Design of a Peanut Hohlraum with Low Gas-Fill Density for the Laser Megajoule

TL;DR

This paper proposes a novel peanut-shaped hohlraum with lower gas-fill density and larger case-to-capsule ratio to improve drive symmetry and ignition prospects in inertial confinement fusion experiments.

Contribution

Introduction of a noncylindrical peanut hohlraum design with optimized parameters for better drive symmetry and energy efficiency in ICF.

Findings

Requires about 2.5 MJ laser energy for ignition.

Maintains low plasma electron density to reduce Raman backscattering.

Ensures high drive symmetry and minimal laser absorption in the inner cone.

Abstract

Recent experiments on the National Ignition Facility [D.E. Hinkel et al., Phys. Rev. Lett. 117, 225002 (2016)] demonstrates that utilizing a long, large case-to-capsule ratio (=3) conventional cylindrical hohlraum at moderate gas-fill density (=0.6 mg/cm3 4He) improves the drive symmetry controaums has a little chance to achieve ignition at an acceptable energy level due to its small margin for the laser cone propagation. In this letter, a noncylindrical hohlraum, called as peanut hohlraum, using a larger case-to-capsule (=3.46) at lower gas-fill density (=0.3 mg/cm3 4He) is proposed to ignite a high-foot pusher capsule with a shorter pulse (~9ns). The peanut hohlraum requires about 2.5 MJ laser energy to achieve 306 eV peak drive temperature while the low-z plasma electron density on the inner cone path is maintained very low which results in little simulated Raman backscattering and the high-z bubble still stays away from the inner cone path without the laser absorption in it, which favor the drive symmetry control. Crossed-beam energy transfer is also neglectable because the crossing position is far away from the LEH. The peanut hohlraum can provide a good drive environment for capsule to achieve ignition, so it is undoubted that it will add to the diversity of ICF approaches.