Counter-streaming plasma flows are a central focus of many research groups seeking to generate high-energy-density (HED) conditions to investigate astrophysical processes such as particle acceleration, shock formation, and energy transfer, as well as to explore inertial confinement fusion (ICF) concepts, for example, field-reversed configurations [1]–[4]. Shock formation in both collisional and collisionless regimes remains not fully understood, particularly with regard to mechanisms of energy dissipation. In the collisional regime-relevant to ICF, supernova explosions, and stellar interior dynamics-investigating shocks is critical for improving our understanding of plasma compression, validating equations of state, and determining fusion cross-section thresholds at relatively low ion energies. The current research aims to develop a platform for HED plasma flow generation spanning regimes from highly collisional, dense, fluid-like plasmas to weakly collisional, mostly kinetic ones. As part of this effort, we have designed and manufactured a pulsed-power plasma gun, with a diagnostic platform currently under development [5]. The plasma gun includes a high-voltage $1.33 \mu ~\mathrm{F}$ capacitor and a spark-gap switch triggered by a ~ 40 kV pulse from a triggering generator. It has two coaxial electrodes: an inner central electrode (cathode) and an outer cylindrical electrode (anode), separated by a dielectric insulator. The plasma is generated via a surface breakdown mechanism and accelerated with the $\mathbf{J} \times \mathbf{B}$ force toward the interaction region. In this poster, we present the results of an experimental characterization of this plasma gun. Lineintegrated density measurements were performed with a laser-based Mach-Zehnder interferometer, bulk plasma flow velocity was measured via a Doppler-shifted line, and discharge current and resistive voltage were measured with a Rogowski coil and voltage divider, respectively.

The electron velocity distribution function in the plasma, formed by gas ionization by a microwave pulse of sub-nanosecond timescale width and hundreds of megawatts power, is studied by a theoretical model and by 3D numerical simulations which confirm quite well the model. It is shown that the distribution function is defined by the field amplitude variation during the entire pulse. After the pulse's passage through the gas, the remaining plasma distribution function follows a decreasing power-law function. Experiments performed in a waveguide filled with helium gas confirm that energetic (from several keV to several tens of keV) electrons remain in plasma long after the pulse has crossed the experimental volume. These electrons continue the gas ionization over extended times up to tens of nanoseconds.

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.

We demonstrate remote thermoacoustic detection of biological tissue and conductive materials using pulsed RF excitation and audible-range acoustic sensing. Short RF pulses (4.5 mu s, 2.865 GHz, 275 kW) generated wideband acoustic signals, which were detected by a calibrated microphone at distances of over 1 m through obstructions. Distinct acoustic signatures were recorded for tissue, saline, steel, and RF absorber targets. Surface measurements indicate that signal amplitudes are sufficient for unaided auditory perception. These results support the feasibility of non-contact RF-thermoacoustic signal generation for remote sensing and communication.

This study investigates the ionization pressure threshold of a gas (air, helium, argon, and SF6 across a wide pressure range) filled dielectric tube when a similar to 300 MW, similar to 0.7 ns, 9.6 GHz high-power microwave (HPM) pulse propagates through it. The thresholds are determined as the pressure for which the energy of the transmitted HPM pulse decreases to similar to 30%, which is close to the same HPM pulse's transmission coefficient when a metal rod fills the tube. These thresholds are found to be 0.4 x 10(5) Pa,10(5) Pa, 1.8 x 10(5) Pa, and 0.2 x 10(5) Pa, for air, argon, helium, and SF6, respectively. The measured intensity of the plasma light emission starts to decrease at a pressure which coincides with the pressure threshold determined by HPM pulse propagation. Additionally, at gas pressures <5 x 10(4) Pa, it is shown that time- and space-resolved images of the light emission display a diffused plasma which at higher pressures >10(5) Pa transforms into streamer like plasma. Simplified numerical simulations of a microwave discharge in air at 1 x 10(5) Pa and 4 x 10(5) Pa are consistent with the experimental plasma light observations. (c) 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

Pulsed-power-driven underwater electrical explosion of cylindrical or conical wire arrays produces supersonic water jets that emerge from a bath, propagating through the air above it. Interaction of these jets with solid targets may represent a new platform for attaining materials at high pressure (>1010 Pa) conditions in a university-scale laboratory. However, measurements of the internal structure of such jets and how they interact with targets are difficult optically due to large densities and density contrasts involved. We utilized multi-frame x-ray radiographic imaging capabilities of the ID19 beamline at the European Synchrotron Radiation Facility to explore the water jet and its interaction with a 50 μm thick copper foil placed a few mm from the surface of water. The jet was generated with a ∼130 kA-amplitude current pulse of ∼450 ns rise time applied to a conical wire array. X-ray imaging revealed a droplet-type structure of the jet with an average density of <400 kg/m3 propagating with a velocity of ∼1400 m/s. Measurements of deformation and subsequent perforation of the target by the jet suggested pressures at the jet–target interface of ∼5 × 109 Pa. The results were compared to hydrodynamic simulations for better understanding of the jet parameters and their interaction with the foil target. These results can be used in future research to optimize the platform, and extend it to larger jet velocities in the case of higher driving currents supplied to the wire array.

