A two-stage model for achieving X-ray amplifying action on photoionized K-transitions pumped by the soft X-ray broadband flux from laser-produced plasma is treated. A simple pumping criterion is considered by the analysis of the positive gain-length product for the two-stage model of innershell transitions (0) → (3) and (2) → (1). The incident photon flux integrated over the effective lifetime T21 of an innershell transition (2) → (1) is related to the photoionization coefficient μ03(ph), stimulated emission cross section σ21(stim), and the effective gainlength L(eff) ≃ 0.63/μ03(abs), where μ03(abs) is the absorption coefficient of a pump soft X-ray radiation. Numerical estimates for the feasibility of K-shell X-ray amplification are given for C, F, Na, Ti, and Sn. For the example of an exponential X-ray carrier pulse (XCP) propagating through a resonant medium, an amplifying effect depending on the ratio of the input pulse temporal bandwidth τband to the effective lifetime T21 is investigated. It is shown that for an XCP with a finite bandwidth τband the logarithm of an energy gain, ln[U(z)], falls below a linear relation with z and thus, from the viewpoint of maximum amplifying effect bandwidths τband with τband/T21 > 1 are preferable. The requirements for achieving X-ray K-shell photoionization amplification for C are discussed in more detail. PACS numbers: 32.80.Hd, 42.50.Md, 42.55.Vc, 42.65.Re, 52.25.Nr.

A new conception of subcritical transition to turbulence in unbounded smooth shear flows is discussed. According to this scenario, the transition to turbulence is caused by the interplay between the four basic phenomena: (a) linear “drift” of spatial Fourier harmonics (SFH) of disturbances in wave-number space (k-space); (b) transient growth of SFH; (c) viscous dissipation; (d) nonlinear process that closes a feedback loop of transition by angular redistribution of SFH in k-space; The key features of the concept are: transition to turbulence only by the finite amplitude vortex disturbances; anisotropy of the process in k-space; onset on chaos due to the dynamic (not stochastic) process. The evolution of 2D small-scale vortex disturbances in the parallel flows with uniform shear of velocity is analyzed in the framework of the weak turbulence approach. This numerical test analysis is carried out to prove the most problematic statement of the conception—existence of positive feedback caused by the nonlinear process (d). Numerical calculations also show the existence of a threshold: if amplitude of the initial disturbance exceeds the threshold value, the self maintenance of disturbances becomes realistic. The latter, in turn, is the characteristic feature of the flow transition to the turbulent state and its self maintenance.

2D, low-frequency, relativistic solitary waves have been found recently in 2D. Particle-in-cell simulations of ultraintense laser pulses propagating in underdense plasmas, in regimes where the laser pulse undergoes energy depletion and downshifting of its frequency. These slowly propagating, spatially localized, electromagnetic structures represent an important channel of energy conversion from the laser pulse to the plasma. Pulse energy can also be converted into fast propagating structures, associated with collimated beams of ultrarelativistic electrons. Lienard–Wiechert magnetic field structures have been observed in PIC simulations to move together with focalized ultrarelativistic electron beams in the plasma.

Magnetohydrodynamic simulation results related to the capillary discharge dynamics are presented. The main physical process that should be taken into account is the ablation of the capillary wall material evaporated by the heat flux from the capillary plasma. The possible applications of the capillary discharges related to the physics of the X-ray lasers and the use of the capillary plasma to provide a guiding for ultrashort high-intensity laser pulses over a distance greater than the defocusing length are discussed.

A detailed investigation of many aspects of the physics of laser–plasma interaction at very high laser intensities is required in order to assess the feasibility and the promise of the fast ignitor scheme for inertial confinement fusion. Relevant results, obtained in a series of experiments carried out at the Rutherford Appleton Laboratory, Chilton (UK) and at the Centre d'Etudes Atomique, Limeil Valenton (France), are presented and discussed here. In particular, the formation of plasma channels was observed following the propagation of relativistically intense, ps laser pulses through underdense plasmas. The channels persist long after the interaction, and their expansion has been measured. Efficient guiding of ultraintense laser pulses, both through preformed density channels and through solid guides, has been demonstrated. Finally, indication of collimated fast electron propagation through solid targets has been obtained from the observation of filamentary ionization tracks in laser irradiated solid targets.

This paper presents an investigation into the behavior of a laser beam of finite diameter in plasma with respect to forces and optical properties, which lead to self-focusing of the beam. The transient setting of ponderomotive nonlinearity in a collisionless plasma has been studied, and consequently the self-focusing of the pulse, and the focusing of the plasma wave occurs. The description of a self-focusing mechanism of laser radiation in the plasma due to nonlinear forces acting on the plasma in the lateral direction, relative to the laser has been investigated in the nonrelativistic regime. The behavior of the laser beams in plasma, which is the domain of self-focusing at high or moderate intensity, is dominated by the nonlinear force. The investigation of self-focusing processes of laser beams in plasma results from the relativistic mass and energy dependency of the refractive index at high laser intensities. Here, the relativistic effects are considered to evaluate the relativistic self-focusing lengths for the Nd glass radiation, at different plasma densities of various laser intensities. A numerical program in c++ that incorporates both the ponderomotive force in self-focusing mechanism and relativistic effects has been developed to explore in depth self-focusing over a wide range of parameters.

