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 similar to 1.1 mu s and a peak amplitude of similar to 28 kA. EuXFEL enabled MHz x-ray radiography of the wires using 20 keV photons and a narrow bandwidth of similar to 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.

The Richtmyer-Meshkov instability (RMI) characterizes mixing of 2 fluids of differing density under shock loading. Quantifying its behavior and exploring methods to mitigate its growth is vital for applications such as Inertial Confinement Fusion (ICF), but most explorations to date have been 1D planar in nature. We present experimental results from a campaign to demonstrate suppression of cylindrically convergent RMI using a novel engineered void technique [1]. A compact pulsed-power generator (120 kA in 600 ns) vaporizes a cylindrical wire array of 35mm diameter, driving a cylindrically converging shock wave traveling at $1.9 \text{km} \mathrm{s}^{-1}$ through a surrounding gelatine dielectric. The shock wave interacts with an azimuthally symmetric pattern of 12 air-filled voids in the gelatin that collapse under compression to split the initial shock into distinct, temporally-separated concentric shocks. These in turn interact with a cylindrical, sinusoidally perturbed interface between gelatin and air; characterized by Atwood number $\mathrm{A}=-1$, wavelength $\lambda=1.2 \text{mm}$, and amplitude $\mathrm{a}_{0}=0.12 \text{mm}$.

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 results of experimental studies and two-dimensional hydrodynamic simulations of underwater electrical explosions of semi-cylindrical wire arrays. In the experiments, a pulse generator delivers a current pulse of 250 kA amplitude rising in 1 µs to a 5-mm radius semi-cylindrical array of copper or aluminum wires. Experiment and simulation results display the generation of a radially symmetric and converging strong shock wave (SSW) and the generation of a blade-shaped supersonic jet (SJ) with a velocity reaching 1.6 km/s. Simulations predict that in the vicinity of the implosion axis, the density, pressure, and temperature can reach 1.5 g/cm3, 6 × 109 Pa and 500 K, respectively. Thus, this approach can be used for studies of properties of materials placed at the convergence axis or by interaction with the blade-shaped SJ with easy access of diagnostics. Additionally, the simulations show that the internal structure of the jet is not uniform, consisting of bubbles and voids that result in significantly smaller density than the normal density of water. Finally, explosions of arrays of different diameter wires result in the generation of converging SSW with satisfactorily uniform azimuthal distribution. This indicates a resistance positive feedback mechanism that stabilizes the explosion process across wires of different diameters.

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.

This paper presents the energy density deposition thresholds required for plasma formation on the surfaces of tungsten, tantalum, copper, and stainless steel targets. Experiments were conducted using the irradiation of targets bombarded with high-current electron beams of up to 600 GW/m2 power density. The electron beam was generated in a vacuum diode powered by a high-voltage pulse of up to ~250 kV with a duration of up to ∼600 ns. The radial distribution of the electron current density was determined using an array of collimated Faraday cups and X-ray diagnostics. The energy deposition thresholds for plasma formation were determined using the ion time-of-flight method, as well as time- and space-resolved spectroscopy. Additionally, scanning electron microscopy and electron backscattering diffraction were used to analyze the morphology of the targets before and after electron beam bombardment. The onset of plasma formation was observed prior to expected material melting across all ‎the investigated materials, and the thresholds of energy density deposition necessary for the plasma formation of the studied materials were determined. The results of these experiments are useful for predicting potential damage to fusion reactor walls and propulsion systems.

In a recent publication [IEEE Trans. Plasma Sci. vol.53, no. 11, pp. 3285-95, Nov. 2025], we reported on the operation of a novel X-band relativistic diffraction output magnetron (MDO) producing over 100MW microwave power at similar to 9.7GHz with similar to 50% maximum electronic efficiency. Here, we present the results of the performance of an MDO with a significantly shorter diffraction output antenna (DOA), which makes the overall dimensions smaller by half. Guided by 3-D particle-in-cell (PIC) simulations, the experiments demonstrate that this MDO produces similar to 9.5GHz high-power microwave (HPM) pulses reaching similar to 120MW maximum power, <= 20 ns pulse duration, and maximum instantaneous electronic efficiency reaching similar to 100%. We also demonstrated that this relativistic magnetron (RM) operates well when four longitudinal slits were cut symmetrically in the vanes of the eight-cavity anode block, transforming it into a four azimuthally segmented MDO. This allows one to decrease drastically the duration requirements of the external magnetic field and, respectively, the energy stored in the magnetic field's pulsed power supply. Finally, we present a novel design of a quarter-wave plate (QWP), which operates without breakdown at HPM powers exceeding 100 MW.

Recent experiments at the Technion Pulsed Power and Plasma Physics Laboratory investigated the properties and applications of dense plasmas generated by microsecond-timescale electrical explosions of wires and foils in water. Studies of explosions of different material single wires, revealed thresholds of the energy density deposition necessary for solid-liquid-vapor phase transitions and led to qualifying the wire materials in three groups of resistance development [1]. Studies of butterfly shaped and other foil geometries showed significantly smaller values of the specific action integral than those obtained in explosions in vacuum. These results are of primary importance for optimizing underwater explosions [2]. In collaboration with X-ray imaging facilities such as the ESRF and EuXFEL, underwater and air wire explosions were examined by flash X-ray radiography to observe electro-thermal and magneto-hydrodynamic instabilities, often encountered in Z-pinch configurations [3]. Complementary experiments with exploding foils demonstrated that acceleration of dielectric flyers by the generated water shocks is less destructive than using thermal shocks [4], [5], while explosions of V-shaped foils and semi-cylindrical wire arrays demonstrate enhanced supersonic blade-shaped jet generation and positive resistance feedback resulting in almost simultaneous explosions of wires of different diameters in wire arrays [6]. Thus, these experiments are very attractive and affordable means to investigating energy deposition, plasma instabilities, and shocks under extreme conditions, bridging laboratory scale pulsed power research and high-energy-density physics studies.

