Reduction of cross beam energy transfer by speckles pattern

Many experiments dedicated to Inertial Confinement Fusion and laboratory astrophysics require the use of several tens of high intensity laser beams. The crossing of these beams through the produced plasma may induce significant energy exchanges, hence modifying the laser energy deposition. This effect, known as cross beam energy transfer (CBET), is a three-wave mixing process in which an electronic wave partially scatter one beam along the direction of the second beam.

Because the laser beams used in these experiments are spatially smoothed with a Random Phase Plate (RPP), their resulting intensities are formed of many hot spots called speckles. However, in most hydrodynamic models, this speckle structure is not taken into account and may explain the difficulties to reproduce the experimental results.

To this end, we performed 2D Particle-In-Cell simulations which describe the exchange between two RPP beams having different frequencies, in a stationnary plasma. A substantial energy exchange drop down by 50% is observed compare to the case where the laser beams share the same frequency while crossing each other in a flowing plasma. Supported by a model, these results suggest that the CBET modelling in most of lagrangian hydrodynamic codes is inaccurate when the laser beams have different frequencies.