Alexander Rososhek

We present the first x-ray images of electrically exploding wires in air and water, captured at the European X-ray Free-Electron Laser (EuXFEL). These images reveal current shunting in a copper wire during its explosion in air, and the development of electrothermal (and possibly magnetohydrodynamic) instabilities in copper and aluminum wires exploding in water. The experiments were conducted at the Single Particles, Clusters, and Biomolecules and Serial Femtosecond Crystallography instrument (SPB/SFX) at EuXFEL, where fine metallic wires were driven by a pulsed-power generator with a current rise time of ∼1.1 μs and a peak amplitude of ∼28 kA. EuXFEL enabled MHz x-ray radiography of the wires using 20 keV photons and a narrow bandwidth of ∼40 eV. Using the specific current action integral for a copper wire explosion, we provide the first quantitative estimate of the fraction of current that is shunted through the surrounding medium of an exploding wire. Fourier analysis of the electrothermal instability spectrum shows that its wavelength remains mostly independent of the wire diameter once it has exploded, and that the temperature perturbation is similar in magnitude to the average temperature in the wire.

An imaging refractometer can be used to describe the properties of a high-energy density plasma by analyzing the transverse intensity distribution of a laser beam that has passed through the plasma. The output of the refractometer can be directly calibrated in terms of beam deflection angles using ray transfer matrix analysis. This paper describes a novel way to calibrate the refractometer output in terms of the spatial wavenumbers of the transverse intensity distribution of the laser beam. This is accomplished by replacing the plasma with a gridded structure that modulates the transverse intensity of the beam, producing an intensity distribution with a known Fourier Transform. This calibration technique will generate a one-to-one mapping of deflection angle to wavenumber and will enable measurement of the size of Fourier space available to the system. The spectrum of wavenumbers generated when the laser beam passes through a high-energy density plasma may contain information about the types of density fluctuation that are present in the plasma.

In this paper, we present the first direct experimental evidence supported by numerical modeling of a turbulent plasma column formed during a gas-puff $z$-pinch implosion generated by the cobra current. Utilizing an imaging refractometer, we showed a significant decrease in spatial autocorrelation of the laser field and the appearance of a laser speckle pattern shortly before stagnation. The intensity distribution of the speckles measured during different shot campaigns while employing long and short cobra pulses follows the speckle statistics satisfactorily. The imaging refractometer signal is proportional to the integral over the electron density gradients; hence, the measured phase randomization of the individual plane waves composing the laser field implies a random density distribution. To validate this, the beam propagation method code simulates the laser beam propagation through different artificial density distributions with various average fluctuation scales and generates synthetic imaging refractometer data. The results reproduce similar trends in the experimental data, such as the increasing vertical width for the decreasing average spatial scale of the fluctuations and the decreasing spatial correlation length of the laser field. Therefore, during the gas-puff $z$-pinch implosion process, it is likely that the plasma flow is almost always turbulent with the average spatial scale of the turbulent density fluctuations decreasing towards stagnation.

We present the results of an energy-inventory analysis for argon gas-puff Z-pinch implosions driven by the 1-MA, $\sim$ 230-ns rise time COBRA generator at Cornell University. Implosions with and without an applied axial magnetic field are examined. The total energy coupled to the plasma is inferred from current and voltage traces, while spatially resolved plasma parameters, such as flow velocity, temperature, and density, are measured at different times and positions across highly repeatable implosions using Thomson scattering (TS) and laser interferometry. This enables the calculation of all kinetic and internal energy components. Directed kinetic energy is also estimated from the radial implosion trajectory and initial mass distribution assuming snowplow accretion. Radiated energy is measured using a calibrated photoconducting diamond detector (PCD) and a bolometer. By including a means of discriminating between thermal and nonthermal broadening of the TS spectra, we observe features consistent with the hypothesis that small-scale turbulence mediates dissipation in a collisionless shock front. Assuming axial and azimuthal symmetry, reasonable agreement between measured and coupled energy is observed until peak current when axial nonuniformity and/or axial flux compression become significant.

