Unlocking Fusion: How Scientists Measure Success at NIF

At the National Ignition Facility (NIF), scientists use activation foils by exposing material samples to neutrons, which induces nuclear reactions and creates radioactive species. By measuring the decay of these radioactive nuclei, they can determine the neutron yield. Specifically, the number of neutrons that passed through the material is calculated using a detailed formula that accounts for factors like the distance from the neutron source, atomic mass of the isotope, decay rate, and various efficiencies. The NIF employs five different Neutron Activation Diagnostic (NAD) methods—Well-NAD, NAD20, DIM-NAD, Snout-NAD, and Flange-NAD—each strategically positioned to capture data from different perspectives around the facility. For instance, Indium is used to detect 2.45 MeV neutrons from deuterium-deuterium (D-D) fusion, while zirconium and copper measure 14 MeV neutrons from deuterium-tritium (D-T) reactions.

The Well-NAD is a specific neutron activation diagnostic (NAD) setup at the National Ignition Facility (NIF) that involves placing three zirconium foils of varying thicknesses inside a diagnostic well. This well is located at specific coordinates (θ, φ) = (64,241) on the NIF chamber, positioning the zirconium 4.48 meters from the implosion site. The strategic placement minimizes neutron scatter while keeping the foils outside the chamber's vacuum for easy retrieval. During deuterium-tritium (D-T) fusion, neutrons interact with the zirconium via 90Zr(n,2n)89Zr reactions, which require neutron energies above 12.1 MeV. The resulting 89Zr undergoes β+ decay, producing 88mY, which emits detectable gamma rays. These gamma rays are then measured using high-purity germanium detectors in a low-background counting facility, providing precise measurements of neutron yield. The Well-NAD setup is crucial because it allows for high-accuracy, independent measurements of yield along a specific line of sight, contributing to a comprehensive understanding of the fusion process.

The equation for calculating neutron yield (Yn) from activation foils includes several key components and considerations. The formula, Yn= (4πR²ANe)/(mfBrfaNaϵiϵd(σ)e−λ(Δts)[1−e−λ(Δtc)]), accounts for: 'R' which is the distance from the neutron source to the activation sample, 'A' is the atomic mass of the isotope, 'Ne' the number of measured decay particles, 'm' represents the sample mass, 'fBR' the branching ratio, 'fa' the isotope's abundance, and 'Na' Avogadro's number. Also included are 'ϵi' and 'ϵd', representing irradiation and detection efficiencies, respectively, (σ) the spectrum-weighted cross section, 'λ' the decay constant, and 'Δts' and 'Δtc' are time intervals. Each of these parameters is essential for accurately determining the neutron yield. The accuracy of each parameter directly impacts the overall calculation. For example, the efficiencies (ϵi and ϵd) account for factors like absorption and scatter, ensuring a more precise neutron yield measurement.

Understanding implosion areal density variations (ρR(Ω)) is critical in fusion research because it provides insights into the uniformity and stability of the implosion process. Areal density is a measure of the mass density integrated along a particular line of sight, and variations in this density can indicate asymmetries or instabilities that can impede fusion. At the National Ignition Facility (NIF), neutron activation diagnostics (NADs) play a crucial role by providing multiple lines-of-sight measurements of neutron yield, which are directly related to areal density. By observing significant anisotropies in ρR on some shots using activation foils in multiple locations around the NIF chamber, scientists can infer the spatial distribution of the fuel and identify areas where the implosion is not uniform. This information is essential for refining the implosion process and optimizing fusion performance. The NADs, including Well-NAD, NAD20, DIM-NAD, Snout-NAD, and Flange-NAD, collectively provide a comprehensive view of the implosion dynamics.

While activation foils offer several advantages for neutron measurements, they also have limitations. One primary limitation is their sensitivity to neutron energy thresholds; reactions only occur above a specific energy, potentially missing lower-energy neutrons. Also, the analysis depends on accurate knowledge of nuclear reaction cross-sections and decay parameters, and uncertainties in these values can affect the final yield calculation. At facilities like the National Ignition Facility (NIF), scientists address these limitations through multiple strategies. They use a suite of activation diagnostics, such as Well-NAD, NAD20, DIM-NAD, Snout-NAD, and Flange-NAD, with different materials (like indium, zirconium, and copper) to cover a range of neutron energies and reaction types. They also employ sophisticated simulation tools like MCNP6 to estimate and account for factors like neutron scatter and absorption within the experimental setup. Careful calibration and cross-validation of results from multiple diagnostics are also used to ensure high confidence in the yield measurements.