Gamma-Ray Measurements of Burning Plasmas achieved in Inertial Fusion

Reaction history measurements, such as the time of peak reaction rate (i.e., bang time) and how long thermonuclear conditions are maintained (i.e., burn width), are fundamental information indicating the dynamics of inertial confinement fusion. Fusion gammas provide a direct measure of nuclear reaction rates (unlike x-rays) without being compromised by Doppler spreading (unlike neutrons), however spectral information of fusion gammas is not readily available. In a deuterium-tritium fusion, the R-matrix nuclear analysis has predicted two branches of gamma rays (i.e., the transition to the ground state (g₀) and the transition to the intermediate excited state (g₁)). There have been large inconsistencies especially when determining state g₁, the team of LANL/AWE were able to confirm for the first time the presence of two gamma branches and reported a ratio of g₀ : g₁ = (2.1 ± 0.4) : 1 and eliminate ambiguity in the fusion gamma measurement.

While GRH has provided the bang time and burn width since the inception of National Ignition Campaign, there has been an unsettling discrepancy especially in the burn width between simulations and measurements. The team made novel design advancements to include pulse dilation technology which resulted in 10x fast temporal resolution of only 10 ps. This discrepancy persisted until the August 2021 record-yield breakthrough where the prior GRH measurements were confirmed, showing a record short burn duration of 90 ps (30% narrower than before), which is a strong indication of burn propagation.

In summary, through innovating temporal response of instrument and deepening understanding of fusion gamma-rays itself, gamma-ray measurements of burning plasmas can challenge the continued improvement in simulation codes so that the impediments to ignition could be truly identified and addressed.