Design Calculations for a NIF Diagnostic

ICF targets are complex and it is important to measure target performance in several ways. One quantity that is fundamental to the confinement is the fuel density (denoted r) and the fuel rR (density times radius), which is directly proportional to the confinement time.

Under a project initiated by Los Alamos National :Laboratory, ANSR has performed design calculations using the Monte Carlo Neutronics Photonics (MCNP) code to help guide diagnostic development for the National Ignition Facility.

The concept for the diagnostic is indicated in the schematic below, showing the target's burn region in two different configurations: low-compression (on the left) and high-compression (on the right). For a given fuel mass, higher compression leads to higher rR and a higher scattering fraction for the fusion neutrons.

A detector in the NIF chamber then measures the arrival time of neutrons. Scattered neutrons are traveling slower, so they arrive later. The plot below shows the MCNP-calculated fluences of neutrons (n) and gamma rays (g) arriving at the detector, as a function of time, for various sources within the target chamber. Since the distance from the target to the Aluminum (Al) NIF target chamber wall is larger than the distance from the target to the detector, neutrons scattered from the chamber walls arrive later than those scattered in the target's fuel region. The desired "signal" is the portion of the blue curve inside the green oval. The most significant sources of background are neutrons scattered by the Beryllium (Be) nosecone of the cryogenic target positioner (magenta curve inside the red oval) and gamma rays produced when neutrons scatter at the chamber walls (green curve inside the orange oval). The Time-of-Flight spectrum of neutrons from the target was calculated by Dr. Douglas C. Wilson, of LANL.

A technical report on this project is available for download.

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