Ron Grikshtas

We present x-ray radiographs of underwater electrically driven exploding copper foils of different thickness, captured at the European X-ray Free-Electron Laser (EuXFEL) using 20 keV (∼30 eV bandwidth) self-amplified spontaneous emission photon beams. The foils were exploded by a ∼30 kA amplitude, ∼1.1 μs rise time current pulse produced by a compact pulsed-power generator. The radiographs reveal rapid frontal expansion of foils accompanied by lateral compression. This occurs due to strong magnetic pressure at the foil edges where, due to lateral skin-effect, the temperature reaches ∼1000 K leading to a fivefold decrease in the yield strength. Furthermore, the explosion begins with the foil edges, consistent with enhanced edge heating due to lateral skin-effect. With relatively thick foils, the bulk of the foil material remains in the liquid phase, and initial longitudinal density non-uniformities, coinciding with the current direction, transform into longitudinal striations. For thinner foils which experience explosion into a plasma, spatially non-uniform density structures appear at the edges of the foil and transverse the current direction. These density modulations, which can be estimated by x-ray attenuation, may be associated with the electrothermal instability (ETI); however, the linear theory of ETI growth does not appear to be applicable at the time of observation.

The Richtmyer-Meshkov instability (RMI) poses a major challenge in inertial confinement fusion (ICF) due to its role in mixing and performance degradation. We report the first experimental observation of passive freeze-out of RMI in a low-pressure surrogate regime, an instability stagnation effect induced without modifying the driving pressure pulse or the target surface geometry. Using additively manufactured subsurface voids in a sinusoidal target, we convert a single shock into a sequence of weaker shocks that suppress instability growth upstream of the surface by over 70%. High-speed x-ray imaging and hydrodynamic simulations suggest that this suppression arises primarily from temporal shaping, with lesser contributions from spatial curvature and shock weakening. Our results demonstrate a driver-independent pathway for controlling shock-driven hydrodynamic instabilities relevant to ICF and other high energy density systems.

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.

Underwater electrical explosions of single metallic wires driven by microsecond current pulses are investigated and compared with previously reported sub-microsecond experiments. Current and voltage waveforms, streak camera shadow imaging, and one-dimensional hydrodynamic simulations are employed to characterize how the energy density, energy density deposition rate, and the generated shock waves in water depend on the wire parameters. It was found that, similar to the sub-microsecond timescale, the solid–liquid phase transition occurs later than thermodynamic calculations predicted, while the liquid–vapor phase transition happens sooner than expected, leading to a two-phase coexistence. Additionally, most materials show a notable resistance peak (Ti, Fe, Ni, Zn, Ag, Sn, Ta, Au) compared to a quasi-plateau for Cu and Mo or a continuous increase for Al and Pt. Moreover, the specific action integral values are significantly smaller than those observed in wire explosion experiments in vacuum. Finally, the plasma formed at peak resistive voltage is non-ideal but exhibits lower electron density, ionization degree, and temperature compared to the sub-microsecond case.

We present results of an experimental study of underwater strip line configurations with exploding or non-exploding foil electrodes, using a sub-microsecond generator delivering a ∼100 kA current pulse rising in 290 ns. For thin (≤15-μm) exploding copper and aluminum foils, the onset of visible and infrared light emissions, obtained by fast photomultiplier tubes, was found to coincide with the onset of melting and boiling, respectively. This suggests that this method can be applied for studies of phase transitions in conductors. For thick (>100-μm) non-exploding foils, streak shadow imaging and photonic Doppler velocimetry reveal that foil vibrations are synchronous with current oscillations, driven by a restoring compressed water pressure of ∼90 MPa. A water flow propagating at the speed of sound is observed ∼100 ns after the onset of the current. Current distribution simulations confirm that mechanical deformations, rather than magnetic or thermal diffusion, are responsible for this effect. This finding provides a means to probe the elastic behavior of conductors under extreme pulsed-current loading.

