Diagnosing Implosions at the National Ignition Facility with X-ray Spectroscopy
نویسندگان
چکیده
X-ray spectroscopy is used on the National Ignition Facility (NIF) to diagnose the plasma conditions in the hot spot and the compressed shell of ignition-scale inertial confinement fusion (ICF) implosions. Ignition of an ICF target depends on the formation of a central hot spot with sufficient temperature and areal density. The concentric spherical layers of current NIF ignition targets consist of a plastic ablator surrounding a thin shell of cryogenic thermonuclear fuel (i.e., hydrogen isotopes), with fuel vapor filling the interior volume. A fraction of the ablator has a Ge dopant to minimize preheat of the ablator closest to the DT ice caused by Au M-band emission from the hohlraum x-ray drive. This paper concentrates on three spectral features of the implosion: Ge Heα emission, Ge Kα emission, and the Ge K-edge. Hydrodynamic instabilities seeded by high-mode (50 < < 200) ablator-surface perturbations on ignition-scale targets can cause mixing of Ge-doped ablator into the interior of the shell at the end of the acceleration phase [1]. As the shell decelerates, it compresses the fuel vapor forming a hot spot. K-shell line emission from the ionized Ge that has penetrated into the hot spot provides an experimental signature of hot-spot mix. The amount of hot-spot mix mass is estimated from the brightness and spectral line shape of the Ge Heα and satellite emission using a detailed atomic physics code. X-ray continuum from the hot spot is attenuated by the compressed shell, and the photoexcitation causes the shell to fluoresce in Ge Kα emission. The contrast at the Ge K-edge and the brightness of Ge Kα emission are used to diagnose the shell areal density. The highlighted spectral features measured in the 9.75-13.1 keV photon energy range
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