Neutron yields and fuel areal densities are measured at the National Ignition Facility by an extensive suite of neutron activation diagnostics. Neutrons interact with materials whose reaction cross sections threshold just below the fusion neutron production energy, providing an accurate measure of primary unscattered neutrons without contribution from lower-energy scattered neutrons. An array of up to seventeen zirconium samples are located on ports around the chamber to measure activation via 90Zr(n,2n) reactions and provide a global map of neutron yield and relative anisotropies. A dipole perturbation in zirconium activation around the target chamber on DT glass-capsule exploding pusher implosions is indicative of bulk core velocity. The relatively low areal density (10-50 mg/cm2) allows measurement of the Doppler energy shift of the primary fusion spectrum, contributing systematic low-mode variation to the activation anisotropy. Quantification of this few-percent perturbation is a challenge as measurements are made on the calibration shots themselves. A calibration technique using a global fit to all exploding pusher shots and vector summing to zero net velocity allows measurement of core velocity as low as tens of kilometers per second. Velocities higher than 200 km/s have been observed on shots with known drive asymmetries due to dropped laser bundles or misaligned capsules.This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratories under contract DE-AC52-07NA27344.

Measuring bulk fuel velocity at the National Ignition Facility with neutron activation

248th National Meeting (2014)

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