In the field of fusion energy, one critical aspect that influences the viability and efficiency of fusion reactions is neutron isotropy. This term refers to the uniformity of neutron energy emitted in all directions during the fusion process. In a perfectly isotropic fusion plasma, the neutrons have uniform energy across all angles, suggesting that the fusion reaction is stable and thermodynamic equilibrium is maintained. On the other hand, anisotropic neutron energy, where certain directions exhibit higher energy concentrations, is indicative of instability, reducing the scalability and sustainability of fusion reactions. Understanding and measuring neutron isotropy is crucial for determining the potential of fusion technologies to generate net energy—that is, more energy output than input.
A recent study by Zap Energy, published in the journal Nuclear Fusion, marks a pivotal achievement in the quest for sustainable fusion. The paper presents neutron isotropy measurements from the FuZE device, offering the most compelling evidence yet that Zap’s innovative fusion approach—sheared-flow-stabilized Z pinches—can produce stable, thermal fusion. This milestone confirms the potential of Zap’s technology for scaling fusion reactions to higher energy outputs and improving its ability to achieve higher performance in future fusion reactors, particularly the FuZE-Q device.
What is Neutron Isotropy and Why Does It Matter?
Fusion, the process that powers the sun and stars, involves fusing lighter atomic nuclei, such as hydrogen, into heavier nuclei, such as helium, releasing vast amounts of energy. This process also produces neutrons, high-energy particles that carry a significant portion of the energy generated in fusion reactions. For fusion energy to be practically scalable, it is vital that the neutrons produced exhibit thermal behavior, meaning they are emitted in a uniform direction with a consistent energy distribution.
In contrast, beam-target fusion occurs when fast-moving hydrogen nuclei collide with stationary nuclei, often creating neutrons with highly anisotropic energy distributions. This type of fusion is less efficient and far harder to scale for practical energy generation.
For fusion to be commercially viable, it is essential to achieve thermal fusion in which the neutrons are isotropically distributed. These neutrons are the hallmark of stable fusion reactions that can be harnessed for net energy production. An anisotropic fusion plasma, on the other hand, indicates an imbalanced or unstable reaction, preventing the system from scaling to produce sufficient energy for practical use.
Zap’s Progress in Achieving Isotropic Neutron Energy
Zap Energy’s research on Z pinches, a form of plasma confinement using magnetic fields to compress plasma, has been a focal point of their efforts to achieve efficient and scalable fusion. The latest results from the FuZE device reveal a breakthrough in their ability to measure and optimize neutron isotropy within the plasma. This was achieved by measuring the neutron energy distribution across 433 plasma shots, all generated under the same machine settings. The results indicated near-perfect isotropy in the neutron emissions, suggesting that the fusion process occurring inside the device is indeed thermal, and the plasma remains in thermodynamic equilibrium.
According to Uri Shumlak, Zap’s Chief Scientist and Co-Founder, these findings validate the concept of scaling fusion energy using Zap’s technology. “Essentially, this measurement indicates that the plasma is in a thermodynamic equilibrium,” he says. “That means we can double the size of the plasma and expect the same sort of equilibrium to exist.”
This isotropic neutron measurement is an essential milestone because it confirms that Zap’s technology is capable of maintaining a stable, thermally balanced plasma—a crucial step toward scaling fusion reactions and achieving net energy production in the future.
The Neutron Energy Profile in Fusion
Inside the FuZE device, the goal is to achieve thermal fusion. This occurs when hydrogen nuclei in the plasma are heated to extremely high temperatures, causing them to overcome the electrostatic repulsion between them and fuse together, producing helium nuclei and neutrons in the process. Thermal fusion reactions produce neutrons with a uniform energy distribution across all directions, known as isotropy. This is important because the more isotropic the neutron energy, the more likely it is that the plasma is stable and that the fusion reaction will be scalable to larger systems.
On the other hand, beam-target fusion, which occurs when a fast-moving hydrogen ion strikes a stationary nucleus, produces neutrons with anisotropic energy distributions. This results in some directions having higher energy neutrons than others, signaling an unstable plasma reaction. For fusion energy to be useful for power generation, thermal fusion is the desired process, as it provides consistent and predictable energy production.
Neutron Isotropy Tests: A Detailed Approach
To confirm the isotropy of the neutrons generated in the FuZE device, Zap’s team used neutron detectors placed strategically around the device to measure the energy distribution of neutrons emitted from the plasma. The measurements spanned a wide range of plasma shots, resulting in an overwhelmingly isotropic neutron profile, with very little variance in neutron energy across different directions.
Rachel Ryan, Senior Scientist at Zap and lead author of the study, explains, “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.” The consistent isotropic behavior of the neutrons provides a strong indication that Zap’s fusion process is indeed thermal, paving the way for future advancements in scalable fusion technology.
The Significance of Zap’s Achievement in Fusion History
Zap’s success in measuring and achieving thermal fusion with isotropic neutrons is significant not only for its technological promise but also in the context of fusion research history. The Z pinch approach, which dates back to the 1950s, has long been an area of interest in the pursuit of fusion energy. However, previous attempts to use the Z pinch method, such as those by the ZETA project in the United Kingdom, were met with failure. The magnetic instabilities created in these devices often resulted in beam-target fusion rather than thermal fusion, which ultimately led to unsuccessful fusion attempts.
In particular, the dense plasma focus (DPF) devices, which share some similarities with Zap’s approach, were similarly dismissed as impractical for energy generation due to their propensity for producing anisotropic neutrons. These historical setbacks make Zap’s achievement of thermal fusion with isotropic neutrons all the more important.
Zap’s approach differs significantly from earlier Z pinch efforts, as it uses sheared-flow stabilization to minimize plasma instabilities. This method prevents the instabilities that previously limited the success of Z-pinch-based fusion devices. By stabilizing the plasma flow, Zap Energy’s technology has overcome a major hurdle in fusion energy research, confirming that thermal Z-pinch fusion could indeed be scaled for net energy production.
Looking Ahead: Scaling Fusion to Higher Energies
Zap Energy’s next steps involve applying their neutron isotropy measurements to the FuZE-Q device, which is designed for even higher performance. These measurements will continue to assess the role of beam-target fusion and ensure that the thermal fusion process remains dominant as the device scales up. The FuZE-Q device is poised to provide further confirmation that Zap’s technology can achieve higher fusion energy yields.
As Rachel Ryan notes, “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.” These ongoing tests will allow Zap to refine their approach and work toward continuous fusion reactions that can sustain the energy needs of future power grids.
A Crucial Step Toward Net Energy Fusion
Zap Energy’s breakthrough in measuring neutron isotropy in their FuZE device represents a crucial step toward realizing commercial fusion energy. By demonstrating stable, thermal fusion reactions that are isotropic and scalable, Zap has brought us closer to a future where fusion power can provide virtually unlimited, clean energy.
This achievement underscores the importance of ongoing research and precise measurements in the pursuit of fusion. By refining the understanding of neutron energy profiles and addressing plasma instabilities, Zap is at the forefront of fusion innovation, making their path toward net-energy fusion one of the most promising in the field.
As the research continues, the understanding of plasma behavior and fusion stability will improve, helping to pave the way for a future where fusion energy becomes a reliable and sustainable source of power for the world. The journey is far from over, but with each new milestone, the dream of clean, unlimited fusion energy becomes one step closer to reality.
Reference: R.A. Ryan et al, Time-resolved measurement of neutron energy isotropy in a sheared-flow-stabilized Z pinch, Nuclear Fusion (2025). DOI: 10.1088/1741-4326/ada8bf