In the context of the direct-drive Laser-Mégajoule (LMJ) fusion-research program, the characteristic long-wavelength nonuniformities produced by irradiating a pellet with multiple overlapping laser beams are studied with or without imperfections coming from power imbalance and pointing errors. The intensity profiles are modeled by super-Gaussian shapes. The beam pattern which minimizes the intrinsic LMJ nonuniformity is determined. It is shown the LMJ beam placement creates untractable nonuniformities from even modes. The laser imperfections create odd and low modes of nonuniformity which can dominate the even modes.

In high flux (1013–15 W/cm2) laser-plasma interaction, a large part of the incoming laser energy is radiated into soft x-rays. To determine both the shape of this spectrum and the conversion efficiency, we have designed and tested an absolutely calibrated broadband soft x-ray spectrometer with a high temporal resolution (100 ps). The detector in this spectrometer is a new coaxial x-ray diode coupled with a fast single shot oscilloscope (French IN7100 with 7 GHz frequency response cutoff). Both absolute calibrations (x-ray response of diodes) and relative calibrations (filters and mirrors) have used the French synchrotron beam lines at Laboratoire pour l’utilisation du Rayonnement Electromagnétique (LURE) in Orsay. The initial version of this instrument was first successfully implemented on laser plasmas experiments at the Phébus facility in France and an improved version is now operating at the Omega laser facility in Rochester, New York. The emitted x-ray spectrum is absolutely measured in 18 broad bands from 50 eV up to 20 keV. The softer bands (<1.5 keV) combine mirror and filter responses coupled with the coaxial diode response to improve hard x-ray rejection. Intermediate energy channels (1.5 keV<hν<5 keV) used only a filter and coaxial diode. For the hardest channels (>5 keV) we replace the x-ray diode (not sufficiently sensitive) with a photoconductive detector (neutron-damaged GaAs). An equivalent instrument will be designed for the future National Ignition Facility (NIF) in the United States and the Laser Mégajoule (LMJ) in France.

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