We present the fmal optics design of the Megajoule Laser (LMJ) using high efficiency transmission gratings at 1053nm and 35mm for broadband tripling, wavelength filtering, laser diagnostics sampling and focusing on target. We also present a 1 :3 scale prototype of our optical system and we summarize the experimental results obtained during the year of 1998.

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The experiments reported in this paper demonstrate that the PHEBUS laser facility is now currently being operated with high performances (4 TW with 250 ps pulses at 0-527 jum wavelength).The output energy of the 2-beam PHEBUS laser system can be focused either in a small focal spot (80% of the incident energy is in a 220 /zm diameter focal spot) for high intensity experiments (3=5 x 10 15 W cm" 2 ) or in very large spots (a few mm in diameter) at moderate intensities (10 13 -2-5 x 10 14 WcrrT 2 ), for large scale experiments. It is shown that the spatial intensity distribution in the target plane is primarily due to intensity independent aberrations and to diffraction. Laser light absorption in plane aluminum and gold targets are interpreted in terms of inverse bremsstrahlung absorption that may account for 70 to 90% of absorbed energy. Finally, the plasma expansion is shown to be very planar and comparison with one-dimensional Lagrangian simulations gives flux limiter values of 0-03 and 0-02 respectively for Al and Au targets. G. Thielletal. Pixels 3072 2048 1024 1024 Pixels FIGURE 1. Temporal profile of the oscillator pulse recorded through a Fabry-Perot etalon with a streak camera.

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