In physics, the term "isotropy" means a system where the properties are the same in all directions. For fusion, neutron energy isotropy is an important measurement that analyzes the streams of neutrons coming from the device and how uniform they are. This is critical because so-called isotropic fusion plasmas suggest a stable, thermal plasma that can be scaled to higher fusion energy gains, whereas anisotropic plasmas, those emitting irregular neutron energies, can lead to a dead end.
A new Zap research paper, published last week in Nuclear Fusion , details neutron isotropy measurements from the FuZE device that provide the best validation yet that Zap's sheared-flow-stabilized Z pinches generate stable, thermal fusion. It's a benchmark milestone for scaling fusion to higher energy yields in Zap's technology and giving confidence in reaching higher performance on the FuZE-Q device.
"Essentially, this measurement indicates that the plasma is in a thermodynamic equilibrium," says Uri Shumlak, Zap's Chief Scientist and Co-Founder. "That means we can double the size of the plasma and expect the same sort of equilibrium to exist."
Reading the neutrons
Inside a Zap core , hydrogen nuclei are fused into helium, a process that kicks out a neutron at high energies. These neutrons carry 80% of the energy that comes from the fusion reaction, so, in general, the more neutrons, the better.
However, not all kinds of fusion reactions are created equal. Thermal fusion is Zap's goal — when nuclei are fused together by the extreme heat and pressure inside its plasmas. Thermal fusion produces energetic neutrons that scale exponentially (at around 10 to the eleventh power) as the amount of current conducting through the plasma is dialed up to reach the levels necessary for fusion to yield net energy.
Less desirable is what's known as beam-target fusion, which happens when a hydrogen nucleus is accelerated to high velocity and strikes a stationary nucleus. Unlike in thermal fusion, beam-target fusion indicates the plasma is out of equilibrium, and therefore doesn't scale as strongly, making a working energy source much more difficult.
Thermal fusion produces neutrons with isotropic velocities, or with the same energy in all directions, while beam-target fusion produces them anisotropically, or such that neutrons in certain directions have higher energies. So, comparing measurements of the neutron energy at different locations is a simple way to see how much of the fusion in the FuZE device is non-thermal.
"If we saw neutrons primarily from a beam-target source, it would mean that our machine wouldn't be scalable. We couldn't get to net energy production," says Rachel Ryan, a senior scientist at Zap and lead author of the new research.
To test the neutron isotropy in FuZE, Zap scientists and engineers ran a series of tests using neutron detectors placed around the device. Measuring 433 plasma shots generated with the same machine settings, the neutrons were found to be almost totally isotropic.
A meaningful measurement, in more ways than one
Besides being a key benchmark for physics progress, neutron isotropy holds extra historical significance for Zap's fusion approach.
The Z pinch is one of fusion's oldest approaches and dates back to the 1950s. When scientists working on the Zero Energy Thermonuclear Assembly (ZETA) device in the United Kingdom began using magnetic fields to "pinch" a plasma strongly enough to create fusion, they thought they had succeeded. But that success didn't come in the way they had hoped. Their device turned out to be creating almost entirely beam-target fusion through the creation of instabilities in the magnetic field. That meant they could never generate net-energy-gain fusion. What had been a hopeful moment for the physics community turned out to be a disappointment and a PR disaster .
And while isotropy became a particular black mark for pinch-based approaches, all fusion technologies risk measuring false positives from beam-target neutrons. For example, a device known as a dense plasma focus (DPF) has also been largely dismissed as a practical path to a fusion power plant. Though they are similar in some ways to Zap's devices and are considered an effective means of generating neutrons, DPF neutrons come primarily from beam-target interactions.
In the shadow of those experiments, Zap is extra conscious of the story its neutrons tell. The company first measured thermal fusion in 2018 and these new tests, done with higher sensitivity and at higher energies, are the latest confirmation that sheared flows can postpone the instabilities that doomed previous Z pinch efforts. Scalable thermal Z-pinch fusion, without requiring any external magnets for confinement, remains promising.
The paper represents a major physics consideration, Shumlak says. "This is why we put so much effort into making these precise measurements," he says.
Preparing for the future
Since joining Zap in 2023, Ryan has played the lead role in planning and carrying out neutron measurements at Zap, building on work previously done by collaborators and co-authors from Lawrence Livermore National Lab. Next up for the team is running the same set of tests at higher energies on Zap's FuZE-Q device. Initial results look promising.
"As we continue to scale up, it's important for us to keep taking this measurement and keep checking whether beam-target fusion is contributing to our yields," Ryan says.
Interestingly, the paper also notes that the neutrons became less isotropic and lost uniformity near the end of each shot. The researchers suggest this is likely a phase where the pinch becomes unstable before it breaks down and stops generating fusion entirely. Understanding that phase may give a better understanding of how to keep the instabilities from cutting fusion short and further increase the duration and performance of the plasma.
