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AbstractUnderstanding the physics of electromagnetic pulse (EMP) emission and nozzle damage is critical for the long-term operation of laser experiments with gas targets, particularly at facilities looking to produce stable sources of radiation at high repetition rates. We present a theoretical model of plasma formation and electrostatic charging when high-power lasers are focused inside gases. The model can be used to estimate the amplitude of gigahertz EMPs produced by the laser and the extent of damage to the gas jet nozzle. Looking at a range of laser and target properties relevant to existing high-power laser systems, we find that EMP fields of tens to hundreds of kV/m can be generated several metres from the gas jet. Model predictions are compared with measurements of EMPs, plasma formation and nozzle damage from two experiments on the VEGA-3 laser and one experiment on the Vulcan Petawatt laser.

An overview of results is given for laser ion sources based on the expanding laser plasma. All these experiments serve as a base for studies of possible, but not yet unambiguously determined mechanisms of ion production under various experimental conditions. The thermal ambipolar acceleration of ions guided either by thermal or suprathermal electrons, are assumed to be responsible for the ion streams expanding backward from the massive targets and registered by standard corpuscular diagnostics methods. A multi peak structure of the ion energy spectra is the most conspicuous phenomenon suggesting the presence of several fast electron groups guiding the plasma expansion and assisting the charge freezing due to the enhanced acceleration. The frequently observed asymmetry of the ion with respect to the laser caustic can be interpreted as the onset of self focusing of the heating laser beam inside the self-created plasma of the developing laser corona (or a pre-pulse plasma either formed by a double pulse or generated spontaneously in the case of a low contrast of the heating pulse) with a dramatic increase in the power density impinging on the target.

Interaction of sub-nanosecond intense laser pulses with foams containing fine and large pores has been studied experimentally. Laser penetration and energy transport in the foam material are measured via streaked side-on x-ray slit images. Shock wave transition through the foam is detected via streaked optical self-emission from foil attached on the foam rear side. The shock transition time increases with the pore size, foam density, and also with the contents of high Z additions in plastic foams. Foil acceleration is observed via 3-frame interferometry. Comparison of experimental results with numerical simulations and an analytical model is underway.

An overview of results is given for laser ion sources based on the expanding laser plasma. All these experiments serve as a base for studies of possible, but not yet unambiguously determined mechanisms of ion production under various experimental conditions. The thermal ambipolar acceleration of ions guided either by thermal or suprathermal electrons, are assumed to be responsible for the ion streams expanding backward from the massive targets and registered by standard corpuscular diagnostics methods. A multi peak structure of the ion energy spectra is the most conspicuous phenomenon suggesting the presence of several fast electron groups guiding the plasma expansion and assisting the charge freezing due to the enhanced acceleration. The frequently observed asymmetry of the ion with respect to the laser caustic can be interpreted as the onset of self focusing of the heating laser beam inside the self-created plasma of the developing laser corona (or a pre-pulse plasma either formed by a double pulse or generated spontaneously in the case of a low contrast of the heating pulse) with a dramatic increase in the power density impinging on the target.

Interactions of sub-nanosecond pulses of kJ-class iodine laser “PALS” with low-density foams and acceleration of Al foils by the pressure of the heated foam matter are investigated here, both experimentally and theoretically. X-ray streak camera is used for evaluation of the speed of energy transfer through the porous foam material. The shock-wave arrival on the rear side of the target is monitored by optical streak camera. Accelerated foil velocities, measured by three-frame optical interferometers, and shadowgraphs, reach up to 107 cm/s. The accelerated foil shape is smooth without any signature of small-scale structures present in the incident laser beam. Conversion efficiencies as high as 14% of the laser energy into the kinetic energy of Al foil are derived. Experimental results compare well with our two-dimensional hydrodynamics simulations and with an approximate analytical model.

Abstract The efficiency of crater creation for different types of Al targets, namely, single massive targets and double targets consisting of a foil or a disk placed before the massive target at a chosen distance (300 and 500 µm), is studied. Targets were irradiated by the PALS facility laser beam with EL = 100 – 400 J at the first harmonic ? = 1315 nm, a focal spot radius of 125 µm, and pulse duration of 400 ps. Velocities of the accelerated foil’s fragments or disks and electron density distributions of the plasma streams are determined by means of three-frame interferometry. Shapes and volumes of craters are obtained using the crater replica technology and microscopy measurements. It is shown that direct laser action is the most efficient way of energy transfer to the massive target and the most efficient method of crater creation. Somewhat lower efficiencies of shock wave loading and crater creation in comparison with direct laser action are found in the case of double targets where the energy is transferred to the massive target by colliding laser-driven foils or disks. The efficiencies of such a colliding energy transfer are close to 60% for foils and 40% for disks. The experimental results are in a good agreement with two-dimensional hydrodynamic models of shock wave generation under direct laser action and laser-driven macroparticle impact.

The results of investigations are presented which are concerned with laser radiation absorption in a target, the plasma state of its ablated material, the energy transfer to the solid target material, the characteristics of the shock wave and craters on the target surface. The investigation involved irradiation of a planar target by a subnanosecond plasma-producing laser pulse. The experiments were carried out with massive aluminium targets using the PALS iodine laser, whose pulse duration (0.4 ns) was much shorter than the shock wave attenuation and on-target crater formation times (50—200 ns). The investigations were conducted for a laser radiation energy of 100 J at two wavelengths of 0.438 and 1.315 mm. For a given pulse energy, the irradiation intensity was varied in a broad range (1013—1016 W cm-2) by varying the radius of the laser beam. The efficiency of laser radiation-to-shock energy transfer was determined as a function of the intensity and wavelength of laser radiation; also determined were the characteristics of the plasma plume and the shock wave propagating in the solid target, including the experimental conditions under which two-dimensional effects are highly significant.

