A Gw-level high-power CO2 laser system has been developed. A short laser pulse is cut out with a double Pockels cell electrooptical shutter from a hybrid CO2 laser oscillator and amplified with a three-stage dual-path TEA preamplifier, a two-stage TEA preamplifier, a large aperture TEA amplifier, and an electron beam-controlled final amplifier. All parts of the laser system are controlled with a control system. The laser output power is up to 3.2 × 109 W and laser pulse width is 4 ns full width at half maximum.

The effects of finite bandwidth and random-phase pumping on parametric instabilities have been studied extensively (Thomson & Karush 1974; Thomson 1975; Laval et al. 1977). The possibilities arising from the use of this method and a time delay to obtain a spatial incoherence uniform illumination have been proven in experiments (Lehmberg & Obenschain 1983; Mima & Kato 1982; Kato & Mima 1982; Richardson 1981). In this paper we investigate this subject in an alternative way and obtain some results on the influence of the gain profile on parametric instabilities.

A three-dimensional model to calculate X-ray intensity distribution on an indirectly driven fusion target is presented. The model includes conversion of laser light into X rays, radiation reemission from X-ray-heated wall of a cavity, and influence of an inner pellet (i.e., a fuel capsule) on radiation redistribution. Intensity distribution of an X ray inside a cylindrical cavity heated by intense blue laser light (wavelength 351 nm, energy 5.2 kJ, duration 0.7 ns) was determined by measuring a burn-through signal from a diagnostic foil integrated onto the cavity. The experimental result is well replicated by the model calculation. By using this model, optimum conditions for uniform irradiation of afusion capsule by X-ray radiation are evaluated for use in Gekko XII laser fusion experiments.

Heavy ion clusters are considered for driving an inertial confinement fusion (ICF) pellet. They are shown to be effective at much lower energy than required for atomic ions. Multifragmentation is discussed with a maximum entropy hypothesis. Direct pressure imparted to a DT pellet is estimated. The stopping of strongly correlated ion cluster debris flowing in a degenerate electron jellium target is calculated for polarized and unpolarized (with respect to velocity) projectiles. A reduced stopping occur for elongated configurations at low velocities (υ ⋍ 1 a.u.) and very dense targets. Transverse geometries experience a very large and enhanced stopping. Systematic trends are stressed out. Largest stopping is achieved when debris correlation lengths are smaller than the target jellium screening length.

Implosion experiments performed at Centre d'Etudes de Limeil-Valenton in the indirect drive scheme using the two-beams Nd:glass laser facility Phebus at the energy level = 6 kJ (blue light) are presented. A final density of compressed DT close to 100 ρ0 has been obtained; it has been deduced from radiochemistry of the activated silicon atoms in the pusher. The best irradiance uniformity on the microballoon was evaluated to = 15% rms. Phebus has also been equipped with an optical fiber oscillator to study the effect of a smoothing technique on coupling processes: It appeared that at 0·53 μm absorption efficiency is increased by =15–20%. With the eight-beams Octal laser, hydrodynamic instabilities development in accelerated planar targets has been investigated both for direct and indirect drives; the mixing zone detected at the light-heavy interface does not present visible bubble-and-spike like structures and is less developed in the indirect configuration. Atomic physics in laser plasmas is also deeply studied; a particular effort has been made on absorption spectroscopy, a powerful diagnostic of ionization dynamics in cold and dense plasmas. Experiments have been realized either in multilayered targets or using rear-side X-ray emission of thin Au foils to heat the samples. To reach fuel ignition conditions, more powerful lasers, in the range of megajoule, will be needed. Their design needs further technological developments to reduce the capital cost in $/W. At Limeil, we work mainly on high-damage threshold optical coatings, using the sol-gel process, high-quality, low-cost mirror fabrication, using the replica technics, and incoherent laser pulse generation for beam smoothing.

In this paper we generalize the filament study to include the Raman side scattering by solving the coupling equations of an electron plasma wave and a side-scattering electromagnetic wave. We also give an explanation of the physical phenomenon observed in the Raman-scattering experiments.

