A method for the automatic, discrete rezoning of two-dimensional Lagrangian codes for inertial confinement fusion (ICF) implosions has been developed. The method, which applies to matrix-ordered, quadrilateral zone meshes, allows the preservation of the interface tracking property of the Lagrangian approach. Total mass and momentum are conserved exactly, total energy is approximately conserved, and numerical diffusion is kept to tolerable levels. The mesh generator, the mapping scheme, and the actual implementation in a two-temperature laser fusion code are described. The performance of the method is illustrated by several sample applications.

The process of Mach wave generation in air is studied in both plane and spherical geometries. The experimental results reported here are theoretically interpreted using the predictions of a self-similar model of strong explosion along with the hydrodynamic equations of a perfect gas, and a good agreement is found.

Femtosecond laser pulses are capable of producing very high light intensities at moderate pulse energies. We study the laser light absorption at flux densities of 1017 W/cm2 and higher during the plasma formation and the plasma heating process. In the first stage multiphoton and collisional ionization dominate. Modification of the inverse bremsstrahlung absorption occur owing to ionization dephasing. In the second stage beam energy conversion is mainly by collisional absorption at normal incidence, and by resonance absorption at oblique illumination. Simultaneously strong electron heat conduction provides for plasma formation and heating in deeper layers of the solid not accessible to the laser light. The penetration depth of the laser beam is modified by the anomalous skin effect. The electron-ion collision frequency at the high laser intensities under consideration is determined by the oscillation energy of the electrons rather than by their thermal motion. Although it reaches high values (1014-1016 S-1), all kinds of solid targets become strongly reflecting (>60%) owing to the formation of electron densities largely exceeding those of solid-state plasmas. Effects modifying the degree of absorption are briefly discussed.

The behavior of the stimulated Raman scattering is investigated in a laser-irradiated plasma with an inhomogeneous density profile, by means of wave-coupling simulations. In the case of a regular hydrodynamic-like profile, a spectral broadening is observed when the laser irradiance is increased and is found to result from a coupling between the convective and the absolute Raman scattering, via pump depletion. In the case where a short-scale modulation of the density is superimposed on the profile, the spectrum is strongly influenced by the propagation velocity of the modulation and by the sinusoidal/chaotic nature of the modulation.

We present some significant results of collisional excitation X-ray laser experiments in plasmas produced by a laser. We studied the amplification in Ne- and Ni-like ions by varying both the nature and the thickness of targets, the irradiation, and the wavelength of the driving laser. Some potentially interesting scalings as a function of the atomic number of the lasing element are demonstrated in the Ne-like system. An order-of-magnitude increase in gain in the Ni-like experiments was determined.

We investigated the case where two laser-produced plasmas collide nearly head on. Special attention was devoted to the fundamentals necessary to realize a coherent X-ray source. A gas-dynamic computational analysis was performed to understand the evolution of the density, the temperature, and the velocity of merging plasmas. The spatial intensity distribution of selected spectral lines reveals that the interaction of plasmas of different nuclear charge and charge state is not strictly collision dominated. Using spectral line intensity ratios, we determined electron temperatures and electron number densities, as well as the intensity inversion on the 4–1 to 3–1 resonance transitions of [He]-like Al. Inversion occurs in the vicinity of the targets if identical materials are used (Al–Al) and is possibly indicated in the interaction zone for different ones (Al–Cu), too. The inversion factors (and the gain coefficient) for the 4–3 transition of [He]-like Al at about 130 Å were estimated.

Neonlike ions have long been recognized as candidates for producing soft-X-ray lasers along 1s22s22p53p → 1s22s22p53s transitions. A series of experiments using exploding-foil targets driven by high-power lasers was performed at CEL-V and LLNL, and demonstrated amplified spontaneous emission in germanium, selenium, strontium, and molybdenum. We present here simulations of these experiments and discuss the importance of refraction effects on the propagation of the lasing X-rays and on the scaling of this scheme to shorter wavelengths.

We present results of X-ray emission by the rear side of gold thin targets irradiated by the Phebus laser at λ = 0.35 μm, τ = 0.7 and 1.3 ns, and EL = 1.5 kJ. A streak camera coupled with a transmission grating gave the time-resolved X-ray emission of the rear side. Also, a streak camera coupled with a slit allowed us to obtain information about the space and time evolution of the plasma. Some other diagnostics gave information about the energy balance and the X-ray conversion efficiency. The results are in good agreement with previous ones obtained with the Octal laser, particularly on optimum thicknesses for X-ray conversion efficiencies. Values of the thermal flux limiter are deduced. Simulations with FCI 1 code with multigroup radiative transfer and non-LTE ionization reproduce the experimental results only about some points. A number of reasons, such as 2-D effects and problems of opacity, are invoked.

Experiments were performed at Ecole Polytechnique in order to study the effect of preheating of targets by X rays during interaction with a powerful 4ω laser. Thin aluminum foils (typically of 5–15-μm thickness) are used as targets. The emission at 40 and 80 eV is recorded by Schwarzchild microscopes and is time resolved. By assuming a blackbody emission of the rear side of the foil just at the beginning of the emission, we deduce the temperature, by measuring the ratio of the emissivity at the two wavelengths. Experimental results are presented for an energy varying between 10 and 30 J delivered at the fourth harmonic of the neodymium laser in a pulse of 0.5-ns time duration focused in a 100-μm-diameter focal spot. Simulations performed with the code MULTI including radiative transfer are presented in order to show the effect of radiation on the process of heating of the target material. Radiation spectra emitted from the laser side and the rear side, as well as the temperature and density evolution as a function of time and mass coordinate, are shown.