Interactions of the laser beam of iodine laser 'PALS' with low-density foam targets and acceleration of Al foils by the pressure of heated foam matter are investigated here, both experimentally and theoretically. An x-ray streak camera was used for the evaluation of the speed of energy transfer through the porous foam material. X-ray streak records show no noticeable emission near the Al foil at the rear side of a 400?µm thick foam and, thus, Al foil is most likely accelerated without significant heating and expansion. Accelerated foil velocities, measured by three-frame optical shadowgraphy, reach up to 107?cm?s?1. Experimental results compare well with our two-dimensional hydrodynamics simulations and with an approximate analytical model.

Efficient energy transfer and smoothing effect in laser-irradiated polystyrene foam targets have been observed in preliminary experiments on the PALS iodine laser facility. A theory of laser light absorption region formation and ablation pressure generation in laser-produced plasma of porous matter has been developed and applied for discussion of the results obtained. In particular, two stages of homogenization of the porous matter, important for comprehension of the anomalously high absorption of laser radiation in supercritical foam matter, have been identified: the first, a considerably fast stage of partial homogenization, followed by a much slower second stage, leading to a uniform medium.

We present results of studies of the behavior of a laser-produced plasma in a strong external transverse magnetic field of above 10 T in induction generated from a flat teflon target at the laser power densities of about 1014 W/cm2. In the measurements of plasma parameters, a two-channel automated interferometric system (Pisarczyk et al. 1994) was applied. The interferometric measurements have shown that the transverse magnetic field induces an asymmetry of the plasma distinctly apparent in the interferograms registered in two mutually perpendicular directions. Due to the lack of axial symmetry of the plasma, the Abel's transformation could not be applied in quantitative processing of the interferograms. To reconstruct a spatial distribution of electron density in a plasma stream of disturbed axial symmetry, a methodology prepared for this experiment was used. Analyzing the spatial and temporal distributions of electron concentration obtained in the transverse magnetic field, it has been proved that the factor responsible for the disturbance of the axial symmetry of the plasma is the Rayleigh–Taylor (R–T) instability corresponding to the ion unmagnetization state.

The paper presents results of investigations of the process of the creation of a diamagnetic cavity by a laser-produced plasma in a strong transverse magnetic field of 10 T in induction. The plasma was produced from a flat, teflon target by an Nd laser of 5 J in energy and about 1 ns in pulse duration. The investigations were carried on by means of a three-frame interferometry and a non-contact magnetic probe, a so called remote magnetic probe (RMP). For the needs of this work, we developed a methodology for the measurement of the dimensions of the cavity during its expansion by means of the RMP, taking into account the plasma expansion geometry that is specific to flat targets. The dynamics of the process of creation of the diamagnetic cavity, its shape and dimensions have been determined. A very good consistency of dimensions of the diamagnetic cavity and plasma boundary in their quasisteady state, obtained with both diagnostics, allows us to apply the RMP method not only for the investigation of diamagnetic properties of various laser-produced plasmas, but also for the evaluation of their initial kinetic energy and its transformation in magnetic field.

The possibility of using the Faraday effect for the diagnostics of magnetic fields in a dense plasma is theoretically demonstrated. A procedure for measuring the plane of polariztion of the probing radiation is examined in detail, with account taken of the wave polarization and of the presence of plasma self-luminosity. A procedure for determining the spatial distribution of the magnetic field is described. A new three-channel polarointerferometer scheme is proposed. The idea behind the method is illustrated with reconstruction of the magnetic fields in a laser plasma as an example.

Observations were made of expansion, in a transverse quasisteady magnetic field (B ≍ 10 T), of a Teflon plasma generated by Nd laser radiation (λ = 1.06 μm, E ≍ 6 J, and τ ≍ 1ns). Time-integrated two-dimensional images of the plasma radiance were obtained in the spectrum of continuous and line x-ray radiation and spectroholiograms were recorded of 2¹P₁–1¹S₀ and 2³P₁–1¹S₀ lines of the He-like F VIII ion. The plasma radiation was found to be anisotropic relative to the expansion axis and the dependence of the intensity on the distance from the target changed as a result of the influence of the transverse magnetic field on the plasma dynamics.

Interaction of sub-nanosecond intense laser pulses with foams containing fine and large pores has been studied experimentally. Laser penetration and energy transport in the foam material are measured via streaked side-on x-ray slit images. Shock wave transition through the foam is detected via streaked optical self-emission from foil attached on the foam rear side. The shock transition time increases with the pore size, foam density, and also with the contents of high Z additions in plastic foams. Foil acceleration is observed via 3-frame interferometry. Comparison of experimental results with numerical simulations and an analytical model is underway.

We present the results of experiments on the short-term irradiation of a solid material by a laser beam. The data testify to a rise in efficiency of the energy transfer from the laser pulse to a shock wave due to the fast-electron energy transfer. The experiments were performed with massive aluminium targets on the PALS iodine laser, whose pulse duration (0.4 ns) was much shorter than the time of shock decay and crater formation in the target (50–200 ns). The irradiation experiments were carried out using the fundamental laser harmonic (1.315 μm) with an energy of 360 J. The greater part of the experiments were performed for the radiation intensity exceeding 1015W cm-2, which corresponded to the efficient generation of fast electrons under the conditions where the relatively long-wavelength iodine-laser radiation was employed. The irradiation intensity was varied by varying the laser beam radius for a specified pulse energy.