A neodymium-glass diode-pumped amplifier with a zigzag laser beam propagation through the active medium was elaborated; the amplifier is intended for operation in a repetitively pulsed laser. An amplifier unit with an aperture of 20 x 25 mm and a similar to 40-cm long active medium was put to a test. The energy of pump radiation amounts to 140 J at a wavelength of 806 nm for a pump duration of 550 mu s. The energy parameters of the amplifier were experimentally determined: the small-signal gain per pass similar to 3.2, the linear gain similar to 0.031 cm(-1) with a nonuniformity of its distribution over the aperture within 15%, the stored energy of 0.16-0.21 J cm(-3). The wavefront distortions in the zigzag laser-beam propagation through the active element of the amplifier did not exceed 0.4 lambda (lambda = 0.63 mu m is the probing radiation wavelength).

A train of a few tens of high-power subnanosecond laser pulses with a repetition period of 10 ns is generated in the Iskra-5 facility. The laser pulse train has an energy of up to 300 J and contains up to 40 pulses (by the 0.15 intensity level), the single pulse duration in the train being ∼0.5 ns. The results of experiments on conversion of a train of laser pulses to a train of X-ray pulses are presented. Upon irradiation of a tungsten target, a train of X-ray pulses is generated with the shape of an envelope in the spectral band from 0.18 to 0.28 keV similar to that of the envelope of the laser pulse train. The duration of a single X-ray pulse in the train is equal to that of a single laser pulse.

The effect which manifests itself in the form of directed electromagnetic pulses (EMP) initiated by an X-ray incident obliquely upon a conducting surface has been confirmed and investigated experimentally in detail. A planar accelerating diode comprising a metallic cathode and grid anode was initiated with an oblique short soft-X-ray pulse from a point laser-plasma source. Then a source of directed EMP-a current of accelerated photoelectrons-was formed whose boundary ran along the anode external surface with a faster-than-light velocity.The plasma was formed when short-pulse (similar to 0.3 ns) laser radiation from ISKRA-5 facility was focused on a plane Au target. The amplitude-in-time and spatial characteristics of radiation emitted by the faster-than-light source have been measured. Parameters of the accelerated electron current have been measured too. (C) 2006 Published by Elsevier Ltd.

ISKRA-5 facility [1] were used to perform investigations of the effect of controlled X-ray field asymmetry on the compression degree and the neutron yield in indirect (X-ray) drive targets [2]. 12 beams of ISKRA-5 facility were input into the spherical gold hohlraum of 2 mm in diameter [3] through 6 laser entrance holes of 600 micron in diameter. The glass spherical target (microballoon) with DT gas was placed inside the box. The laser radiation input was on the inner surface of the hohlraum. The typical input laser energy was between 6.3-7.8 kJ and the FWHM pulse duration was 0.28-0.39 ns. X-ray generated on the wall of the box irradiates the glass microballoon. The field asymmetry was induced by shift the central glass capsule with respect to the center of hohlraum forward to the special 7 hole 600 micron in diameter. In different experiments relative shift ∆/Rbox was varied as 0, 1/3, 1/2, 2/3, 1 (here ∆ is a shift, Rbox is a hohlraum radius). The experiments demonstrated neutron yield reduction with increasing of asymmetry degree (see figure 1). In parallel with this fact, shifting of microballoon about central position caused the rise of compression time τγn, which was determined by indirect method using the compressed region X-ray luminescence [4]. To carry out analysis and comparison with 2D calculations we show on figure 2 the time delay of neutron generation as a function of target shift calculated relative the time delay of experiment with zeros (symmetrical case) shift.

The effect which manifests itself in the form of directed electromagnetic pulses initiated by an X-ray incident obliquely on the conducting surface has been confirmed and investigated experimentally in detail. The existence of the above-mentioned phenomenon was predicted more than 25 years ago. So, experimentally, a planar accelerating diode comprising a metallic cathode and grid anode was initiated by an oblique short soft X-ray pulse from a point laser plasma source. Then, the directed EMP source, i.e., the accelerated photoelectron current, was formed, whose boundary ran along the external surface of the anode at a faster-than-light velocity. The plasma was formed when a short-pulse (/spl sim/0.3 ns) laser radiation from ISKRA-5 facility was focused onto a plane Au target. The amplitude-in-time and spatial characteristics of the radiation emitted by the faster-than-light source are measured. The parameters of the photo-emission current and accelerated electron current are measured.