The X-ray emission from planar targets made of aluminum, copper, or gold irradiated by a frequency-doubled Nd laser (530-nm wavelength and 1012–1014-W/cm2 laser intensity) was measured at two pulse durations: 3 ns and 30 ps. We absolutely measured the X-ray emission with spectral, temporal, and spatial resolution in the wavelength range 3 Å < λ < 250 Å by using filtered bolometers, a transmission grating spectrometer, X-ray diodes, and an X-ray streak camera as diagnostics. In addition, the absorption of laser light was measured. For the short, 30-ps laser pulse the conversion of incident laser energy into X rays was considerably less than that with the long, 3-ns pulse. This is caused by less absorption of laser light and, in addition, by less conversion of absorbed laser energy into X rays in the case of the short pulse. The results are compared with numerical simulations performed with the MULTI hydrocode.

Planar coated targets of Aluminum and Gold overlayers were irradiated with 1.06-μm laser light at intensities 1011–5 × 1013 W/cm2. The energy penetration depth, the ablated plasma, and the X-ray emission were characterized. Results show significant energy transport through the overlayer.

X-ray spectroscopy is a widely used means of diagnosing densities and temperature within laser-produced plasmas. At X-ray energies above approximately 1 keV, Bragg crystal spectrometers are routinely used to record X-ray spectra. Quantitative measurements of plasma conditions can be obtained with a knowledge of crystal reflectivity and film or detector response. In such data analysis it is always assumed that the crystal response is constant in time. However, we show that under certain adverse experimental conditions the X-ray fluxes incident on the crystal are so high as to significantly transiently modify the reflection characteristics of the crystal. Such degradation need not necessarily be accompanied by a loss of observed spectral solution. The transient (nanosecond-time-scale) change in crystal reflectivity is due to a change from dynamical to more kinematic diffraction caused by an X-ray-induced thermal strain gradient in the surface layer of the crystal. The decay time of this strain is typically several nanoseconds. Calculations of some specific crystal reflectivities and rocking curves under such conditions are presented, and methods of minimizing the effect by appropriate filtering are discussed.

A program is under way to develop methods and instrumentation based on charge-coupled device (CCD) sensors for hot plasma diagnostics. We have developed a new X-ray spectrometer in which a freestanding X-ray transmission grating is coupled to a CCD linear array detector with electronic digitized readout replacing film and its wet processing. This instrument measures time-integrated pulsed X-ray spectra with moderate spectral resolution (δλ ≤ 0.6 nm) over a broad spectral range (0.3–2 keV) with high sensitivity, linearity, and large dynamic range. The performance of the device was tested using laser plasma as the X-ray source.

For spectral diagnosis, a repetitive laser-produced soft-X-ray source was examined in the range 2–40 nm using a spectrometer containing a pinhole transmission grating, an image converter, and a CCD camera. Via digital recording the detection system provides fast on-line data accumulation with spectral and spatial resolution. With this detector the performance of the light source was studied with respect to spectral emission characteristics, long-term stability, and reproducibility by recording spectra for different solid targets and liquid lead.

A very intense laser field polarizes the virtual electron-positron pairs that populate the vacuum. This provides for a coupling between different modes of the electromagnetic field, giving rise to effects such as scattering of light by light, a refractive index of the vacuum, vacuum birefringence, etc. Given enough energy in a sufficiently small spacetime region, the virtual pairs can become real, which leads to pair production in the intense field under the action of a third agent. These, as well as related effects, are summarized with respect to their orders of magnitude and conditions under which they might become accessible to experiment. Some other processes that are normally mentioned in this context, such as Thomson (Compton) scattering at high intensities, are considered, too, even though they are unrelated to the vacuum structure of quantum electrodynamics.

The laser beat wave programs at Rutherford Appleton Laboratory and Ecole Polytechnique are reviewed. The techniques used to generate by multiphoton ionization the highly uniform plasmas needed for the beat wave are described. Evidence of the generation of a 2% plasma wave using laser beams at 1.064 and 1.053 μm is presented. The plasma wave suffers damping, possibly by the modulational instability. The use of a high-power short laser pulse for plasma-wave generation by the wake-field process is discussed. This process has the advantage that there is no resonance as in the beat wave, and since the plasma wave is generated on the time scale of the plasma period, instabilities are unlikely to be important.

We propose a new time-resolved laser-induced breakdown spectroscopy (TRELIBS) system for quantitative determination of small amounts of pollutants in gas. Some experimental results showing the high resolution of the method are reported, and possible improvements of the system are discussed.

Various gas lenses are briefly reviewed, together with their imperfections. We discuss the applicability of these lenses as objectives for telescopes and as the final focusing element of high-power laser systems: cw lasers in industry and pulsed lasers for plasma research. A novel phase conjugate experiment employing a gas lens is described. Pulsed gas lenses are proposed as a solution to the problems of high laser fluence and radiation damage in post-break-even fusion experiments.