Impact of flow-induced beam deflection on beam propagation in ignition scale hohlraums*

Modeling of spectroscopy experiments within ignition scale hohlraums is in broad agreements with experimental measurements when including self-generated magnetic fields and the corresponding reduction in heat flux. However, these simulations cannot explain a recent measurement of specular reflection or "glint" from the waist as observed by a witness plate. An inline beam deflection model has been developed which provides a means to unify a description of both the heat transport and the laser coupling observed in these experiments. This removes the need to tune the heat flux limiter to experimental measurements. Observations show that the glinted laser power is roughly 2 - 3 TW. The common hohlraum model produces < 0.4 TW of glinted power, while simulations including beam deflection enhance the glinted power to 2 - 4 TW in agreement with measurements. Further, the beam deflection model matches many of the observed features: the scaling with gas fill density and the location of the glinted power on the witness plate. Here, we discuss the development of this model and these comparisons with the measurements. Early in the rise to peak power, beam deflection enhances glinted light due to altering the incidence angle at the waist of the hohlraum, driving portions of the beam into areas of the wall that possess a steeper density gradient, and shortening the distance that the laser spends within gold material. Later in time when the gold bubble generated by the outer beams begins to obscure the path of the inner beams, beam deflection experienced within the gold bubble similarly enhances glinted light. Finally, the models broader impact as applied to a variety of laser driven hohlraum shots performed at the National Ignition Facility is discussed.