The experiments measuring the density of DT mixture compressed in indirect drive targets (X-ray targets) were conducted on the ISKRA-5 facility. The density was determined from the line broadening of H- and He-like Ar doped in DT-gas as a diagnostic substance. A series of three experiments with the X-ray targets having different shell thickness of capsule filled with DT + Ar mixture were carried out. In two of the three experiments, radiation spectra of Ar were recorded and the density of compressed gas was determined. The analysis of the experimental results for the X-ray target with a 280-μm diameter and a 7 μm wall thickness revealed that the density of the compressed gas may be estimated as ∼1 g/cm3.

Two shells with the diameter of 0.8–0.9 mm and a wall thickness of ≅1 μm were produced at the Lebedev Physics Institute for the experiments conducted at the ISKRA-5 facility. The results of two experiments with the aforementioned shells conducted at the ISKRA-5 facility with the use of an indirect-drive set up. In one of the experiments, the diameter of the golden hohlraum was D = 2 mm while in the other it was D = 4 mm. In these experiments it was observed to be ≅4 times the difference of the average laser intensity on the hohlraum surface. The results of computational analysis of the experiments are also presented here.

The first experiments measuring the density of a compressed deuterium and tritium mixture in microtargets of indirect irradiation (x-ray targets) were performed at the Iskra-5 facility. The density was determined according to the broadening of the lines of hydrogen-and helium-like argon added to the DT gas as a diagnostics material. A series of three experiments was performed with x-ray targets in which the central capsule filled with a DT + Ar mixture over a range of shell thicknesses. In two of the experiments, argon emission spectra were recorded and the density of the compressed gas was determined. For a microtarget approximately 280 μ m in diameter with a wall approximately 7 μ m thick, an analysis of the experimental results yielded an estimated density in the compressed gas of ∼1 g/cm 3 . Gas-dynamic calculations using the SNDA (spectral nonequilibrium diffusion with absorption) program show that argon emission takes place just after reaching maximum temperature, but much sooner than maximum compression. The results of a calculation for an experiment with low relative Ar concentration are in overall agreement with the experimental data. Additional investigations are needed to interpret experiments at a relatively high concentration.

Experiments on the indirect (x-ray) irradiation of high-aspect-ratio capsules (with a diameter-to-thickness ratio ≈900) filled with DT gas are performed on the Iskra-5 laser facility. It is shown that all the characteristics measured (neutron yield, ion temperature, shell implosion time, etc.) are faithfully reproduced in calculations based on the one-dimensional SNDA (spectral nonequilibrium diffusion of absorption) program for nonequilibrium radiation gas dynamics. The calculations provide an explanation for the experimentally detected generation of a smaller number of neutrons in an experiment with a higher measured value for the ion temperature of DT gas.

The production and characteristics of diagnostic europium-doped glass targets are described. They are made to calibrate the collector for measurements of the product of the compressed target density and its radius [rhoR] in laser thermonuclear fusion experiments. Europium is added to the initial solution of the liquid glass in the citrate form to ensure a high concentration and uniform distribution in the glass microspheres.

The first experiments have been carried out on the hot plasma produced during indirect illumination of microballoons containing a gaseous DT mixture at the Iskra-5 laser installation. The integrated yield of DT neutrons reached 6 X 10(9) per pulse.

The numerical simulation for the target compression exploding shell-regime experiments performed in the system "Spark-4" with the principal frequency pulse of the iodine lambda = 1.315-mu-m laser demonstrates that correlation is observed in the increase of the divergence of the calculated neutron yield compared with the experimental one and growth of the aspect ratio of the target shell used in the experiment. A series of experiments on the high aspect target A(s) > 300 compression is performed after switching the system "Spark-4" onto the second harmonic and improving the beam structure in the region of the target mounting. In this series the neutron yield record N = 6.10(7) for the system in experiments with the glass-shelled targets is observed.

It is established that the activation of phase transitions by stresses can impart nonlinearity to resonance vibrations of materials near diffusionless phase-transition temperatures. Such nonlinearity is manifested in the asymmetry of the resonance peaks, a dependence of their shape on the direction in which the frequency is varied in passage of the sample through resonance, and a periodic variation of the vibration amplitude. The conditions for the onset of various vibration regimes are determined. It is noted that nonlinearity is attributable to diffusionless second-order phase transitions in magnets, ferroelectrics, and superconductors.