Theory and simulation of Cross Beam Energy Transfer between laser beams smoothed with Random Phase Plates

Two laser beams can exchange their energy through a driven acoustic wave. This three-wave coupling, known as Cross beam energy transfer (CBET), frequently occurs in experiments related to inertial confinement fusion (ICF). As the beams are smoothed with phase plates, their resulting intensities are formed of many hot spots called speckles. CBET thus occurs at each of the numerous crossing speckles and may be affected by these inhomogeneities. In previous works [2,3], both 2D Particle-In-Cell (PIC) simulations in an idealised situation and a reduced model have shown the importance of accounting for the speckle structure of the beam to quantify the energy exchange. These results are now improved by accounting for realistic smoothed laser beams -assuming random phase plate (RPP)- in our large scale PIC SMILEI simulations. The exact RPP field of two lasers is imposed as initial conditions, in the case of a strongly (CH) and a weakly (Carbon) Landau-damped plasmas. These simulations are compared with an improved theory of CBET that takes into account the RPP field structure at the focal spot [1]. The energy exchange is calculated  in a large variety of ICF plasmas. CBET induced by a wavelength shift leads to a decrease of the exchange of respectively about 30% / 80% for a strongly/weakly damped plasma. These results confirm the need to account for the speckle structure in inline CBET models for hydrodynamic codes.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.