Soft X-Ray Spectroscopic Measurements of Plasma Conditions at Early Times in ICF Experiments on OMEGA. Final technical report

Griem, Hans R. ; Elton, Raymond C.

University of Maryland at College Park. Institute for Plasma Research.

关键词: Laser Targets;    Irradiation;    70 Plasma Physics And Fusion Technology;    Laser Radiation;    Omega Facility;   

DOI  :  10.2172/761595
RP-ID  :  DOE/SF/21783-2
RP-ID  :  FG03-99SF21783
RP-ID  :  761595

美国|英语

来源: UNT Digital Library

【 摘 要 】

Our previously-reported observation of a disruptive prepulse on OMEGA, possibly as large as 1% of the extreme ultraviolet (euv) radiation measured from the main pulse, has recently been substantiated by the measurement of euv absorption spectra, prior to the main pulse. The absorption features have been identified with n=2 photoionization in aluminum atoms and ions up to Al5+. Cold aluminum is originally present as a 0.0125-um thick sealant coating applied to a neon-filled (10 atm) CH microballoon, with an euv transmission at the L-absorption edge of {approx}50%. The aluminum in turn is overcoated with 2 um of Mg. The spectra which show the absorption also include continua as well as line emissions from Mg9+ to Mg11+ ions. These occur prior to the onset of laser target irradiation by at least 10 ns, and imply a prepulse irradiance of about 10{sup 12} W/cm{sup 2}. Since the neon and CH are opaque to euv radiation from the rear, a likely scenario is early (prepulse) vaporization of the outer Mg layer, perhaps in hot spots, followed by laser radiation transmitted through the thin Al layer, thereby heating the CH surface. This could provide an euv continuum backlighter for the aluminum that leads to the euv absorption features in various ionic species, perhaps from different points on the target surface.