Historical research guide · 0691 · 2019

Sustained Neutron Production from a Sheared-Flow Stabilized Z Pinch

Y. Zhang · U. Shumlak · B. A. Nelson · R. P. Golingo · T. R. Weber · A. D. Stepanov · E. L. Claveau · E. G. Forbes · Z. T. Draper · J. M. Mitrani · H. S. McLean · K. K. Tummel · D. P. Higginson · C. M. Cooper

In one minute

The Z pinch is the simplest fusion machine anyone ever drew: run a large current straight down a column of gas and the column’s own magnetic field squeezes and heats it — no external magnets, no auxiliary heating. It was largely abandoned in the 1960s because the column kinks and necks apart in about a nanosecond. Uri Shumlak’s group at the University of Washington, working with Lawrence Livermore, report the repair holding. Give the column a sheared axial flow — plasma moving faster on the axis than at the edge — and those instabilities stay suppressed. On their FuZE device, fed one part deuterium to four parts hydrogen, the plasma goes quiet for about sixteen microseconds and fusion neutrons come out steadily for five of them at a pinch current near 200 kiloamps. That is roughly five thousand times as long as the column should have survived. The yield, about 125,000 neutrons a pulse, rises with the square of the deuterium fraction and disappears entirely with no deuterium — the signature of thermal fusion rather than a stray accelerated beam.

Connection

Chapter 12 treats fusion as the energy substrate the rest of the programme runs on, and this is the paper that put the cheapest geometry in fusion back on the table: a bare current column, stabilised by its own flow, making neutrons for thousands of instability lifetimes. Chapter 9 gets the cleanest laboratory case of a self-organising plasma column that confines and compresses itself with no external field coils at all. The same concept scaled into a power-plant mock-up is on this site at /library/stm-49324933a9.

From the source

The Fusion Z-pinch Experiment produced sustained, quasi-steady-state neutron emission lasting approximately 5 microseconds at pinch currents of approximately 200 kiloamps, during an approximately 16 microsecond period of plasma quiescence, with an average yield of 1.25 plus or minus 0.45 times ten to the fifth neutrons per pulse — the first such demonstration from a sheared-flow stabilized Z pinch.Abstract; Figure 5(a) and 5(b); summary paragraph
Sheared axial flow suppresses the instabilities that ended the Z pinch as a fusion concept. The m equals 1 kink fluctuation amplitude falls below the empirical threshold of 0.2 from 22 to 38 microseconds along the assembly region, and the absence of voltage spikes above 20 kilovolts shows the m equals 0 sausage mode is also absent; the 5 microsecond emission is about 5000 times the theoretical kink growth time of roughly 1 nanosecond.Figures 5(c) and 5(d); the paragraph beginning ‘Mitigating the growth and development of the m equals 0 sausage mode’
The neutrons come from thermal fusion in the bulk plasma rather than from a beam of ions accelerated by an instability: the yield follows the square of the deuterium ion density across 20, 10 and 0 percent deuterium fills, no signal at all appears with pure hydrogen, and the emission lasts microseconds rather than the tens or hundreds of nanoseconds characteristic of beam-target production.Figure 6(a) and 6(b); the paragraph on deuterium concentration
Two independent diagnostic routes give the same plasma. Digital holographic interferometry measures a peak electron density of about 1.1 times ten to the seventeenth per cubic centimetre with a pinch radius of about 0.3 centimetres, and Doppler broadening of the carbon-V triplet gives ion temperatures of 1 to 2 keV; running the measured neutron yield back through the thermonuclear yield integral independently implies 1.1 to 1.3 keV.Figures 3 and 4; Equation (1) and the paragraph following it
The concept scales steeply with drive current. Because the Bennett equilibrium makes temperature proportional to the square of the pinch current and the deuterium-deuterium reaction rate parameter goes as temperature to the fourth power in this range, the neutron production rate should rise as the pinch current to a power greater than eight — the observed increase from 80 to 200 kiloamps alone predicts more than three orders of magnitude in rate.The scaling paragraph following Equation (1)
The measurement that would sharpen the result is named in the paper: the energy spectrum of the emitted neutrons has not yet been characterised, and it is the spectrum that distinguishes a thermal distribution from any residual directed component.The paragraph closing the deuterium-scaling discussion; summary paragraph

The way in

https://doi.org/10.1103/PhysRevLett.122.135001

Historical source-access record. This guide links to the original; it does not reproduce the paper text.

LICENCE. Published as Physical Review Letters 122, 135001 (2019); the Crossref record carries the APS default licence and the APS default accepted-manuscript licence, not a Creative Commons one. The accepted manuscript is public on arXiv as 1806.05894v4, dated 24 February 2019, but that posting carries the arXiv.org perpetual non-exclusive distribution licence rather than a Creative Commons statement, so this page holds the summary, the claims and the authors’ own abstract and sends the reader to the source. REGISTRY CORRECTIONS. The record reached the library with the journal’s MathML markup left in the title, so the title is restored here as printed; and with eight of the fourteen authors, so the author list is completed from the paper’s own by-line. The work is from the Aerospace and Energetics Research Program at the University of Washington, Seattle, and Lawrence Livermore National Laboratory, funded in part by ARPA-E under award DE-AR-0000571 and LLNL contract DE-AC52-07NA27344. Two of the authors, Uri Shumlak and Brian A. Nelson, are co-founders of Zap Energy, whose later 100-kilowatt-scale Century machine has its own sheet at /library/stm-49324933a9.

How to cite it

Y. Zhang, U. Shumlak, B. A. Nelson, R. P. Golingo, T. R. Weber, A. D. Stepanov, E. L. Claveau, E. G. Forbes, Z. T. Draper, J. M. Mitrani, H. S. McLean, K. K. Tummel, D. P. Higginson, C. M. Cooper (2019) Sustained Neutron Production from a Sheared-Flow Stabilized Z Pinch doi:10.1103/PhysRevLett.122.135001

Where it sits in the curriculum

Fusion machines: pinches, focus devices and inertial drivers