Diagnosing inertial confinement fusion ignition


 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 from a target driven by 1.9MJ of laser energy, 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. On the NIF, ICF research primarily uses laser indirect drive in which the fusion capsule is surrounded by a high-Z enclosure (“Hohlraum”) used to convert the directed laser energy into symmetric x-ray drive on the capsule. Precise measurements of the plasma conditions, x-rays, γ-rays and neutrons produced is key to understanding the pathway to higher performance. The 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 Inertial Fusion Energy (IFE).

Fusion ignition by inertial confinement requires compression and heating of the fusion fuel to temperatures in excess of 5 keV and densities exceeding several hundred g/cc.In hot-spot ignition this must be achieved while controlling radiation and thermal conduction losses, so that heating by the a-particles generated in the [D+T→ n (14.1MeV) + 4 He (3.5 MeV)] fusion reaction dominates and the plasma begins to self-heat, burn, and ignite [1,2].On the NIF, ICF research primarily uses laser indirect drive in which the fusion capsule is surrounded by a high-Z enclosure ("Hohlraum") used to convert the directed laser energy into symmetric x-ray drive on the capsule.Precise measurements of the plasma conditions, x-rays, g-rays and neutrons produced is key to understanding the pathway to higher performance.This paper will discuss the diagnostics and measurement techniques developed to understand these ICF experiments and will focus 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 Inertial Fusion Energy.features such as the tube used to fill the capsule with DT gas as significant degradations and have driven capsule quality improvements.X-ray radiography of implosions together with downscattered neutron energy measurements have provided evidence of lower-than-expected DT fuel compression limiting performance, and 2D shock velocity measurements have characterized the impact of diamond crystal structure on implosion quality.In combination advances in optical, xray nuclear diagnostics [3,4,5], have all played a key role in improving understanding of Hohlraum energetics, capsule symmetry and mix, and overall implosion performance, which together impacted decisions and design choices that resulted in fusion ignition.
Signatures of the onset of burn and ignition are both evident in and connected across multiple types of measurements; this consistency was critical for an unambiguous determination of target gain > 1.In addition to the increased fusion neutron yield, Fig. 1 shows the consistent picture evidenced by multiple diagnostics: (a) an increased neutron emission region due to burn propagation, (b) increased ion temperatures measured by neutron spectroscopy (>10keV), and (c) reduced burn durations (< 100ps).Fig. 2 shows an independent, yet clear signature measured by the diagnostic usually used to monitor the Hohlraum x-ray emission.In these igniting experiments the fusion energy emitted by the igniting capsule is seen to reheats the hohlraum after laser turns off, and actually exceeds the x-ray power emitted during laser drive.
Having entered the ignition regime there are naturally capability gaps in diagnostics that need to be overcome to understand this novel regime.Fundamentally, instruments must be radiation hardened for the survivability of detectors and diagnostics.From a physically motivated perspective the onset of a-heating and burn occurs on sub-100ps timescales and over multiple orders of magnitude in fusion reaction yield, requiring a new generation of ultrafast time-resolution diagnostics that have >10 3 dynamic range, all while operating in the harsh ICF environment.
Finally, with the grand goal of clean inertial fusion energy there are substantial scientific and technological challenges that diagnostics will need to overcome [6].Operation at the even higher yields (~10 20 ) required for power generation, high repetition (~10Hz), minimized solid aperture, and coexisting in the environment of a power plant are several.Stretching the scope of current diagnostic efforts, these challenges will be a grand undertaking requiring many years of development -this paper will outline some of these challenges.This work was performed under the auspices of Lawrence Livermore National Security, LLC (LLNS) under Contract DE-AC52-07NA27344.LLNL-ABS-844482 precision and 3D reconstruction of neutron hot-spot shape and motion have shown that imbalanced drive can degrade fusion performance.Quantification of x-ray emission imaging has identified target

Fig. 1 .
Fig. 1.In addition to increased fusion neutron yield, signatures of the onset of a-heating, burn propagation and ignition are observed by multiple independent measurements: (a) neutron imaging, (b) ion temperature from neutron spectroscopy and (c) the fusion burn duration.

Fig. 2 .
Fig.2.As the ignition threshold is reached, the x-ray emission due to the laser-heating of the Hohlraum, used to drive LID-ICF experiment, is exceeded by the energy from the igniting fusion capsule that reheats the hohlraum