A two-temperature equation of state (EOS) for a plasma medium was developed. The cold-electron temperature is taken from semiempirical calculations, while the thermal contribution of the electrons is calculated from the Thomas–Fermi–Dirac model. The ion EOS is obtained by the Gruneisen–Debye solid–gas interpolation method. These EOS are well behaved and are smoothed over the whole temperature and density regions, so that the thermodynamical derivatives are also well behaved. Moreover, these EOS were used to calculate the average ionization 〈Z〉 and 〈Z2〉 of the plasma medium. Furthermore, the thermal conductivities have been calculated with the use of an extrapolation between the conductivities in the solid and plasma (Spitzer) states. The two-temperature EOS, the average ionization 〈Z〉 and 〈Z2〉, and the thermal conductivities for electrons and ions were introduced into a two-fluid hydrodynamic code to calculate the laser-plasma interaction in carbon, aluminum, copper, and gold slab targets. It was found that the two EOS are important mainly from the ablation surface outward (toward the laser). In particular, the creation of cavitons in the distribution of the electrons is predicted here, especially for light materials such as aluminum. These studies enable us also to establish that the commonly used exponential scaling laws of the type Pa = A[I/(1014 W/cm2)]α for the ablation pressure and similar laws for the temperature are valid only for absorbed laser intensities in the range 3 × 1012-3 × 1014 W/cm2, while the degree of ionization (at the corona) follows a quite different scaling law. We also found that the parameters A and α. in the above expression are dependent on material, laser wavelength, and pulse shape. Thus we determined for the ablation pressure, using a trapezoidal Nd laser pulse, that α varies between 0.78 and 0.84 and that A varies between 14 and 8 Mbar for 6 ≤ Z ≤ 79. Beyond the range of validity the scaling laws may give values at least twice as large as those obtained by the simulation.

New diagnostics were implemented on the implosion experiments performed at LULI to improve our measurements of hydroefficiencies: Neutron chronometry gives the time of emission of the fusion reaction products as measured from the peak of the laser pulse; thereby making it possible to correlate the neutron emission with X-ray emission. Core imaging, based upon a maximum entropy reconstruction technique, leads to core size determination and also is a promising diagnostic for wall nonuniformities induced by irradiation conditions. A simple model is developed to retrieve experimental spectra of α-particles.

The ion flux produced by an obliquely incident Nd Q-switch pulse on a graphite target has been analyzed with regard to its kinetic energy, charge, and angular distribution. The laser intensity has been varied in a range between 109–5·1010 W/cm2, appropriate for many low-irradiance applications. It is observed that for ions of charge state n the emission cone of the number of ions scales with cos2n+1. The angular emission probability of the kinetic energy of the individual ions is found to be independent of their charge and scales as a cosine function. Due to the asymmetrical heating of the expanding plasma by the obliquely incident laser pulse, the maximum of emission is rotated away from the target normal toward the incoming laser, depending upon the ion's charge and the laser energy. The measured kinetic energy spectra are determined by the recombination during the plasma expansion: There are no low-energetic highly charged ions and no high-energetic lowly chargedions. If the laser energy (intensity) is enhanced, it is observed that the additional heating essentially serves only to increase the velocity of the higher charged ions; the energy of the individual singly charged ions is not altered.

Expressions for the stopping power are derived from the Lenard-Balescu and Boltzmann kinetic equations. Numerical evaluation is carried out for the stopping of light and heavy particles in a multicomponent plasma.

We present a diffusion approximation describing the flow of thermal radiation that preserves several important features of the underlying equation of radiative transfer. Specifically, this diffusion description: (1) is flux limited; (2) reduces to the correct transport weak gradient limit; (3) allows correct and simultaneous exponential growth and Decay for a certain class of problems; (4) gives correct transport results for certain contiguous half-space problems; and (5) allows the radiative flux and the gradient of the radiation energy density to point in independent directions. This treatment extends and generalizes earlier flux-limited diffusion approximations that are widely used in radiation–hydrodynamics calculations.

Improved measurements of ion-correlation effects in a dense shock-compressed plasma are presented. The extended X-ray-absorption fine-structure (EXAFS) technique on the aluminum K edge is used to observe the short-range order within a dense plasma. Densities of about three times solid density were measured with good accuracy. The experimental measurements of density give results that are in good agreement with the MEDUSA onedimensional fluid code predictions. The improved quality of the data enabled us to calculate the ion coupling parameter during the compression and the subsequent heating of the plasma. An estimation of the temperature is given on the basis of published models, and an approximate agreement is obtained with the MEDUSA code predictions.

