Selenium X-ray K-shell source optimization for the Crystal Backlighter Imager at the National Ignition Facility

The Crystal Backlighter Imager (CBI) is a backlit quasi-monochromatic x-ray radiography system designed to image ICF implosions with the Se He_α line at 11.652 keV. This diagnostic has the capability to image late in an implosion because it only requires the source brightness to be larger than that of the capsule self-emission in a narrow bandwidth around the backlighter line energy. We present here a series of NIF experiments that aim to optimize the CBI configuration by increasing the number of Se He_α photons to the detector. Data will be shown from various backlighter target positions and laser configurations. Additionally, we will present NIF data from recent source development efforts to specifically improve laser to Se He_α x-ray conversion efficiency with a small (1 mm height, 1mm dia) Se-lined cylinder target. Larger cavity targets have shown an increase in temperature and emission from plasma stagnation on axis compared to open-geometry foils. Characterization and performance of the new source will be discussed and compared to the flat foil approach currently used with CBI.