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.

This study investigates the plasma formed by a ∼300 MW, ∼0.8 ns, ∼9.6 GHz high-power microwave (HPM) pulse propagating inside a waveguide filled by helium or hydrogen gas at different pressures. It was found that HPM traverses He and H2 gas at pressures <1 and <0.5 kPa for gas, respectively, does not form sufficient plasma density to attenuate the pulse. The maximal plasma light emission intensity for helium and hydrogen plasmas at different gas pressures is reached within ≤5 ns, but for helium, the duration of the emitted light (∼20 ns) was significantly longer than for hydrogen (>7 ns). Using visible spectroscopy, the time-resolved density of the helium and hydrogen plasmas was determined. The plasma density continues to increase after the HPM pulse has left the system. Additionally, it was found that the polarization of the emitted light coincides with the direction of the HPM propagation, which is explained by the existence of an axial electric field in the plasma. This field accelerates free electrons, which transfer momentum to bound atomic electrons. Radial emission of electrons with energies up to 100 keV was also registered. This validates the results with the results of numerical simulations, which show that a large proportion of the electron energy spectrum at the tail of the HPM pulse is with energies >10 keV.

Explosive emission from an axial cathode of a relativistic magnetron produces plasma, the radial expansion of which can cause pulse shortening. In a split cathode fed magnetron, the electron source and its explosive plasma are outside the space where the high power microwave producing interaction occurs. This electron source is a longitudinal annular electron column expanding radially. This expansion simulates the radial emission from an axial cathode. A mathematical model and numerical simulations are presented which enable to calculate the parameters of this electron column, its density, angular velocity, and potential distributions.

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 similar to 300 kA and similar to 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 similar to 120 kA and similar to 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.

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, which is not described by the models of self-similar shock propagation far from and in the vicinity of the piston. 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. The model 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 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 is in agreement with known experimental data.

Experiments of a target accelerated by the shockwaves and water flow generated by underwater sub-μs timescale electrical explosion of a planar wire array are presented. The results of this experiment are compared with previous results [Maler et al., J. Appl. Phys. 129, 034901 (2021)] in which efficient target acceleration by μs-timescale underwater explosions of planar wire arrays was obtained. Although less energy is deposited into the wire array in the present experiments, the target acquires similar and even higher velocities compared to the previous research. This is considered to be associated with the higher energy density deposition rate, inducing faster radial wire expansion, and, consequently, the generation of a stronger shockwave and faster water flow behind its front.

The results of experimental studies together with numerical and analytical modeling showed that the acceleration of a target by employing the shock compression and water flow generated by the underwater electrical explosion of a wire array can be considered an efficient (up to ∼20%) approach. In experiments, a pulse generator with stored energy of ∼6.5 kJ, current amplitude of ∼380 kA, and rise time of ∼1.2 μs was used for underwater electrical explosion of a copper wire planar array. Streak shadow imaging and photonic Doppler velocimetry were applied to study the time-resolved velocity of the shock in water and an aluminum target in air, respectively. The targets, having different thicknesses and designs, were positioned at variable distances from the array. Experimental results showed that the target velocity evolution is characterized by an ns-timescale rise time peak with a subsequent decrease, which transfers to a μs-timescale increase up to its saturated value. Target velocities of up to 1360m/s were measured. The experimental, numerical, and analytical modeling results showed that a temporally unmovable barrier, located between the exploding array and the target, allows one to increase the pressure in that location, which leads to higher shock velocity in the target.

Studies of the equation of states (EOS) of different materials at extreme pressures and densities are of great importance for various fields of research. A commonly used method to achieve extreme conditions is shock compression. Shock compression can be achieved by numerous techniques, one of them is the flyer plate which can be accelerated using either explosives, gas guns or pulse magnetic field gradients. In our presentation we will describe the experimental setup and results of accelerating flyer plates using a strong shock wave generated by the underwater electrical explosion of a planar wire array. Explosion of a planar Cu wire array was produced using a µs and sub-µs-timescale generators with stored energies of up to ~6 kJ, delivering to the array a current pulse with amplitudes of up to ∼500 kA with rise time of ~400 ns and ~1 µs, respectively. A Photonic Doppler Velocimetry (PDV) is applied for determining the velocity of a free moving target.

A Photonic Doppler Velocimetry (PDV) is applied for determining the velocity of a free moving target accelerated by a strong shock wave and water flow generated by underwater electrical explosion of a planar wire array. Explosion of a planar Cu wire array was produced using a $\mu \mathrm{s}$-timescale generator with stored energies of ~3 kJ and ~6 kJ for different charging voltages, delivering to the array a current pulse with amplitudes of ~230 kA and ~330 kA and rise time of $-1\ \mathrm{\mu s}$.

A Photonic Doppler Velocimetry (PDV) is applied for determining the velocity of a free moving target accelerated by a strong shock wave and water flow generated by underwater electrical explosion of a planar wire array. Explosion of a planar Cu wire array was produced using a $\mu \mathrm{s}$-timescale generator with stored energies of ~3 kJ and ~6 kJ for different charging voltages, delivering to the array a current pulse with amplitudes of ~230 kA and ~330 kA and rise time of $-1\ \mathrm{\mu s}$.