National Ignition Facility

Quantum mechanics

Scale (ratio)

Linguistics

Engineering

Philosophy

Materials science

Nuclear physics

Energy (signal processing)

Omega

Atomic physics

Plasma

Classical mechanics

Thomson scattering

Scaling

Thermodynamics

Laser

Physics

Electron density

Nuclear engineering

Fusion

Electron temperature

Inertial confinement fusion

Mathematics

Optics

Hot electron

Inertial frame of reference

Geometry

Electron

Intensity (physics)

Mechanics

Ignition system

Hohlraum

Physical Review Letters

Planar laser-plasma interaction (LPI) experiments at the National Ignition Facility (NIF) have allowed access for the first time to regimes of electron density scale length (∼500 to 700 μm), electron temperature (∼3 to 5 keV), and laser intensity (6 to 16×10^{14} W/cm^{2}) that are relevant to direct-drive inertial confinement fusion ignition. Unlike in shorter-scale-length plasmas on OMEGA, scattered-light data on the NIF show that the near-quarter-critical LPI physics is dominated by stimulated Raman scattering (SRS) rather than by two-plasmon decay (TPD). This difference in regime is explained based on absolute SRS and TPD threshold considerations. SRS sidescatter tangential to density contours and other SRS mechanisms are observed. The fraction of laser energy converted to hot electrons is ∼0.7% to 2.9%, consistent with observed levels of SRS. The intensity threshold for hot-electron production is assessed, and the use of a Si ablator slightly increases this threshold from ∼4×10^{14} to ∼6×10^{14} W/cm^{2}. These results have significant implications for mitigation of LPI hot-electron preheat in direct-drive ignition designs.

Origins and Scaling of Hot-Electron Preheat in Ignition-Scale Direct-Drive Inertial Confinement Fusion Experiments