The work is devoted to direct numerical simulation of turbulent mixing by shear driven instability at an interface of two plane-parallel gas flows. The work presents the results obtained in 2D simulations of turbulence being developed at the interface of two almost incompressible gases using the MAX program package. Spatial and temporal evolution of the turbulence zone resulted from shear driven instability is studied. We calculated the constant of shear driven turbulence mixing and investigated how the rate of turbulence zone growth depended on density difference of mixed fluids. Heterogeneity coefficient of the mixture was calculated for all considered density differences.

This article reports on the interaction between slow ions and a partially ionized plasma. Temporal evolutions of energy loss and charge distribution of 2.4 MeV oxygen beams in the laser-induced polyethylene plasma were measured. The charge distribution showed strong stripping ability in the early phase of the plasma. Stopping power deduced from the experimental energy loss was 1.9 times larger than that for the solid. The effective charge of the projectile ion was estimated from the yields of 4+ and 6+ states. The peak value of the effective charge was 1.4 times larger than that of the solid. The stopping power equation given by Sigmund was extended for the partially ionized plasma and it could reproduce the measured energy loss.

This paper presents a numerical and theoretical study of the generation and propagation of oscillation in the semiclassical limit τ → 0 of the nonlinear paraxial equation at laser–plasma interaction. In a general setting of both dimension and nonlinearity, the essential differences between the focusing and defocusing cases is identified due to the nonlinearity, and dispersion effects involved in the propagation of solitons at laser plasma interaction. A sequence of codes has been developed in mathematics to explore the focusing and defocusing of the soliton formation and propagation.

An energy loss of 240 MeV argon ions in a Z-pinch helium plasma has been for the first time observed throughout the entire pinching process. Standard Stark broadening analysis gives an electron density ranging from 4 to 6 × 1017 cm−3 during the pinch. To deduce stopping power from the energy loss, the target thickness of the helium plasma has been evaluated assuming the mean charge of helium based on thermal equilibrium. The observed electron density and the mean charge of helium give a target thickness of 30 ± 3 μg cm−2 from 1 μs to 1.8 μs after the discharge ignition. The measured stopping power exceeds a tabulated value for cold helium gas by a factor of 2 to 3 around the time of the first pinch. The experimental stopping power is compared with theoretical values calculated using an equation of stopping power for a partially ionized plasma.

The paper addresses Rayleigh–Taylor instability (RTI) problems and presents a method of describing an interface with markers on a Eulerian mesh, which is implemented in the MAH-3 code. The proposed method allows a more accurate description of the evolution of the interface caused by Rayleigh–Taylor perturbations and preserves symmetry of the interface under appropriate symmetry of the problem (planar, cylindrical, and spherical). The method employs an unstructured triangular mesh of makers. Method capabilities are demonstrated on 2D and 3D Rayleigh–Taylor instability problems.

An investigation of the growth of a radially symmetrical ripple, superimposed on a Gaussian laser beam in a plasma is presented. Based on WKB and paraxial ray approximation the phenomenon of relativistic self-focusing (RSF) is analytically investigated. The differential equation for beamwidth parameter of rippled laser beam is evaluated. The ripple gets focused when the initial power of the ripple is greater than the critical power for focusing. The focusing is found to be considerably affected by the power of the main beam and the phase angle between the electric vectors of the main beam and the ripple. At higher intensities the saturation effects of nonlinearity become predominant, making the nonlinear refractive index in the paraxial region have slower radial dependence, and thus the ripple extract relatively less energy from its neighborhood. The case of magnetized plasmas is also preliminarily discussed.

Frequency doubling a 140 × 110 mm 40 J sub-ps 1054 nm beam for laser matter interaction studies was investigated at the Central Laser Facility, with efficiencies greater than 60% being achieved. The conversion characteristics (efficiency, beam quality, focusability, and pulse length) for two large 157 mm diameter aperture high quality KDP crystals of thickness 2 mm and 4 mm were studied. Using a fundamental drive beam of two times diffraction limited quality, focal spots of four times the diffraction limit in the frequency doubled beam were achieved and beam degradation effects shown to be minimal.

The paper considers the equation for heterogeneity coefficient within the turbulent mixing area in the approximation of big Reynolds numbers and small Mach numbers. A mechanism is studied of the heterogeneity coefficient dissipation due to molecular diffusion. The Kolmogorov's hypothesis on developed turbulence is used to calculate a dissipative term. The model presented allows us to take into account the heterogeneity degree in LV- and KE-models of turbulent mixing. A system of equations allowing us to calculate directly the heterogeneity degree is derived for the case of the LV-model with the turbulent diffusion coefficient which is constant over the turbulent mixing area. A self-similar solution is derived for the heterogeneity coefficient which is in good agreement with the results of experiments and direct numerical simulations. The heterogeneity coefficient averaged over the mixing area is shown to depend weakly on the density drop between the mixing materials. Thus, it is kH = 0.25 at the drop n = 1–3, and at the drop n = 20 − kH = 0.23.

