Engineering Papers⌕ Search

Engineering topics

Divol, L.

Publications and source records attributed to Divol, L..

3 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Diagnosing inertial confinement fusion ignition

Abstract Fusion ignition by inertial confinement requires compression and heating of the fusion fuel to temperatures in excess of 5 keV and densities exceeding hundreds of g/cc. In August 2021, this scientific milestone was surpassed at the National Ignition Facility (NIF), when the Lawson criterion for ignition was exceeded generating 1.37MJ of fusion energy (Abu-Shawareb et al 2022 Phys. Rev. Lett. 129 075001), and then in December 2022 target gain >1 was realized with the production of 3.1MJ of fusion energy from a target driven by 2.0MJ of laser energy (Abu-Shawareb et al 2024 Phys. Rev. Lett. 132 065102). At the NIF, inertial confinement fusion research primarily uses a laser indirect drive in which the fusion capsule is surrounded by a high-Z enclosure (‘hohlraum’) used to convert the directed laser energy into a symmetric x-ray drive on the capsule. Precise measurements of the plasma conditions, x-rays, γ -rays and neutrons produced are key to understanding the pathway to higher performance. This paper discusses the diagnostics and measurement techniques developed to understand these experiments, focusing on three main topics: (1) key diagnostic developments for achieving igniting plasmas, (2) novel signatures related to thermonuclear burn and (3) advances to diagnostic capabilities in the igniting regime with a perspective toward developments for intertial fusion energy.

Moore, A. S. (ORCID:0000000221258882)↗
DOE OSTI2024

Inverse bremsstrahlung absorption rate for super-Gaussian electron distribution functions including plasma screening

Here we provide analytic expressions for the effective Coulomb logarithm for inverse bremsstrahlung absorption which predict significant corrections to the Langdon effect and overall absorption rate compared to previous estimates. The calculation of the collisional absorption rate of laser energy in a plasma by the inverse bremsstrahlung mechanism usually makes the approximation of a constant Coulomb logarithm. We dispense with this approximation and instead take into account the velocity dependence of the Coulomb logarithm, leading to a more accurate expression for the absorption rate valid in both classical and quantum conditions. In contrast to previous work, the laser intensity enters into the Coulomb logarithm. In most laser-plasma interactions the electron distribution function is super-Gaussian [Langdon, Phys. Rev. Lett. 44, 575 (1980)], and we find the absorption rate under these conditions is increased by as much as ≈ 30% compared to previous estimates at low density. In many cases of interest the correction to Langdon's predicted reduction in absorption is large; for example at Z = 6 and Te = 400 eV the Langdon prediction for the absorption is in error by a factor of ≈ 2. However, we also account for the additional effect of plasma screening, which predicts a reduction in absorption by a similar amount (up to ≈ 30%). These two effects compete to determine the overall absorption, which may be increased or decreased, depending on the conditions. The corrections can be incorporated into radiation-hydrodynamics simulation codes by replacing the familiar Coulomb logarithm with an analytic expression which depends on the super-Gaussian order “M” and the screening length.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