The Laser Mégajoule (LMJ) facility has about 40 large optics per beam. For 22 bundles with 8 beams per bundle, it will contain about 7.000 optical components. First experiments are scheduled at the end of 2014. LMJ components are now being delivered. Therefore, a set of acceptance criteria is needed when the optical components are exceeding the specifications. This set of rules is critical even for a small non-conformance ratio. This paper emphasizes the methodology applied to check or re-evaluate the wavefront requirements of LMJ large optics. First we remind how LMJ large component optical specifications are expressed and we describe their corresponding impacts on the laser chain. Depending on the location of the component in the laser chain, we explain the criteria on the laser performance considered in our impact analyses. Then, we give a review of the studied propagation issues. The performance analyses are mainly based on numerical simulations with Miró propagation simulation software. Analytical representations for the wavefront allow to study the propagation downstream local surface or bulk defects and also the propagation of a residual periodic aberration along the laser chain. Generation of random phase maps is also used a lot to study the propagation of component wavefront/surface errors, either with uniform distribution and controlled rms value on specific spatial bands, or following a specific wavefront/surface Power Spectral Distribution (PSD).
This paper investigates hot spot characteristics generated by the superimposition of multiple laser beams in the Laser
MegaJoule configuration. First, properties of speckle statistics are studied in the context of the superimposition of several
laser beams. The case of a single quadruplet is studied. Values of the speckle width and of the speckle length as well as
of the abundance of the speckles are compared to the results given by numerical simulations. Application to the speckle
patterns generated in the Laser MegaJoule configuration in the zone where all the beams overlap is then presented. The
case of three different polarizations is investigated: P polarization, S polarization and the case of Double Polarization
smoothing (DPS). It is found that the sizes of the speckles and their abundance depend on the choice of the polarization
and that DPS seems to be the best option.
We investigate theoretically the stimulated Brillouin scattering (SBS) which occurs during damage experiments on thick fused silica samples. A 3D time dependent model shows that for conditions equivalent to the experimental ones, the Stokes fluence at the front surface is of the same order as the incident laser fluence increasing in this way the damage probability of this face. The effect of self-focusing on this process is presented.