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.

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.

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.

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.

Experimental observations are reported on the interaction of 1-μm laser light with underdense plasmas (n ≤ 0·25 nc) from thin foil plastic targets. Nominal laser intensity on target was up to 3 × 1013 W/cm2 in a 3-ns pulse, but much higher intensity was reached due to spiky laser pulses. We studied forward-emitted second harmonic light as a diagnostic of the interaction and in particular of the occurrence of filamentation. Measurements included: energy monitoring of 2ω forward emission vs. target position and laser energy; time resolved (120-ps gate) imaging of the interaction region cross section. The second harmonic energy level was found to be sensitive to target position. In addition, the images obtained with the target in position of maximum second harmonic generation showed unstable structures whose scale length is comparable with the expected one for maximum filamentation growth. These results are shortly discussed in the framework of stationary filamentation theory and second harmonic generation in inhomogeneous media.

An experimental analysis is conducted to visualize sidescattered second harmonic spectra originating from the critical surface of a plasma produced from a 1, 064-nm laser beam. It is shown that longitudinal and transverse wave-scattering mechanisms producing the second harmonic may also alter the local plasma parameters. These irregular plasma parameter variations and the perturbed spatial uniformity of the incident laser beam can, in turn, be visualized through the second harmonic behavior. This work confirms the origin of the second harmonic production in an inhomogeneous plasma. Time evolution of the optical density of this harmonic showed spectral shifts due to the Doppler effect related to the critical surface dynamics. On the time-integrated spectra, shifted secondary peaks have been observed, indicating that the second harmonic takes its origin also from parametric decay as well as electron decay instability. Other properties of the interaction physics are deduced from the present second harmonic study.

The time-resolved X-ray emission recorded from a plasma produced by a Nd laser at high intensity (>1014 W/cm2) shows successive bursts. They are correlated with the pulsation of the back-scattered second harmonic spectrum obtained during the same shot. Both results show that the plasmons resonantly excited at the critical surface by the laser waves can undergo quasiperiodically the collapse process.

The quasisteady expansion of a plasma ablated from a laser-irradiated pellet with classic (Spitzer) heat flux is reconsidered with a nonclassic heat flux law. The nonphysical flux factor (calculated flux vs. free streaming), going to infinity as one goes away from the pellet in the above transition regime, is corrected.

In previous works, we developed several atomic physics models for calculating opacities for low-Z materials. In this work, we used those models for obtaining opacities for Au plasmas in local thermodynamic equilibrium. In this particular case, a great number of configurations must be considered for a full treatment of atoms at plasma conditions. Thus, we included several changes in the aforementioned models, which are extensively reported here.

We report on KrF laser-plasma interaction studies at focused intensities up to 4 × 1014 W/cm2 for pulse durations of 1–2 ns and up to 1015 W/cm2 for pulse duration of 100 ps. The longer-pulse experiments are concerned with quantifying two important features of the ablating plasma. Stimulated Brillouin scattering at moderately large L/λ has been measured in detail as a function of intensity, target Z, and angle of incidence θ to compare with modeling calculations of backscatter in inhomogeneous plasma. In addition, electrodynamic charge analyzer measurements have been made for varying intensity and target Z to compare with hydrodynamic calculations of ion expansion and recombination. In the short-pulse experiments, we report on X-ray conversion measurements for 100-ps laser-irradiated targets of varying Z at laser intensities of 1·5 × 1014 and 1015 W/cm2. In particular, it is shown that higher laser intensity leads to a substantial increase in X-ray conversion efficiency.

Experimental treating of hydrodynamic instabilities in laser-accelerated planar targets have pointed out the creation of a mix between a heavy material and a light one. The experiment described here is intended to answer the question: “Is this mix homogeneous or is it an interpenetration of both materials in the form of bubbles and spikes?” The method chosen to study the mix is self-backlighting, that is, backlighting of the target by its own X-ray emission. Comparison of the front and rear X-ray pinhole images of the target allows us to obtain information about the local areal density. Indeed, if 20-μm diameter Au dots are deposited on the target rear face some structures corresponding to these dots are observed on the rear image. The resolution imposed by the pinholes is about 10 μm. By using stable targets Au/CH, we obtained a good coincidence between front and rear pinhole images of the target. By doing the same comparison for unstable targets Au/CH/Au, the coincidence of the images is not significantly different from that obtained with stable targets. Therefore, in this experiment hydrodynamic instabilities do not create structures with dimension larger than 10μm.