Recently, in a series of articles Mumtaz, Choi, and others [i.e., S. Mumtaz and E. Choi, IEEE Electronic Device Lett. 43, 1756 (2022)] studied a relativistic gridded vircator with one or more dielectric anodes, in which considerable increase in the efficiency of high-power microwave (HPM) generation compared to ordinary vircators was observed. This is very important since the main disadvantage of vircators is their low efficiency. It was suggested that the repelling of electrons by the initial impacting electron charge collected on the dielectric reflectors forms a multi-vircator, which increases HPM production. The present article reports the results of experiments and numerical simulations, which show that this mechanism does not result in high multi-vircator efficiency. Similar dielectric anodes were investigated 50 years ago in a configuration known as the Luce diode, in which collective acceleration of ions was studied. It was also known that the operation of the Luce diode is accompanied by a HPM burst, which was never measured. Results of experiments and simulations in various configurations based on a gridded vircator and dielectric reflectors confirm the existence of high-energy ion acceleration. Unfortunately, none of these configurations produce HPM of higher efficiency than a regular gridded vircator.

It was observed experimentally that after crossing a waveguide filled with a neutral gas a short powerful microwave pulse leaves a periodic glow of plasma along the waveguide, persisting for several tens of nanoseconds. A theoretical model is presented which in combination with numerical simulations proposes a possible explanation for this phenomenon.

A relativistic magnetron (RM) with diffraction out-put, modified from the original Kovalev, FuksX-band magnetron(MDO) presented more than 50 years ago, has been designedand studied experimentally using pulse generators with voltageamplitudes <= 300 kV and pulse duration <= 200 ns.X-bandmagnetrons are naturally small devices, and for high voltages,microwave pulse shortening occurs because of cathode explosiveemission plasma expansion. A split cathode can potentiallysolve this problem, but in this research, the MDO with splitcathode experienced pulse shortening as well. It is suggestedbased on preliminary results of simulations using the MAGICparticle-in-cell (PIC) code that the reason for this is that thismagnetron needs optimization by simulations that were notavailable originally

The semiconductor opening switch (SOS) diode, invented in Russia around three decades ago, is attractive for a variety of pulsed power applications due to its ability to interrupt currents of thousands of amperes in several nanoseconds in inductive storage generators. This allows delivering hundreds of megawatts into the load. The SOS has a p(+)-p-n-n(+) structure, where the p-n junction depth could be similar to 200 mu m. Thus, the fabrication process of such structure requires a long diffusion time at a very high temperature, and was not suited for mass production in regular fabs. We present the design, simulation, fabrication, and testing of a Si SOS diode developed at Soreq NRC. It consists of a 180 mu m epitaxial structure with a p-n junction depth of similar to 110 mu m. In a single die testing, we used a fast driving circuit. A peak negative voltage of 1515 V with a rise time of 1.56 ns was obtained on a matched 50 Omega load. The voltage rise rate, of 0.97 kV/ns, is the highest record obtained for a single Si current interruption die. In order to test the diode at high voltages (HVs), we used two setups of SOS-based generators based on magnetic compression. We compared its original SOS diode developed in Russia with a stack of epi-SOS diodes. In the first setup, an epi-SOS made of 65 dies in series with a cross section of 25 mm(2) was used. The reverse current was 927 A. The peak load voltage on a 46 load was 37.4 kV with a rise time of 25 ns. In the second setup, an epi-SOS made of 130 dies in series with various cross sections of 1, 2, and 3 cm(2) was used. For the 3 cm(2) stack, a 173-kV, 7.5-ns rise time pulse was obtained on a similar to 224 Omega low-inductance resistive load.

In earlier research Phys. Plasmas 27, 103102 (2020), a split cathode was proposed to avoid pulse shortening of microwave generation in relativistic $\text{S}$ -band magnetrons. Experiments confirmed the generation of microwave pulses limited only by the power generator’s pulselength ( $\sim$ 200 ns) J. Appl. Phys. 131, 023301 (2022). In the current research, the results of experiments with the same magnetron but powered by a generator producing up to $\sim$ 500-ns-long high-voltage pulses are presented. It is shown that the power and duration of the microwaves depend strongly on the applied magnetic field, anode–cathode gap length, and the applied voltage amplitude. In the experiments, $\sim$ 400-ns, $\sim$ 100-MW microwave pulses were measured. It was also identified that impedance matching between the relativistic magnetron (RM) load and the high-voltage (HV) pulse generator is an important factor in the operation of these devices.

Results of an experimental research and one-dimensional hydrodynamical simulations of critically damped sub-microsecond timescale underwater electrical explosions of wires made of 12 different materials are presented. Using current and voltage waveforms, streak shadow images of the shocks generated in water and wire expansion obtained by one-dimensional hydrodynamic simulations, the maximal values of the energy density, energy density deposition rates, and specific action integrals were determined. It is shown that for all study materials, the deposited energy density significantly exceeds the energy density required for the solid–liquid phase transition but is substantially smaller to induce a full liquid–vapor phase transition of the wire. At the time when the maximal value of the deposited power is realized, the deposited energy densities were found to be larger than the atomization energy for all materials. Estimates of the plasma parameters show that the explosion of the wires can be characterized by a high resistance and lowly ionized weakly coupled plasma. Three groups of materials were distinguished by either decrease, plateau, or increase in the resistance after the maximum of the deposited power. It was confirmed that the observed maximum Planckian temperature for all wire material does not exceed 6000 K due to the “bath” effect and that there is a correlation between the wire radial expansion and the strong shock wave velocities.

