We examine the effect of laser focusing on the effectiveness of a recently discussed scheme [M. F. Ciappina et al., Phys. Rev. A 99, 043405 (2019) and M. F. Ciappina and S. V. Popruzhenko, Laser Phys. Lett. 17, 025301 (2020)] for in situ determination of ultrahigh intensities of electromagnetic radiation delivered by multi-petawatt laser facilities. Using two model intensity distributions in the focus of a laser beam, we show how the resulting yields of highly charged ions generated in the process of multiple sequential tunneling of electrons from atoms depend on the shapes of these distributions. Our findings lead to the conclusion that an accurate extraction of the peak laser intensity can be made either in the near-threshold regime, when the production of the highest charge state happens only in a small part of the laser focus close to the point where the intensity is maximal or through the determination of the points where the ion yields of close charges become equal. We show that for realistic parameters of the gas target, the number of ions generated in the central part of the focus in the threshold regime should be sufficient for a reliable measurement with highly sensitive time-of-flight detectors. Although the positions of the intersection points generally depend on the focal shape, they can be used to localize the peak intensity value in certain intervals. Finally, the slope of the intensity-dependent ion yields is shown to be robust with respect to both the focal spot size and the spatial distribution of the laser intensity in the focus. When these slopes can be measured, they will provide the most accurate determination of the peak intensity value within the considered tunnel ionization scheme. In addition to this analysis, we discuss the method in comparison with other recently proposed approaches for direct measurement of extreme laser intensities.

Investigations were carried out at the multistage hybrid Ti:sapphire–KrF laser facility GARPUN-MTW on the direct amplification of TW-power picosecond UV laser pulses in e-beam-pumped KrF amplifiers and propagation along a 100 m laboratory air pass. The experiments identified the main nonlinear effects and their impact on the amplification efficiency, amplifier optics degradation, beam quality and focusability, and the evolution of radiation spectra. The research was performed towards an implementation of the shock-ignition concept of inertial-confinement fusion using krypton fluoride laser drivers.

We present here experimental results on the optimization of the mega-electronvolt ion source from the target front surface by using relativistic (10¹⁸ W/cm²) interactions with ultra-short laser pulses (50 fs). The source perturbation in the accelerated proton/ion beam was primarily controlled by the addition of a pre-pulse to main pulse contrast ratio. The 2D particle-in-cell simulations agreed well with the observed experimental results for the ion source perturbation and mitigation. This work provides insights into ion source perturbations (temporal and spatial) and the need to control them in intense laser–plasma interactions. Our results may assist in the efficient guiding of proton/ion beams to the core of fusion fuel or of ions in cancer therapy.

Specimens of materials for prospective use in chambers of nuclear fusion reactors with inertial plasma confinement, namely, W, ODS steels, Eurofer 97 steel, a number of ceramics, etc., have been irradiated by dense plasma focus devices and a laser in the Q-switched mode of operation with a wide range of parameters, including some that noticeably exceeded those expected in reactors. By means of 1-ns laser interferometry and neutron measurements, the characteristics of plasma streams and fast ion beams, as well as the dynamics of their interaction with solid-state targets, have been investigated. 3D profilometry, optical and scanning electron microscopy, atomic emission spectroscopy, X-ray elemental and structural analyses, and precise weighing of specimens before and after irradiation have provided data on the roughening threshold and the susceptibility to damage of the materials under investigation. Analysis of the results, together with numerical modeling, has revealed the important role of shock waves in the damage processes. It has been shown that a so-called integral damage factor may be used only within restricted ranges of the irradiation parameters. It has also been found that in the irradiation regime with well-developed gasdynamic motion of secondary plasma, the overall amount of radiation energy is spent preferentially either on removing large masses of cool matter from the material surface or on heating a small amount of plasma to high temperature (and, consequently, imparting to it a high velocity), depending on the power flux density and characteristics of the pulsed irradiation.

The paper discusses a possible energy transformation that leads to the acceleration of fast ions and electrons. In plasma-focus discharges that occur during deuterium filling, which have a maximum current of about 1 MA, the accelerated deuterons produce fast fusion neutrons and fast electrons hard X-ray emissions. Their total energy, which is of the order of several kilojoules, can be delivered by the discharge through a magnetic dynamo and self-organization to the ordered plasma structures that are formed in a pinch during the several hundreds of nanoseconds of the pinch implosion, stagnation, and evolution of instabilities. This energy is finally released during the decay of the ordered plasma structures in the volume between the anode face and the umbrella front of the plasma and current sheath in the form of induced electric fields that accelerate fast electrons and ions.

Underwater shock waves generated by pulsed electrical discharges are an effective, economical, and environmentally friendly means of stimulating reservoirs, and this technology has received much attention and intensive research in the past few years. This paper reviews the main results of recent work on underwater electrical wire explosion (UEWE) for reservoir stimulation. A platform is developed for microsecond single-wire explosions in water, and diagnostics based on a voltage probe, current coil, pressure probe, photodiode, and spectrometer are used to characterize the UEWE process and accompanying shock waves. First, the UEWE characteristics under different discharge types are studied and general principles are clarified. Second, the shock-wave generation mechanism is investigated experimentally by interrupting the electrical energy injection into the wire at different stages of the wire-explosion process. It is found that the vaporization process is vital for the formation of shock waves, whereas the energy deposited after voltage collapse has only a limited effect. Furthermore, the relationships between the electrical-circuit and shock-wave parameters are investigated, and an empirical approach is developed for estimating the shock-wave parameters. Third, how the wire material and water state affect the wire-explosion process is studied. To adjust the shock-wave parameters, a promising method concerning energetic material load is proposed and tested. Finally, the fracturing effect of the pulsed-discharge shock waves is discussed, as briefly are some of the difficulties associated with UEWE-based reservoir stimulation.

A divergent gas-puff Z pinch has been devised for the realization of an efficient soft x-ray point source. In this device, a divergent hollow annular gas puff is ejected outward from the surface of the inner electrode, and the plasma is compressed three-dimensionally to generate a soft x-ray point source. In the SHOTGUN III-U device at Nihon University, the power supply was enhanced, and experiments were conducted over a larger current range. The peak current at the charging voltage of -25 kV was -190 kA. Ar was used as the discharge gas. The self-contraction process of the plasma was investigated in detail using a gated camera. Near the peak current, local contraction occurred in front of the inner electrode. The contraction velocity of the plasma was 5.5 × 10⁴ m/s. As the plasma contracted, the discharge current decreased. The energy input was analyzed by induction acceleration. The net input energy was found to be 750 J, which corresponded to 13.3% of the stored energy of the capacitor, 5630 J. The soft x-ray source was observed using a soft x-ray CCD camera. A point source was observed 7 mm in front of the inner electrode. The size of the source was 35 μm in the axial direction and 14 μm in the radial direction.

Revised simulations of ALT-like devices are presented. The results from these simulations closely match those from experiments and demonstrate the capabilities of the devices as applied to ramp compression of metals to pressures of 20 Mbar by imploding liners driven by ～10 MG azimuthal magnetic fields (with currents up to 55 MA). These results can be applied to the design of experiments on isentropic compression of materials.