Numerical Modeling of the Sensitivity of X-Ray Driven Implosions to Low-Mode Flux Asymmetries

TL;DR

This paper uses numerical modeling to explore how low-mode flux asymmetries, especially P4 shapes, impact the uniformity and efficiency of inertial confinement fusion implosions, revealing challenges in diagnosing and correcting these asymmetries.

Contribution

It demonstrates the effects of low-mode flux asymmetries on implosion dynamics and neutron yield, highlighting the difficulty in quantifying P4 asymmetries through x-ray imaging.

Findings

Large P4 asymmetries cause non-uniform capsule deceleration.

P4 asymmetries reduce neutron yield by disrupting energy transfer.

P4 shapes may be hidden as P2 distortions in x-ray images.

Abstract

The sensitivity of inertial confinement fusion implosions of the type performed on the National Ignition Facility (NIF) to low-mode flux asymmetries has been investigated numerically. It is shown that large-amplitude, low-order mode shapes (Legendre polynomial P4), resulting from associated low order flux asymmetries, cause spatial variations in capsule and fuel momentum that prevent the DT ice layer from being decelerated uniformly by the hot spot pressure. This reduces the transfer of kinetic to internal energy of the central hot spot, thus reducing neutron yield. Furthermore, synthetic gated x-ray images of the hot spot self-emission indicate that P4 shapes may be unquantifiable for DT layered capsules. Instead the positive P4 asymmetry aliases itself as an oblate P4 in the x-ray self emission images. Correction of this apparent P2 distortion can further distort the implosion while creating a round x-ray image. Long wavelength asymmetries may be playing a significant role in the observed yield reduction of NIF DT implosions relative to detailed post-shot 2D simulations.