CEL-V is investigating techniques for production of low cost components for a multimegajoule laser concept.

The increase of the normal-state resistance of Nb-Al/AlOx-Nb tunnel junctions by annealing at elevated temperatures is reported. Junctions with an area of 4 μm2 or smaller have been investigated. The resistance could be increased up to a factor of 5 with only a small influence on the quality of the quasiparticle characteristic. The effect can be used to adjust the resistance after fabrication. The increase of the AlOx barrier thickness has been estimated. Annealing experiments in nitrogen atmosphere and after nitridation have been carried out in order to find out from where the additional oxygene stems. The annealing properties of anodization curves have been investigated. A clear increase of the oxide peak could be observed. The results also show that the resistance is sensitive to temperature increases in the various fabrication steps.

The Phebus Neodymium glass Laser system located at Centre d'Etudes de Limeil Valenton (CEL-V) is the most powerful in Europe (20 Id, 1 ns at = 1053nm and up to 8 U, 1 ns at = 351 nm). A major concern in the design of this system was the damage thresholds of the 1 meter diameter turning mirrors and of the large polarizers (0.3 m2). MATRA DEFENSE S .A. , in a joint effort with CEL-V, using the well known reactive e-beam process, has investigated and produced a new generation of efficient ZrO/SiO and HfO/SiO high reflective and polarizing coatings, leading to very high laser-conditioned damage thresholds. In addition to good flatness figures (t/4 to ?i /10 at He-Ne wavelength), excellent roughness values (5-10 A RMS) and excellent spectral characteristics, laser-conditioned damage thresholds were in the range of 25-30 J/cm2 and 12- 15 J/cm2 respectively for mirrors and polarizers at 1 .064 m wavelength for a 3 ns pulse. Such performances are respectively 2 and 6 times higher than those of same components in the Ti02/5i02 conventional design. Laser-conditioning effect, enhancing the damage threshold by at least a factor of 2, was observed as a durable improvement

Velocities up to 108 cm s−1 have been obtained in laser implosion of gold-coated microballoons driven by irradiation at short wavelength. Despite a strongly nonuniform illumination, high neutron yield (i.e., high fuel temperature) has been obtained. Results analysis suggests that the glass shell is first decompressed by x-ray radiation from the gold coating and then accelerated by the ablation pressure.

Streak cameras are now currently used as a basic tool in the diagnostics instrumentation or laser-plasma interaction studies. We have recently shown at the 15th congress which was held in San Diego the properties of the the P 650 soft X-ray cameral having both optimized spatial and temporal resolutions (16-20 1p.mm-1, 3 to 5 ps). During 1983-1984 we have continued our efforts to improve and extend the performances of these diagnostics. New developments include : - the used of a cylindrical mirror with the P 650 X-ray streak camera, in order to obtain a streaked soft X-ray imaging device which will be describe with some more details in the following paper. - study of a new X-ray streak camera using a modified image converter tube P 750 X with a best lamellar optics. - enhancement of photocathode sensitivity by testing new materials. - development of new photocathode-tube window arrangement in order to record both laser and X-ray signals. - another important improvement in the same field is the C.C.D. readout which is presented in other papers.

The X-ray emission from laser irradiated high-Z targets has received much attention in the recent years. The two main centers of interest were : spectral studies for the determination of plasma parameters and conversion efficiencies for understanding the plasma energetics. We have performed these kinds of experiments at Limeil during the three past years using spherical and planar targets (Glass, Aluminum, Gold, Copper) and modifying irradiation conditions (laser wavelength and flux).

Microballoons filled with an equimolar deuterium-tritium mixture and coated with a plastic ablator of variable thickness are imploded by the eight-beam Octal laser (λ = 1.06 μm; 0.6 TW). An X-ray diagnostic with space-time resolution is used to analyse the implosion of the targets designed to the highest ∫ ρdr. The experimental results are compared with numerical simulations performed by a one-dimensional Lagrangian code. A DT density of about 2 g·cm−3 is obtained; the transition from an exploding-pusher regime to a more ablative one is analysed on the basis of the evolution of the preheat, the hydrodynamic efficiency and the density and temperature performance.

In laser implosion studies, the data collected from X-ray radiation of the target itself provide information on density and temperature as well as on the implosion mechanism. The radiography of the imploding target with an external X-ray source is a diagnostic tool who can complete or replace this more conventional technique, particularly to analyze the relatively cold and dense core of a target imploded in the ablative scheme. In the introduction we present the experimental device currently used at Limeil and we recall the main phenomena and mechanisms involved in implosion of spherical target by intense lasers. The main characteristics of the laser bear facilities OCTAL are given : λ = 1.06 μm Eight beams with a total power of : 1 TW - 50 ps (E = 50 J) 500 GW - 600 ps (E = 300 J) The targets are glass spherical microballoons with radius from 30 to 150 pm and thickness from .6 to 3 μm. Targets are pressurized until few tens of bars with an equimolecular compo-sition of D-T. With some results obtained in typical experiments we discuss the different possibilities of imagery in the X-ray domain : . Imagery by emission of the target . Radiography of the target with an external X-Ray source obtained by a laser beam (backliahting) . A device is described who combine these two possibilities used simultaneously in the same experiment. . Imagery by self-emission or backlighting with time resolution. Some results are given which are in agreement with numerical modelization.

1.3-keV X-ray backlighting of microballoons imploded with 0.8-TW, 50-ps Nd-glass-laser pulses has been performed early during the irradiation. Transmission measurements have been interpreted in terms of fast-electron energy deposition. Targets with the thinnest walls are shown to be pre-heated up to more than 200 eV.

In laser implosion experiments, X-ray shadowgraphy can replace conventional techniques of self emission recording, to probe the compressed core in the case of low temperature and high density. A simple experimental device using pinholes and X-ray streak camera is presented, as well as adjusting methods. Possible ways of applications are discussed, illustrated by specific examples.

Nous rendons compte de resultats obtenus sur l'experience d'implosion IRIS realisee a Limeil au moyen des quatre faisceaux emis par le systeme laser a verre dope au NdC6 en impulsion nanoseconde et dans une gamme de flux inferieure a 1014 W/cm2. Dans la premiere partie nous donnerons les resultats d'une etude systematique de l'efficacite d'absorption en fonction des conditions de focalisation des faisceaux laser sur la cible. La seconde partie concernera l'etude spatio-temporelle de l'implosion de microballons en impulsion nanoseconde et a flux laser moderes (< 1013 W/cm2).