We are developing neutron diagnostics to characterize the implosions performed with the Phebus laser, operating at 5 kJ blue light delivered in 1·3 ns. For measuring the glass pusher areal density, (ρΔR), of the target, a silicon radiochemistry diagnostic has been implemented and is currently being used. We describe the diagnostic and its performance. Pusher areal density measurements, (ρΔR) and calculated values of fuel density are given. Deuterium (D)-tritium (T) final densities as high as 100 × D-T liquid density (20 g/cm3) have been achieved.

We carried out a numerical analysis on the stability of targets designed to produce fusion energy gains close to 100 when irradiated with an appropriate heavy ion beam. To reach such performances, we found that a high-Z radiation shield was necessary to screen the DT fuel from the radiation coming from the surrounding hot material. As opacity of high-Z materials is only weakly density dependent, we considered targets with lead shields both at solid density and with a density equivalent to that of the contiguous external material. The behaviour of both kinds of targets has been studied by introducing a small spatial nonuniformity on the external surface of the lead shield. Targets with low-density shields have been tested with a beam intensity perturbation, too. Because density gradients are very sharp and ablation is practically absent, we have found these implosions to be Rayleigh-Taylor unstable. The evolution has been followed in the nonlinear regime because the full hydrodynamical-radiative model with a nonlinear heavy ion energy deposition and mesh correction was used.

A steady-state analytical model is used to calculate the operational parameters of a coaxial-type, self-magnetically insulated ion-beam diode, called a plasma focus diode (PFD). The electrons are described by nonlaminar flow with a constant radial density distribution instead of the density profile determined self-consistently by one-dimensional laminar flow. The calculated results were compared with the experimental results obtained at the peak power, and good agreement was obtained between them. From the theoretical understanding of the PFD we obtained the scaling relations of diode impedance and the ion-current efficiency and used them to discuss the possible ways to increase the coupling efficiency and the ion-current efficiency and to reduce the instabilities in the diode operation.

Three-wave parametric instabilities, in which an incident, laser-light wave resonantly decays into two decay waves, have long been recognized to potentially play a large role in the physics of laser-produced plasmas. Many effects have been predicted, but only gradually and with difficulty have experiments developed the ability to observe some of them. One obstacle has been that the behavior of the instabilities often depends upon the scale length and planarity of the plasma. My collaborators and I have worked for several years to overcome this specific obstacle and when necessary now work routinely with plasmas that are thousands of laser wavelengths in the axial and transverse scale lengths of the plasma parameters. This has allowed us to report the first observations of some instabilities and study the scaling behavior of several instabilities as well.

The interaction of ultrashort laser pulses with a fully ionized plasma is investigated in the plane geometry by means of numerical simulation. The impact of the space oscillations in the amplitude of the laser electric field on the shape of the electron distribution function, on laser beam absorption, and on electron heat transport is demonstrated. Oscillations in the absorption rate of laser radiation with the minima coincident to the maxima of the laser electric field lead to a further decrease in the absorption of laser radiation. Heat flux in the direction of increasing temperature in the underdense region is caused by the modification of the electron distribution function and by the density gradient. A limitation of heat flux to the overdense plasma isobserved with the flux limiter in range 0.03–0.08, growing moderately with the intensity 1014–1016 W/cm2 of the incident 1.2-ps laser pulse.

The plasma composition, equation of state, and phase diagram of dense helium plasma were calculated for temperatures of 104...105 K, total atom densities of 1015... 1025 cm−3, and pressures up to 102 TPa, including the region of partial ionization and strong Coulomb coupling. The basic thermodynamic potential was chosen to be the free energy density with contributions due to Coulomb interaction, hard-core repulsion, and van der Waals-like attraction for a mixture of differently charged atoms and free electrons. For the first time, we show the potential occurrence of a sequence of plasma phase transitions. In helium, they correspond to the ionization steps He0→He+ and He+→He++ respectively. The properties of the coexisting phases were determined by a Maxwell construction based on the combined chemical potential.