss the models of a radiating Z-pinch in plasma composed of matter with a large atomic number that disregard the cold-start and prolonged plasma-production effects.

Measurements of the azimuthal magnetic field within imploding multiwire arrays in the Angara-5-1 facility

Results are presented from measurements of the azimuthal magnetic fields within imploding multiwire tungsten arrays in the Angara-5-1 facility at currents of 2.5–4 MA. It is shown that the penetration of the magnetic field into the axial region of the wire array lags behind the discharge current pulse. The current of a precursor produced at the array axis prior to the implosion of the bulk array mass is measured. It is found that the magnetic field in the initial stage of implosion is azimuthally nonuniform. The mass distribution inside the array is calculated from the measured magnetic field.

Characteristics of high-power radiating imploding discharge with cold start

A qualitative model of the dynamics of a multiterawatt radiating Z-pinch with cold start and high rate of current rise is proposed. The model is used to analyze discharges with currents I ∼ 2–5 MA (with dI/dt > 1013 A/s) through uniform or structured plasma-producing loads, including wire arrays. The most important consequence of cold start is that spatially nonuniform plasma production is prolonged to almost the entire current rise time. Under these conditions, the Ampére force begins to play a dominant role in the plasma dynamics before the plasma-producing load is completely transformed into an accelerated plasma. The results of computations of wire-array vaporization are presented. A formula is proposed for estimating the highest attainable velocity of plasma flow into a heterogeneous liner driven by the Ampére force. It is shown that local imbalance between radial motion of the produced plasma and supply of the plasma-producing substance to be ionized leads to axially nonuniform breakthrough of magnetic flux into the liner, which precedes plasma collapse. The magnetic-flux breakthrough gives rise to a chaotic azimuthal-axial plasma structure consisting of radial plasma jets of relatively small diameter, which is called a radial plasma rainstorm. The breaking-through azimuthal magnetic flux obstructs further current flow in the breakthrough region. Analyses of Z-pinch implosion based on the theory of Rayleigh-Taylor instability or the snowplow model are incorrect under the plasma-rainstorm conditions. The processes taking place in a stagnant Z-pinch include conversion of the energy carried by the current-generated magnetic field into turbulent MHD flow of the ion component of the plasma, its convective mixing with magnetic field, heating, energy transfer from ions to electrons, and emission from the plasma. Under typical experimental conditions, emission plays a key role in the energy balance in an imploding pinch. Z-pinch is modeled by an electric-circuit component that has a time-dependent nonlinear impedance and consumes the magnetic energy supplied by a generator through a magnetically insulated transmission line (MITL). The peak power reached in the circuit is comparable to the peak soft X-ray power output emitted by the pinch in terms of magnitude and timing. Optimum matching conditions are formulated for the generator-MITL-pinch circuit.

Prolonged plasma production and dynamics of implosion of multiwire arrays

Brief review of our recent results concerning prolonged plasma production (P3) in multiwires arrays and its influence on their dynamics in pulse power facilities is presented. P3 means that owing to overheating effects hot plasma with high conductivity is being produced from the relatively cold material almost during the whole period of current rising. P3 results in formation of a plasma shell with a low aspect ratio and without any skin effect. It means that magnetic flux and electric current are distributed more or less uniformly along the shell and there is no magnetic piston. The latter fact is responsible for the more stable implosion of the plasma. P3 is unavoidable effect for initially cold liners of all types in pulse power facilities, and it is useful for production of high power X-ray pulses with multiwires arrays because the process of P3 leading to more stable implosion is well controlled in this case due to well defined periodic structure of the array. Theoretical evaluation of rate of plasma production is obtained, and it is compared with experiments. Simple simulation of plasma dynamics during implosion of multiwire arrays with P3 is presented. Combining of such simulation with probe magnetic measurements inside cylindrical liners we obtain a certain information about dependence of the plasma production rate on time and about characteristics of depletion of plasma source. It appears that process of the depletion may be rather gradual, so that some amount of plasma is being placed at initial array position even till the moment of X-ray pulse maximum. It may lead to partial shunting of electric current and to some depression of X-ray pulse intensity. These results discover new opportunities to control plasma implosion and X-ray pulse parameters.

Experimental study of wire array implosion in presence of prolonged plasma production on Angara-5-1 facility

The study of the wire array implosion on installation Angara-5-1 is devoted. Magnetic probes and the x-ray backlighting technique were used to investigate a current and mass distribution during the implosion. The experimental data confirm the concept of prolonged plasma production: the long time of implosion the hot plasma flows from dense fixed wire cores and is accelerated by the Lorentz force to the array center together with frozen magnetic field.

Research of Nested Wire Array Emission and Implosion on Angara-5-1

The results of the experiments performed on the Angara-5-1 facility of Z-pinch produced from wire arrays are presented. The models of liner implosion, i.e. prolonged plasma production and plasma rainstorm have found their confirmation. The effect of the inner array on the current distribution in the space between the arrays has been revealed.

Physics of ICF related multiwire array implosion

Spatial distributions of mass and current inside current-driven wire array during implosion were investigated on Angara-5-1 facility. Radiography of dense cores of imploding wire array, measurements of internal azimuthal magnetic field in wire array and electron density measurements on the periphery of initial array position at the moment of stagnation give new experimental evidences of prolonged plasma production phenomenon. The measurement of current density inside the array shows that about half the current flows on the liner periphery at the moment of maximum compression. At this time up to 10% of the liner mass is present on the periphery of the initial wire array.