Time- and space-resolved radiation emitted by the plasma produced by a 0.8 ns duration at full width half maximum, ~600 MW maximum power microwave (~9.6 GHz) pulse traversing a hydrogen-, helium-, or air-filled circular waveguide, is studied. Gas ionization by microwaves is an old subject but the regime investigated in the present experimental research, of very high-power microwaves and very short pulses using modern diagnostic tools, is new and follows a series of new studies performed so far only in our laboratory, revealing non-linear phenomena never observed before. In the present research, plasma radiation is observed along a slit made in a circular waveguide wall by either an intensified fast frame camera or a streak camera. Using calibrated input and output couplers, the transmission and reflection coefficients of the high-power microwaves were determined over a broad range of gas pressures, 0.1 kPa < P < 90 kPa. It was found that the intensity of the plasma light emission increases significantly after the high-power microwave pulse has left the waveguide. Depending on pressure, the radiation is either uniform along the slit, while the front of the emitted light follows the microwave pulse at a velocity close to its group velocity, or it remains in the vicinity of the input window, indicating that the plasma density is above critical density. It was also found that the radial distribution of radiation depends on pressure. At pressures 80 kPa, light emission is most intense on the axis where the electron oscillatory energy is ~100 eV and the ionization rate is maximal. We also studied the UV radiation from the plasma, the duration of which was found to be longer than the duration of visible light emission. This indicates the existence of energetic electrons for tens of ns after the high-power microwave pulse has left the observation region. Considering that the emitted light intensity depends on the plasma density and temperature, the observed data may be used for a comparison with the results of collisional radiative models if the electron time and spatial energy distribution is known.

We present the results of two studies: (1) on the specific action integral h and (2) on the electrothermal instability (ETI) during underwater electrical explosions of different material foils. Values of h were studied in experiments with the explosion of tapered (“butterfly”) foil geometries using current pulses with an amplitude of ∼300 kA and ∼400 ns rise time using the MAGEN [Kovalchuk et al., Rev. Sci. Instrum. 80, 083504 (2009)] generator at Technion. Shadow images of foils, together with strong shock waves generated in the surrounding water by their explosion and results of COMSOL [Asmedianov et al., J. Appl. Phys. 136, 133303 (2024)] numerical simulations, were used to determine the values of h. These values were found to be significantly smaller than those stated in earlier research of wire explosions in vacuum. ETI was studied with butterfly and rectangular shaped foils explosion using pulsed driver generating current pulses with an amplitude of ∼120 kA and ∼450 ns rise time and multi-frame x-ray radiography at the European Synchrotron Radiation Facility . In the case of the butterfly, x-ray radiography showed the ETI instability initially appeared at the “waist,” where later explosion starts. In the case of the rectangular foil, it was found that the ETI perturbations have a minimum wavelength and a broad spectrum of larger wavelengths.

We present measurements of the wavelength of electrothermal instabilities (ETI) formed during underwater electrical explosions of aluminum (Al), silver (Ag), and molybdenum (Mo) wires. Wires were exploded using a ∼450 ns rise time and ∼120 kA amplitude current pulse delivered by a pulse generator. Images of the exploding wires were captured by multi-frame synchrotron radiography at the ID19 beamline of the European Synchrotron Radiation Facility. Resolvable ETI was observed only in Al and Ag wires after the vaporization phase, whereas no such instabilities were detectable in Mo wires. Fourier analysis revealed that the ETI wavelengths in Al and Ag wires were comparable within the spatial resolution error, despite their different minimal instability wavelengths, which were predicted to develop during the melting phase. These minimal wavelengths were calculated using the linear ETI development theory and the simulated average wire temperature. The latter was calculated using one-dimensional hydrodynamic simulations, considering uniform current density across the wire cross-sectional area.

The results of experiments and numerical modeling show that a polyoxymethylene (Delrin) flyer is accelerated to nearly the same velocity (∼800 m/s) by either the underwater electrical sub-μs timescale explosion of 15 μm thick Al and Cu foils, or by the shock and water flow generated by the explosion of these foils. Experiments were carried out on the high-current generator delivering to the foil, a current of ∼280 kA with a rise time of ∼450 ns. The velocity of the flyer was determined using a photonic Doppler velocimeter and multi-frame shadow images of the flyer. It was shown that thermal expansion of the foil leads to the destruction of the flyer but it is not so when the flyer is accelerated by the shock and the water flow. Additionally, spallation of the methyl methacrylate disk used as a support to the flyer was observed, with a spall velocity of up to ∼1200 m/s.