Results of experimental research and numerical simulations related to two applications of the underwater electrical explosion (UEWE) of various symmetric wire arrays for the study of high energy density physics are presented. We explored how the rise-time of the generator affected energy deposition into underwater wire explosions, and the subsequent effects this had on shockwave generation and water flow. One application is the acceleration of flyer plates (targets) realized by strong shockwaves and subsequent Waterflow, generated by the UEWE of a planar wire array. In experiments, targets were accelerated to velocities of ≤1.3 km/s with energy efficiency transfer of ≤20% from the deposited energy. Another application is the generation of supersonic (~3-4 km/s) water jets by the UEWE of cylindrical/conical wire arrays. Jet generation occurs due to extremely high pressure and density of water formed in the vicinity of the axis by imploding shockwave and cumulation effect of converging shockwave at the edge of the cylindrical/conical array.

The study of the properties of matter at extreme conditions (>10 9 Pa, >10 4 K 0 ) is imperative for understanding phenomena where such states are reached in planetary astrophysics and confined fusion. To achieve such states of matter, what is often described as warm dense plasma, extremely high energy density deposition should be realized. One of the techniques used to explore matter under extreme conditions is the underwater electrical explosion of wire and wire arrays accompanied by the generation of strong shock waves.

Analytical modeling of the evolution of cylindrical and spherical shock waves (shocks) during an implosion in water is presented for an intermediate range of convergence radii. This range of radii was observed in experiments when the exploding wire expansion dynamics does not influence on shock propagation, but not yet described by well-known self-similar solutions. The model is based on an analysis of the change in pressure and kinetic energy density as well as on the corresponding fluxes of internal and kinetic energy densities behind the shock front. It shows that the spatial evolution of the shock velocity strongly depends on the initial compression, the adiabatic index of water, and the geometry of convergence. The model also explains the transition to a rapid like a self-similar increase in the shock velocity at only a certain radius of the shock that is observed in experiments. The dependence of the threshold radius, where the shock implosion follows the power law (quasi self-similarity), on the initial compression is determined. It is stated that in the entire range of the shock radii, the internal and kinetic energy density fluxes are equal, which agrees with known experimental data.

The commonly applied self-similar solution of the problem of the converging shock wave (shock) evolution with constant compression of the medium behind the shock front results in an unlimited increase in the medium velocity in the vicinity of the implosion. In this paper, the convergence of cylindrical shocks in water is analyzed using the mass conservation law, when the water compression behind the shock front is a variable. The model predicts a finite range of radii, which depends on the adiabatic index of water and where the increase in pressure exceeds the sum of the change in the kinetic and internal energy densities behind the shock front. In this range of radii, only the finite increase in the shock and water flow velocities is realized.

Underwater electrical explosion experiments of cylindrical or conical wire arrays accompanied by the generation of fast (up to ∼4500 m/s) water jets are presented. In these experiments, a pulse generator with a stored energy of up to ∼5.7 kJ, current amplitude of up to ∼340 kA, and rise time of ∼0.85 μs was used to electrically explode copper and aluminum wire arrays underwater. Streak and fast framing shadow imaging was used to extract the space–time resolved velocity of the ejected jet from the array while it propagates in air. The jet generation occurs due to high pressure and density of water formed in the vicinity of the array axis by the imploding shockwave. It was shown that the velocity of the jet ejected from the array depends on the array geometry and the thickness of the water layer above the array. The results suggest that ≥50% of the energy deposited into the array is transferred to the kinetic energy of this jet and the axial waterflow.

We report experimental and numerical results relevant to the dynamics in the vicinity of the implosion axis of a strong shock wave generated by an underwater electrical explosion of a cylindrical array. Experiments were conducted using both sub-microsecond and microsecond timescale pulsed generators with stored energy up to ∼5 kJ, delivering ∼420/350 kA with a 320/1000 ns rise time pulses to the array, respectively. The backlighted images of the converging shock wave and the light emission around the cylindrical axis indicate strongly that the shock wave front keeps its azimuthal uniformity as far as r ≈30 μm. Also, images obtained almost simultaneously with the implosion suggest symmetric convergence <2 ns prior to the shockwave's arrival to the axis. In addition, the light emission obtained by a photo-multiplying tube suggests the existence of a ∼200 ns long, almost constant, strong afterglow immediately following the light emission peak related to the implosion.