Measurements of specular reflections (“glint”) of the inner beams in a hohlraum*

At the National Ignition Facility (NIF) indirect-drive inertial confinement fusion (ICF) experiments aim to obtain controlled thermonuclear burn by firing 192 laser beams, grouped into 4 beams per quad and 4 cone angles, onto the inner surface of a high-Z hohlraum. The goal is a sufficiently uniform and intense radiation bath generated at the hohlraum wall to compress a spherical capsule containing the fuel at the center of the hohlraum.  There are several processes that can absorb or re-direct the laser energy, reducing the x-ray drive and symmetry, reducing thermonuclear burn effiiciency. In this work we quantify one of these processes – inner beam specular reflection of laser light off the cylindrical hohlraum wall at peak power that escapes through the opposite laser entrance hole of the hohlraum. The amount of reflection "glint" is inferred from the x-rays generated by the bottom glinted inner beams that hit a Ti witness plate capping the top part of the hohlraum.  The x-ray signal is calibrated by a second Ti witness plate hit by a known laser intensity and placed outside the hohlraum. The x-ray emission from both witness plates is imaged onto a gated x-ray camera to evaluate the inner beam glint power as a function of time and hohlraum gas fill pressure. Large variations in the glint level were observed between inner beams even within the same quad indicating that inner cone glint is largest from the most grazing incidence angle beams (21.2°) with the lowest turning point density.  Further understanding of the potential roles of intraquad CBET was gained by changing the polarization arrangement of the 23.5° quad beams.