Speaker
Holger Schmitz
Description
See the full Abstract at http://ocs.ciemat.es/EPS2018ABS/pdf/P5.2009.pdf
Transverse Beam Structure Formation in Crossed Beam Energy Transfer
H. Schmitz1 , R. Trines1 , B. Bingham1
1 STFC Rutherford Appleton Laboratory, Didcot, United Kingdom
Crossed Beam Energy Transfer (CBET) plays a large role in both direct drive as well as
indirect drive inertial confinement fusion. CBET is responsible for redirecting energy from one
beam into another and reshaping the profiles of the laser beams. For this reason CBET has been
at the focus of research activities for a number of years. Despite this, the details of the energy
transfer and beam reshaping are not yet fully understood.
Usually it is assumed that transverse beam reshaping is due to the effect that the matching
conditions for Brillouin or Raman scattering are only met at certain locations in the non-uniform
flowing plasma around the target. In order to interact the beams and the plasma wave must meet
matching conditions for the frequency and wave vector, corresponding to energy and momen-
tum conservation. However, for the case of Brillouin scattering the bandwidth of the laser beams
can be larger than the frequency of the ion acoustic oscillation. For finite bandwidths the match-
ing conditions are relaxed and don’t have to be fulfilled exactly at the beams’ central frequency
and wave vector. As an example of this case, two beams with identical frequency can exchange
energy through an ion acoustic wave in a uniform plasma at rest.
We present 2D, direct numerical simulation of Brillouin Scattering using a hydrodynamic
code coupled to a full Maxwell solver. In this way we avoid the expense of full PIC simulations
while, on the other hand, not restricting the model to a single frequency envelope approximation.
The simulations show transverse structuring of the outgoing beams caused by the depletion of
the incoming beam energy. For these conditions beam reshaping can not be explained by local
fulfilment of the matching conditions and an alternative explanation must be found. In order
to investigate the cause of the structure formation a theoretical model is developed. The model
shows that the beam reshaping is caused by the same effects as pulse compression in Brillouin
backscattering.
