Observation of hohlraum-wall motion with spectrally selective x-ray imaging at the National Ignition Facility

Hohlraum x-ray preheat asymmetry measurement at the ICF capsule via Mo ball fluorescence imaging

In inertial confinement fusion, penetrating asymmetric hohlraum preheat radiation (>1.8 keV, which includes high temperature coronal M-band emission from laser spots) can lead to asymmetric ablation front and ablator-fuel interface hydrodynamic instability growth in the imploding capsule. First experiments to infer the preheat asymmetries at the capsule were performed on the National Ignition Facility for high density carbon (HDC) capsules in low density fill (0.3 mg/cc ⁴He) Au hohlraums by time resolved imaging of 2.3 keV fluorescence emission of a smaller Mo sphere placed inside the capsule. Measured Mo emission is pole hot (P₂ > 0) since M-band is generated mainly by the outer laser beams as their irradiance at the hohlraum wall is 5× higher than for the inner beams. P₂ has a large swing vs time, giving insight into the laser heated hohlraum dynamics. P₄ asymmetry is small at the sphere due to efficient geometric smoothing of hohlraum P₄ asymmetries at large hohlraum-to-capsule radii ratios. The asymmetry at the HDC capsule is inferred from the Mo emission asymmetry accounting for the Mo/HDC radius difference and HDC capsule opacity.

Development of a gated X-ray imager with multiple views and spectral selectivity for observing plasma evolution in hohlraums

In the study of indirect drive Inertial Confinement Fusion (ICF), X-ray images through the laser entrance hole of the hohlraums can provide key information. In this paper, we present the advances on the development of a gated X-ray imager with multiple views and spectral selectivity. A diagnostic snout which contains four X-ray planar mirrors has been designed and coupled to a gated X-ray detector which includes an air box and has a spatial resolution better than 35 µm and a temporal resolution better than 70 ps. The two inner mirrors in the diagnostic snout operate at M band (1.6-3.5 keV), while the two outer mirrors operate at N band (0.5-1 keV). The four mirrors define four observation angles. Each mirror can reflect 4 X-ray images to the four microstrips of the gated X-ray detector with almost the same observation angle. The imager enables us to observe the spatial and temporal evolution of plasma in the hohlraums from four different observation angles based on both N band and M band images. The imager has been used in the ICF experiments conducted at Shenguang laser facility in China, and the obtained X-ray images of Au cylindrical hohlraum are presented and discussed in this paper.