Results of experimental research and two-dimensional hydrodynamical simulations of close to critically damped microsecond timescale underwater electrical explosions of butterfly-shaped foils for six different materials are presented. Using current and voltage waveforms along with multi-frame shadow images of the shocks generated in water, the values of the specific action integral, h, were determined. It is shown that values of h can be calculated based on the average current density and that its value (within error bars) does not change in the range of current densities (0.5-1) x 10(8) A/cm(2). The values of h were found to be consistent with those obtained for sub-microsecond underwater electrical explosions of wires made of the same material but differ from those obtained in earlier research with explosion of wires in vacuum. (c) 2024 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

Pressure ionization threshold of the high-power microwave pulse (~ 250 – 300 MW) for air, He, Ar and SF 6 gases has been defined in the range of pressure ~30 – 170 kPa. Diffusive or streamer-like plasma formation was observed depending on the gas pressure in the range 10 – 350 kPa. Time evolution of the plasma density produced by the microwave pulse itself in the gas filled waveguide was investigated by time-resolved visible spectroscopy of He I spectral lines at the gas pressure corresponding to the most effective plasma formation ~ 2 kPa. The plasma density is estimated ~ 10 15 cm -3.

Theta pinch is a well-known approach applicable for inertial fusion research. The theta pinch operation is based on plasma generation through gas ionization by induced electric field. The plasma radial compression occurs due to magnetic field pressure produced by an azimuthally driving current. In our research, we use a $1 \mu \mathrm{F}$ capacitor charged to 17 kV. This capacitor is discharged to a singly loop, encompassing a glass tube filled with either hydrogen or helium gas, producing underdamped discharge with $1.5 \mu \mathrm{s}$ rise-time and a peak current of 17.5 kA. We will report on time- and space resolved parameters of the hydrogen and helium which are obtained using frame images of the plasma light emission, microwave cutoff, laser interferometry, plasma spectroscopy, Thompson scattering and Laser Induced Florescence at pressures of $0.1-3$ Torr during plasma compression.

An experimental method to measure the electric fields existing in a plasma wake produced by a ∼0.24 GW, ∼0.5 ns, 9.5 GHz microwave pulse traversing a plasma-filled waveguide is presented. The intensity of the second harmonic of a 30 fs 800 nm laser generated inside a gas-filled dielectric tube placed inside the waveguide is used to characterize the wakefield parameters. Three distinct decaying oscillations of the plasma wakefield, with peak amplitude of ∼20 kV/cm, were observed. The experimental results were confirmed by 3D large-scale plasma particle-in-cell simulations.

We present results exploring various methods of aluminum flyer acceleration. One method uses the shock wave generated by underwater electrical explosions of thin foils supplied by a pulse generator with stored energy of ∼4.7 kJ. Utilizing the shock created by an exploding foil, a maximal free flyer velocity of ∼2000 m/s is obtained. This acceleration method is compared to results exploiting only magnetic pushing to accelerate flyers using a common strip-line configuration, resulting in much lower velocities of ∼300 m/s. We also present a modified strip-line configuration, for which a significant increase in the flyer velocity to ∼1200 m/s is measured. Finally, a hybrid strip configuration, incorporating both the effects of magnetic pushing and acceleration by exploding foil and its subsequent shock wave, results in ∼1400 m/s flyer velocity. These experimental results are analyzed by numerical simulations and analytical modeling of the conservation equations of mass and momentum.

We present results on underwater electrical explosions of thin aluminum and copper foils using a generator delivering ∼200 kA current amplitude, ∼0.9 μs rise time pulses. Time-resolved shadow imaging displays the generation of a strong planar shock wave in water in the vicinity of the exploding foil. Using time-resolved spectroscopy, aluminum oxide (AlO) absorption bands were observed in a Planckian-like spectrum, indicating that aluminum combustion starts when aluminum vaporizes. It is also shown that the strongest shock wave is obtained for the largest linear energy deposition rate to the foil.

Two interacting supersonic water jets and collisions of a water jet with an aluminum target are studied experimentally and by hydrodynamic simulations. Supersonic water jets form, when shocks generated by underwater electrical explosions of conical wire arrays converge. The arrays are supplied by a ∼250 kA, ∼1 μs rise time current pulse. Underwater explosion of two conical arrays placed face to face produces jets propagating in air with velocities of ∼2.5×103 m/s leading to hot plasma formation at a temperature of ∼2200–3000 K, pressure ∼1.7×1010 Pa, and density >1029 m−3. When a single array explodes underwater in front of an aluminum target, the collision of the jet with the target produces a local pressure of ∼3×1010 Pa on the surface of the target.