Modeling of energy transfer between two crossing smoothed laser beams in a plasma with flow profile

Abstract

Abstract. We study the crossed beam energy transfer (CBET) between laser fields generated by optical smoothing methods. The energy transfer, as well as the angular distribution of the outgoing light fields are investigated for two incident smoothed laser beams in a plasma with a flow gradient, allowing for resonant transfer close to the sonic point. Simulations with the code Harmony based on time-dependent paraxial light propagation are compared to simulations using a new approach based on paraxial complex geometrical optics (PCGO). Both approaches show good agreement for the average energy transfer past a short transient period, which is a promising result for the use of the PCGO method as a module within a hydrodynamics code to efficiently compute CBET in mm-scale plasma configurations. Statistical aspects related to role of laser speckles in CBET are considered via an ensemble of different phase plate realizations. IntroductionFor both Direct Drive and Indirect Drive concepts of laser fusion, a reliable modeling of laser propagation is of crucial importance. Such a modeling has to take into account the potential energy exchange between laser beams, and between groups of laser beams.For this purpose we show here results of a comparison between two approaches to model crossed beam energy transfer (CBET), namely (i) a standard modeling of laser plasma interaction using a paraxial wave solver coupled to non linear fluid code for the plasma motion and (ii) a new approach that is based on paraxial complex geometrical optics (PCGO) and which can be implemented in radiation hydrodynamics codes. In this approach the path of multiple beamlets is integrated by a set of model equations [1,2] for each time step of the hydro code.In particular we investigate the crossing of laser beams, whose fields have been generated by optical smoothing methods as random (or kinoform) phase plates (RPP, KPP). The energy transfer considered in this comparison corresponds hence to a realistic situation that may appear in laser fusion configurations where wide beams with speckle structure overlap in a plasma profile [3]- [8]. We concentrate to one of the most likely configurations where resonant exchange between beams of equal laser frequency occurs, namely in a plasma with a flow gradient in the vicinity of sonic point. We compare the amplification of the beam that receives transfer from its counterpart between both models and analyze the outgoing laser fields behind the crossing zone with respect to their angular distribution and direction, and we investigate the statistical role of speckles.

Crossed beam energy transfer in the presence of laser speckle ponderomotive self-focusing and nonlinear sound waves

“…At beam intensities slightly above the moderate intensity values considered in our previous work in Ref. 19,28 the difference in the description of CBET with and without speckle structure becomes already striking.…”

“…In this interval no significant differences in the transfer between the beams was seen when comparing regular and RPP beams, even when accounting for selfinteraction. The differences seen between the results for different RPP realizations are merely due to speckle statistics 28 .…”

“…4,16,27 In the regime of moderate laser intensity of the crossing beams, i.e. when no self-focusing in laser speckles arises, recent studies 19,28 have shown that the role of speckles in the energy transfer is merely of statistical nature: the deviation in the energy transfer arising from different RPP realisations decreases with the number of speckles in the crossing volume, and the expectation value of the transfer corresponds to the value obtained when assuming the average intensity of each beam.…”