Time-resolved measurement of neutron energy isotropy in a sheared-flow-stabilized Z pinch

TL;DR

This study measures neutron energy isotropy in a sheared-flow-stabilized Z pinch, finding deuteron beam energies below 7.4 keV and observing time-dependent isotropy variations, supporting thermonuclear neutron production.

Contribution

It provides the first time-resolved measurements of neutron energy isotropy in a Z pinch, with improved detector characterization and background analysis.

Findings

Deuteron beam energy less than 7.4 keV with uncertainties

Neutron production predominantly thermonuclear in origin

Time-dependent isotropy variations suggest instability growth

Abstract

Previous measurements of neutron energy using fast plastic scintillators while operating the Fusion Z Pinch Experiment (FuZE) constrained the energy of any yield-producing deuteron beams to less than $4.65 keV$. FuZE has since been operated at increasingly higher input power, resulting in increased plasma current and larger fusion neutron yields. A detailed experimental study of the neutron energy isotropy in these regimes applies more stringent limits to possible contributions from beam-target fusion. The FuZE device operated at $-25~kV$ charge voltage has resulted in average plasma currents of $370~kA$ and D-D fusion neutron yields of $4\times10^7$ neutrons per discharge. Measurements of the neutron energy isotropy under these operating conditions demonstrates the energy of deuteron beams is less than $7.4 \pm 5.6^\mathrm{(stat)} \pm 3.7^\mathrm{(syst)}~keV$. Characterization of the detector response has reduced the number of free parameters in the fit of the neutron energy distribution, improving the confidence in the forward-fit method. Gamma backgrounds have been measured and the impact of these contributions on the isotropy results have been studied. Additionally, a time dependent measurement of the isotropy has been resolved for the first time, indicating increases to possible deuteron beam energies at late times. This suggests the possible growth of $m$=0 instabilities at the end of the main radiation event but confirms that the majority of the neutron production exhibits isotropy consistent with thermonuclear origin.