Sustained Neutron Production from a Sheared-Flow Stabilized $Z$ Pinch

The sheared-flow stabilized $Z$ pinch has demonstrated long-lived plasmas with fusion-relevant parameters. We present the first experimental results demonstrating sustained, quasi-steady-state neutron production from the fusion $Z$-pinch experiment, operated with a mixture of 20% deuterium/80% hydrogen by pressure. Neutron emissions lasting approximately $5\mu \mathrm{s}$ are reproducibly observed with pinch currents of approximately 200 kA during an approximately $16\mu \mathrm{s}$ period of plasma quiescence. The average neutron yield is estimated to be $(1.25\pm 0.45)\times {10}^5\mathrm{neutrons}/\mathrm{pulse}$ and scales with the square of the deuterium concentration. Coincident with the neutron signal, plasma temperatures of 1–2 keV and densities of approximately $10^{17}{\mathrm{cm}}^{-3}$ with 0.3 cm pinch radii are measured with fully integrated diagnostics.

Figure 1

Side view of FuZE. The diameters of the coaxial inner and outer electrodes are 10 and 20 cm, respectively. Multiple axial arrays of surface-mounted magnetic probes measure the azimuthal magnetic field and mode amplitude. A digital holographic interferometer measures plasma density from a side view of the plasma. The plasma temperature is measured by Doppler broadening via impurity ion emission spectroscopy from a top view of the plasma. Neutron yields are measured by a cylindrical plastic scintillator (Eljen, EJ-204), coupled to a photomultiplier tube. The experimental data presented here are obtained at the $z=15\mathrm{cm}$ plane.

Figure 2

A simplified schematic of an SFS $Z$-pinch plasma formation showing an overlay of five times (a)–(e). (a) Neutral gas (blue) is injected into the annular acceleration region and then ionized. (b) The $\mathbf{J}\times \mathbf{B}$ force (driven current and resulting magnetic field) accelerates the plasma (red) axially along the coaxial accelerator. (c) At the end of the accelerator, the plasma transitions from the inner electrode to the axis. (d) The $Z$-pinch plasma forms in the assembly region. (e) A deflagration process supplies continuous plasma flow into the assembly region. The current is indicated with green arrows.

Figure 3

(a) Line-integrated plasma electron number density along the $z$ axis and vertical impact parameter of a FuZE plasma pulse. A plasma pinch structure is observed. (b) Abel-inverted plasma electron particle density radial profiles at $z=13.8\mathrm{cm}$ and $z=15.0\mathrm{cm}$ with a peak of approximately $1.1\times {10}^{17}{\mathrm{cm}}^{-3}$ and a pinch radius of approximately 0.3 cm.

Figure 4

(a) Raw intensified charge-coupled device image shows the spatially resolved spectra of the carbon-V triplet lines (227.1, 227.2, and 227.8 nm). (b) Radial profile of the ion temperature indicates temperatures of 1–2 keV. Included error bars are associated with the measurement accuracy and Gaussian-fit analysis. Temperatures from the plasma edge chords are not included since the emission intensity is too low for accurate fits.

Figure 5

(a) Characteristic signal is observed on the scintillator detector, showing neutron production during the quiescent period. For a 20% deuterium/80% hydrogen gas mixture, the signal is sustained for approximately $5\mu \mathrm{s}$, which is 5000 times the $m=1$ mode growth time. (b) Time-integrated neutron yield is calculated from measured scintillator detector signal. An illustrative linear fit plot is added to the figure, which indicates the neutron production rate is constant. (c) The normalized magnetic field fluctuation amplitude for the $m=1$ mode, at locations $z=5$, 15, 25, 35, and 45 cm, decreases below an empirical threshold of 0.2, which indicates a quiescent plasma from 22 to $38\mu \mathrm{s}$ along the assembly region. The evolution of the plasma pinch current at $z=15\mathrm{cm}$ shows a high pinch current plateau of approximate 200 kA during the sustained neutron production. (d) The voltage evolution shows no evidence of large voltage spikes during the quiescent period, indicating the absence of $m=0$ instability.

Figure 6

(a) Comparison of plastic scintillator detector signals during the quiescent period of a $Z$-pinch plasma for three different deuterium concentrations: 20%, 10%, and 0%. For nonzero deuterium concentrations, clearly detectable neutrons signals are observed. No neutron signal is detected in the absence of deuterium fill gas. (b) The average number of neutron counts observed at three different gas mixtures agree with an expected ${n_D}^2$ scaling relation. Error bars indicate the uncertainty in the neutron counts and the shot-to-shot variation.