Reaction-in-Flight Neutrons as a Test of Stopping Power in Degenerate Plasmas

TL;DR

This study measures reaction-in-flight neutrons in inertial confinement fusion to test and constrain plasma stopping power models in dense, degenerate conditions, providing new experimental insights into plasma behavior.

Contribution

First experimental measurement of RIF neutrons in ICF, offering a novel test of stopping power models in dense, degenerate plasmas.

Findings

RIF neutrons constitute about 10^{-4} of total neutrons.

Data indicates the plasma is moderately to strongly coupled and electron degenerate.

Some stopping power models are inconsistent with the experimental data.

Abstract

We present the first measurements of reaction-in-flight (RIF) neutrons in an inertial confinement fusion system. The experiments were carried out at the National Ignition Facility, using both Low Foot and High Foot drives and cryogenic plastic capsules. In both cases, the high-energy RIF ($E_n>$ 15 MeV) component of the neutron spectrum was found to be about $10^{-4}$ of the total. The majority of the RIF neutrons were produced in the dense cold fuel surrounding the burning hotspot of the capsule and the data are consistent with a compressed cold fuel that is moderately to strongly coupled $(\Gamma\sim$0.6) and electron degenerate $(\theta_\mathrm{Fermi}/\theta_e\sim$4). The production of RIF neutrons is controlled by the stopping power in the plasma. Thus, the current RIF measurements provide a unique test of stopping power models in an experimentally unexplored plasma regime. We find that the measured RIF data strongly constrain stopping models in warm dense plasma conditions and some models are ruled out by our analysis of these experiments.