This paper considers various channels of electron–positron pair, neutron, and γ-quantum generation via the ultrashort high-power laser pulse interaction with targets made from different materials. The positron yields in materials with different Z are estimated for the positron production by high-speed electrons and γ-photons. It is shown that a picosecond 100-TW laser pulse gives rise to formation of about 1010 positrons, possible channels of the neutron production in light- and heavy-atom materials being studied as well. It is ascertained that the formation of up to 107 laser neutrons per shot is possible when the laser intensity is ∼1021 W/cm2 on a heavy-atom target.

Calculations of optimal parameters for the e-beam excited Xe spontaneous radiation source (excilamp) at λ ∼ 172 nm have been done. There have been defined optimal values of gas density and specific excitation power. Gas temperature dependence of radiation conversion efficiency related to the injected media energy have been also computed. It has been shown that there is an optimum in excitation pulse duration at frequency operation mode.

The achievement of ignition from an Inertial Confinement Fusion capsule will require a detailed understanding of a wide range of high energy density phenomena. This paper presents some recent work aimed at improving our knowledge of the strength and equation of state characteristics of low-Z materials, and outlines data which will provide quantitative benchmarks against which our predictive radiation hydrodynamics capabilities can be tested. Improvements to our understanding in these areas are required if reproducible and predictable fusion energy production is to be achieved on the next generation of laser facilities.

In particular, the HELEN laser at AWE has been used to create a thermal X-ray source with 140 eV peak radiation temperature and 3% instantaneous flux uniformity to allow measurements of the Equation of State of materials at pressures up to 20 Mbar to an accuracy of <±2% in shock velocity. The same laser has been used to investigate the onset of spallation upon the release of a strong shock at a metal-vacuum boundary, with dynamic radiography used to image the spalled material in flight for the first time. Finally, a range of experiments have been performed to generate quantitative radiation hydrodynamics data on the evolution of gross target defects, driven in both planar and imploding geometry. X-ray radiography was used to record the evolving target deformation in a system where the X-ray drive and unperturbed target response were sufficiently characterized to permit meaningful analysis. The results have been compared to preshot predictions made using a wide variety of fluid codes, highlighting substantial differences between the various approaches, and indicating significant discrepancies with the experimental reality. The techniques developed to allow quantitative comparisons are allowing the causes of the discrepancies to be identified, and are guiding the development of new simulation techniques.

The smoothing mechanism, that is due to the thermal conductivity in the laser plasma generated by a double pulse, is examined. Plasma preformed by a second harmonics prepulse serves as a low density gradient plasma for impacting the main pulse, the frequency of which was tripled for improving the laser-target coupling. The effect of the preformed plasma on the thermal smoothing of the heating pulse, which was split to create two foci on the target surface, was explored by varying the time delay between the prepulse and the main pulse. The smoothing effect was monitored by a pair of pinhole cameras: one viewing side-on and the other one placed at the rear side of the target. Spatially resolved X-ray emission spectra were recorded to determine the density and temperature distributions in the plasma. The maximum smoothing effect was observed for the time delay t = 0.4–0.7 ns.

We derive optimum values of parameters for laser-driven flights into low Earth orbit (LEO) using an Earth-based laser, as well as sensitivity to variations from the optima. These parameters are the ablation plasma exhaust velocity vE and specific ablation energy Q*, plus related quantities such as momentum coupling coefficient Cm and the pulsed or continuous laser intensity that must be delivered to the ablator to produce these values. Different optima are found depending upon whether it is desired to maximize mass m delivered to LEO, maximize the ratio m/M of orbit to ground mass, or minimize cost in energy per gram delivered. Although it is not within the scope of this report to provide an engineered flyer design, a notional, cone-shaped flyer is described to provide a substrate for the discussion and flight simulations. The flyer design emphasizes conceptually and physically separate functions of light collection at a distance from the laser source, light concentration on the ablator, and autonomous steering. Approximately ideal flight paths to LEO are illustrated beginning from an elevated platform. We believe LEO launch costs can be reduced 100-fold in this way. Sounding rocket cases, where the only goal is to momentarily reach a certain altitude starting from near sea level, are also discussed. Nonlinear optical constraints on laser propagation through the atmosphere to the flyer are briefly considered.

We investigate numerical effects related to “single-cycle” ionization of dense matter by an ultra-short laser pulse. The strongly nonadiabatic response of electrons leads to generation of a MG steady magnetic field in laser–solid interaction. By using two-beam interference, it is possible to create periodic density structures able to trap light and to generate relativistic ionization fronts.