ALBERT

Description: Pressure, by definition, characterizes the conditions within an isobaric implosion core at peak compression [Gus'kov et al. , Nucl. Fusion 16 , 957 (1976); Betti et al. , Phys. Plasmas 8 , 5257 (2001)] and is a key parameter in quantifying its near-ignition performance [Lawson, Proc. Phys. Soc. London, B 70 , 6 (1957); Betti et al. , Phys. Plasmas 17 , 058102 (2010); Goncharov et al. , Phys. Plasmas 21 , 056315 (2014); and Glenzer et al ., Phys. Plasmas 19 , 056318 (2012)]. At high spectral energy, where the x-ray emission from an imploded hydrogen core is optically thin, the emissivity profile can be inferred from the spatially resolved core emission. This emissivity, which can be modeled accurately under hot-core conditions, is dependent almost entirely on the pressure when measured within a restricted spectral range matched to the temperature range anticipated for the emitting volume. In this way, the hot core pressure at the time of peak emission can be inferred from the measured free-free emissivity profile. The pressure and temperature dependences of the x-ray emissivity and the neutron-production rate explain a simple scaling of the total filtered x-ray emission as a constant power of the total neutron yield for implosions of targets of similar design over a broad range of shell implosion isentropes. This scaling behavior has been seen in implosion simulations and is confirmed by measurements of high-isentrope implosions [Sangster et al. , Phys. Plasmas 20 , 056317 (2013)] on the OMEGA laser system [Boehly et al. , Opt. Commun. 133 , 495 (1997)]. Attributing the excess emission from less-stable, low-isentrope implosions, above the level expected from this neutron-yield scaling, to the higher emissivity of shell carbon mixed into the implosion's central hot spot, the hot-spot “fuel–shell” mix mass can be inferred.