Transverse Beam Structure Formation in Crossed Beam Energy Transfer

Crossed Beam Energy Transfer (CBET) is important in direct drive and indirect drive inertial confinement fusion. Along with the energy transfer between the beams, their profiles are reshaped. While CBET has been at the focus of research for a number of years, the details of the energy transfer and beam reshaping are not yet fully understood.

In many situations transverse beam reshaping is interpreted as an effect of the matching conditions for Brillouin or Raman scattering which are met only at certain locations in the non-uniform flowing plasma around a target. This confines the energy exchange to well-defined regions of the plasma and imposes a structure on the beam envelopes. However, this does not explain the beam envelope scalloping or "bursty" behavior that can even be seen in homogeneous plasma, where this argument does not apply.

We present 2D numerical simulations of CBET of two perpendicular beams, using a hydrodynamic code coupled to a full Maxwell solver. The simulations exhibit transverse structuring of the outgoing beams caused by pump depletion and subsequent repeated exchange of energy between pump and seed beam. For these conditions beam reshaping can not be explained by local fulfillment of the matching conditions; alternative explanations will be discussed.