The article reviews experiments on flash X radiography of laser-accelerated foils. The spatial resolution, sensitivity, spectral range, and signal-to-noise ratio of measurements were carefully optimized and characterized. The method was used at the Mishen facility to measure a distribution of mass ablative rate across the focal spot and for observation of the transverse plasma flows during the drive laser pulse.

This article presents the tendency of the changes of Nd-laser plasma parameters in dependence upon increasing target atomic numbers. In the experiment, the following targets — (C8H8)n, SiO2, Al, Cu, Ta, and Au — were investigated. For targets with Z higher than 13, the two-temperature plasma is observed with temperatures Te =500–600 eV and Te≅ 100 eV. Energy carried by ions from the low-temperature plasma can be higher than half the total energy carried by ions. The number of ions from low-temperature, X-ray-heated plasma can be by an order of magnitude higher than the number of ions from thermal plasma (Te = 500–600 eV).

Massive carbon targets were irradiated with a pulsed ruby laser and the laser-produced plasma electron densities were simultaneously evaluated using Nomarski interferometry and the refractive fringe diagnostic. An agreement of half an order of magnitude between the two diagnostics was obtained.

Experimental and theoretical studies on the evolution of shock waves in air plasma induced by laser spark have been carried out. The systematic study of the shock wave has been performed experimentally and 1-D numerical code of radiation hydrodynamics (1-DRHC) has been used to simulate the later stage of laser spark in air. The numerical results on the propagation of shock waves and the expansion of hot plasma are presented and subsequent results on the first divergent and convergent shock waves are found to be in good agreement with the experimental data.

Sodium laser blow-off plasma of low temperature (in the 1-eV range) is generated by laser intensities of 108–5.109 W cm−2. Imprisonment of resonant laser light has been observed. These experiments show that basic processes of interaction of radiation with level populations can be studied in the visible range, where the atomic levels have longer lifetimes than the ionic ones in hot plasmas, corresponding to X-ray generation. The imprisonment and resonant effects with various experimental parameters were investigated together with the nonresonant scattering on fragments.

Laser-produced plasma cloud evolution in a magnetic field is investigated up to 3 kg using photographs and spectroheliograms. Generation of a plasma jet, oriented across the magnetic field, was detected, whose emission increases with the field strength and is more distinct for multicharged ions. The effects of the field on the interaction of closely located laser-produced plasma clouds were studied as well. The results of numerical analysis comply with experimental data on plasma behavior in the magnetic field.

Spectroscopic study of plasma produced through bombardment of the 1 × 3 mm2 area on the surface of a solid Mg target with Kr+ ions is carried out. Spectral lines of Mg I and Mg II ions were observed in the visible range (200–600 nm). The plasma parameters Ne = 1.4.1017 cm−3 and Te = 0.8 eV are calculated from electron impact broadening of the 4f–3d line of Mg II and from the ratio of intensities for the 4f–3d and 4s–3p lines of Mg II. The ionic composition of the plasma is determined. The detected X-ray emission is shown to be the characteristic emission of the target.

The first harmonics beam generated by an iodine laser system was focused by an f/2 optics on an Al foil target. The X-ray output from the laser plasma both in the line and broad-band spectra was registered over an interval around the “ideal” focus. It was found that the maximum X-ray power is not obtained in the focus itself but for a somewhat larger focal spot outside the focus. To explain this phenomena, temperature and density measurements were in addition made. The plasma temperature evaluated from both the line (He-like Al XII resonant line and j, k, l satellites) and broad-band spectra (two foil method) was also measured and found to be largely constant in the vicinity of the focus. The line and broad-band temperatures differ, the broad-band temperature being about 25% higher. The electron density was equally determined using an intercombination line.