The sheared-flow Z pinch and the repetition problem
University to national lab. · 2 min read
What it proposes
Revive the simplest fusion geometry ever drawn — run a large current straight down a column of gas and let the column's own magnetic field squeeze it, with no external magnets and no auxiliary heating — by fixing the instability that killed it in the 1960s. Give the column a sheared axial flow, faster on the axis than at the edge, and the kinks stay suppressed. Then face the problem nobody in fusion talks about enough: doing it again, and again, next to hardware that survives.
Who it is forPulsed-power engineers · Plasma-facing materials engineers · Liquid-metal handling specialists
Why the library suggests it
Run the clock on two columns of the same plasma: one kinks apart on the magnetohydrodynamic time, the other carries the sheared flow and stays quiet long enough to make neutrons.
The kink, and the flow that holds it off
Two columns of the same plasma, side by side. The left one is a plain Z pinch: its own current squeezes it, and the kink grows until the column necks apart. The right one carries a sheared axial flow — plasma moving faster on the axis than at the edge — and stays quiet for the interval FuZE measured. Move the clock and watch the two separate.
- Steady neutron production
- Measured quiet interval
At this instant: The column is still holding together. The sheared column is quiet, and the kink has not grown. Fusion neutrons are coming out steadily.
- Measured quiet interval:
- 1.6E-5 s
- Steady neutron production:
- 5E-6 s
- Pinch current:
- 2E5 A
- Pinch radius:
- 3E-3 m
- Neutron yield per pulse:
- 125,000
- Instability lifetimes survived:
- 5,000
Read the diagnostic the way the authors do: the yield rises with the square of the deuterium fraction and vanishes with no deuterium, which is the signature of thermal fusion rather than a stray accelerated beam.
Uri Shumlak's group at the University of Washington, with Lawrence Livermore, report a plasma going quiet for about sixteen microseconds on the FuZE device, with fusion neutrons coming out steadily for five of them at a pinch current near 200 kiloamps — roughly five thousand times as long as the column should have survived. The yield of about 125,000 neutrons a pulse rises with the square of the deuterium fraction and disappears entirely with no deuterium, which is the signature of thermal fusion rather than a stray accelerated beam (Sustained Neutron Production from a Sheared-Flow Stabilized Z Pinch, 2019). The engineering half is Zap Energy's Century: a five-container pulsed-power driver capable of half a million amps firing one shot every ten seconds downward onto a curtain of molten bismuth, with three continuous runs of 1,080 shots each at 14 kilojoules per shot and the liquid metal visibly protecting the cathode tip from erosion (Century: Zap Energy's 100-kW-Scale Repetitive Sheared-Flow-Stabilized Z-Pinch System with Liquid Metal Cooling, 2025).
The experiment or build
The scientific rung is a small sheared-flow pinch with the shear profile measured rather than assumed — the flow profile is the whole mechanism and it is the hardest thing to diagnose. The engineering rung, which is where most of the value now is, is electrode life: run a bench-scale pinch for ten thousand shots and measure the electrodes. The settling measurement is neutron yield per shot held constant across a run of thousands of shots, with electrode mass loss measured before and after. A machine whose yield decays with shot number is a machine whose electrodes are the real experiment.
Research context
The physics result is peer-reviewed, the repetitive machine is built and commissioned, and the run-length data that would turn it into a power plant is being taken.
Take it up
- The measurement that settles it
- The settling measurement is neutron yield per shot held constant across a run of thousands of shots, with electrode mass loss measured before and after.
- What it costs to start
- University to national lab.
- The engineer it grows
- An unglamorous speciality that is on the critical path for every fusion architecture in this programme.
What it rests on
Where it sits in the curriculum
Fusion Machines: Pinches, Focus Devices and Inertial Drivers