Hot-spot mix in ignition-scale implosions on the NIF

Abstract

Ignition of an inertial confinement fusion (ICF) target depends on the formation of a central hot spot with sufficient temperature and areal density. Radiative and conductive losses from the hot spot can be enhanced by hydrodynamic instabilities. The concentric spherical layers of current National Ignition Facility (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 [S. W. Haan et al., Phys. Plasmas 18, 051001 (2011)]. The Rev. 5 ablator is doped with Ge to minimize preheat of the ablator closest to the DT ice caused by Au M-band emission from the hohlraum x-ray drive [D. S. Clark et al., Phys. Plasmas 17, 052703 (2010)]. Richtmyer–Meshkov and Rayleigh–Taylor hydrodynamic instabilities seeded by high-mode (50<ℓ<200) ablator-surface perturbations can cause Ge-doped ablator to mix into the interior of the shell at the end of the acceleration phase [B. A. Hammel et al., Phys. Plasmas 18, 056310 (2011)]. 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 Ge emission from tritium–hydrogen–deuterium (THD) and deuterium–tritium (DT) cryogenic targets and gas-filled plastic-shell capsules, which replace the THD layer with a mass-equivalent CH layer, was examined. The inferred amount of hot-spot–mix mass, estimated from the Ge K-shell line brightness using a detailed atomic physics code [J. J. MacFarlane et al., High Energy Density Phys. 3, 181 (2006)], is typically below the 75-ng allowance for hot-spot mix [S. W. Haan et al., Phys. Plasmas 18, 051001 (2011)]. Predictions of a simple mix model, based on linear growth of the measured surface-mass modulations, are consistent with the experimental results.

Observation of strong electromagnetic fields around laser-entrance holes of ignition-scale hohlraums in inertial-confinement fusion experiments at the National Ignition Facility

“…Such an exciting scientific breakthrough is being vigorously pursued at the National Ignition Facility (NIF) [5] through the indirect-drive approach, in which the capsule implosion occurs in response to tremendous radiation pressure generated by the thermal x-rays in a high-Z enclosure, i.e. hohlraum, when the enclosure's inner wall is irradiated by high-power lasers [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. The lasers would have a temporal pulse shape designed to launch four radially convergent shock waves which would coalesce at the capsule center, creating a self-igniting 'hot spot' which would generate a self-sustaining burn wave that propagates into the main fuel region.…”

“…The National Ignition Campaign (NIC) aims to eventually realize this important goal at the NIF, which has unique capabilities including 192 laser beams and 1.8 MJ of 3ω (0.35 µm) laser energy, enhanced pulse-shaping capabilities and pulse durations and greatly improved irradiation symmetry, etc [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. Several ongoing experimental campaigns have been conducted to tune implosion conditions and to address a wide range of critical physics issues relevant to ignition sciences, including laser coupling, implosion dynamics and symmetry, hot spot formation and burn physics, etc.…”

Technical Applications of the Physics of High Energy Densities

“…The instability of mixing processes was spectroscopically studied in [98,117,198,237]. Measurements of the hard (>100 keV) bremsstrahlung emanating from the fusion target made it possible to estimate the change in adiabatic exponent caused by plasma degeneracy.…”

National Ignition Campaign Program Completion Report

“…Mix signatures include a reduced yield and ion temperature due to radiative cooling and high x-ray brightness from the hot core due to the higher Z ablator mixing into the compressed fuel. Line emission from the hot spot arising from mixing of dopants in the ablator into the fuel is also used to quantify mix [42].…”