Multichannel, triaxial, neutron time-of-flight diagnostic for experiments at the Z facility

Abstract

Neutron bang times ${t}_{\text{bang}}$ and mean neutron speeds $\overline{u}$ have been measured at the $Z$ facility for a series of ${D}_{2}$-filled targets, in magnetized liner inertial fusion experiments. Measurements were made by a novel neutron time-of-flight (nTOF) diagnostic, adapted for use at this facility, and consisted of detecting the neutron times in flight at seven independent scintillator--photomultiplier tube detectors (channels), located on three noncoplanar lines of sight, with distances to the neutron source varying between 690 and 2510 cm. The nTOF signals were analyzed by identifying fiducials on the detector traces to quantify the time in flight to each distance, using a nonrelativistic model for a uniformly thermalized, Maxwellian plasma distribution. The measured neutron arrival times were then linearly regressed on distance with the bang time and mean speed estimated from the fit parameters. A particular shot, 2584, is analyzed here to illustrate the method and the issues encountered in these measurements. On this particular shot, six usable channel traces were obtained. The standard errors of the parameter fits were as follows: ${t}_{\text{bang}}=3102.95\ifmmode\pm\else\textpm\fi{}0.97\text{ }\text{ }\mathrm{ns}$ (standard error) with six nTOF traces on the system clock and $\overline{u}=2.1524\ifmmode\pm\else\textpm\fi{}0.0032\text{ }\text{ }\mathrm{cm}/\mathrm{ns}$ (standard error), from which the mean, nonrelativistic, kinetic energy $\overline{E}$ of the neutrons was $2.4216\ifmmode\pm\else\textpm\fi{}0.0144\text{ }\text{ }\mathrm{MeV}$ (standard error). The estimates of $\overline{u}$ and $\overline{E}$ here agree within 1% of the published values for the $D{(d,n)}^{3}\mathrm{He}$ reaction. Hence, these measurements are consistent with the production of a thermalized, Maxwellian D-D fusion plasma in this experiment. The source duration was estimated to be $3.25\ifmmode\pm\else\textpm\fi{}0.84\text{ }\text{ }\mathrm{ns}$ (standard error) from six pulse-width measurements.