In the relentless pursuit of sustainable energy, nuclear fusion stands out as a beacon of hope, promising a virtually limitless power source by replicating the processes that fuel the Sun. Central to achieving this ambition is the ability to maintain plasma—an ultra-hot, ionized gas—at temperatures exceeding one hundred million degrees Celsius, all while confining it efficiently using powerful magnetic fields. A critical and often elusive parameter in this endeavor is the internal electric potential of the plasma. This potential fundamentally controls how particles and energy move within the plasma, influencing stability and confinement quality. Precise, high-resolution measurements of this electric potential are indispensable for advancing fusion reactor design and performance.
One of the most sophisticated tools developed to probe the internal workings of fusion plasmas is the heavy ion beam probe (HIBP), a non-invasive diagnostic system capable of directly measuring the plasma potential. The technique involves generating a beam of negatively charged gold ions (Au⁻), accelerating them, and injecting this beam into the core of the plasma. As these ions interact with the plasma, they undergo charge state changes. By carefully detecting and analyzing these changes, scientists can infer the plasma’s internal electric potential profile with exceptional sensitivity and spatial resolution.
Despite substantial advancements in negative ion sources, which have pumped out higher current ion beams, a significant technical challenge has persisted: efficiently transporting these intense, high-current ion beams into the tandem accelerator, the critical apparatus that further accelerates the ions to mega-electron volt energies required for plasma injection. Previously, increases in the ion source output did not translate proportionally into higher injection currents due to beam losses caused by space-charge effects, limiting the quality and precision of plasma potential measurements.
At the Large Helical Device (LHD)—one of the world’s most advanced stellarator-type fusion experiments—the HIBP system has been meticulously refined for high-precision electric potential diagnostics. In this setup, Au⁻ ions are initially injected into a tandem accelerator where they are stripped to positive charge states (Au⁺) and boosted to energies up to 6 MeV. Upon entering the plasma, further ionization produces Au²⁺ ions. By measuring the energy differential between the incident Au⁺ beam and the post-plasma Au²⁺ beam, researchers can pinpoint the local plasma potential with high accuracy. However, this process demands an ultra-stable and intense ion beam, motivating researchers to investigate and overcome the bottlenecks limiting ion beam current transport.
Using ion-beam transport simulations coded within IGUN, the team analyzed the early-stage journey of the Au⁻ beam—from its creation in the negative ion source through its passage to the tandem accelerator entrance. These studies revealed a crucial phenomenon: at beam currents below approximately 10 microamperes, the ions are well-collimated and easily pass through the accelerator’s entry slit. However, as beam intensity increases beyond this threshold, electrostatic repulsion among the closely packed charged ions—the space-charge effect—causes the beam to expand dramatically, leading to substantial ion losses before acceleration even commences.
This space-charge-induced beam divergence presents a substantial hurdle, especially when working with heavy ions like gold, whose mass and charge exacerbate these effects. Increasing ion source output alone thus proved insufficient to improve the injection current. To counteract this, the research team turned their attention to an existing, but underutilized, component in the ion transport line: a multistage accelerator module situated between the ion source and the main tandem accelerator.
Rather than merely accelerating ions, this multistage section was optimized to act as a sophisticated electrostatic lens by fine-tuning the voltage distribution across its multiple electrodes. Through careful adjustment of these voltages, the ion beam could be effectively focused and compressed, minimizing the space-charge-induced expansion and steering a larger fraction of the beam into the accelerator slit. Simulation results were striking, showing that such voltage optimization could push ion beam transmission efficiency above 95%, a monumental improvement over previous configurations, which had far lower throughput.
These calculated efficiencies were not just theoretical. Experimental validation in the LHD facility demonstrated a two- to threefold increase in Au⁻ beam current successfully injected into the tandem accelerator. This breakthrough effectively doubled down on the initial advances in ion source technology, culminating in a much stronger and more stable beam suitable for high-precision diagnostic tasks.
The impact of this advance became clear in subsequent plasma experiments. With higher Au⁻ beam currents feeding the fusion plasma, the resulting Au⁺ ion beam within the plasma was correspondingly intensified. This led to an expanded operational range, with plasma potential measurements now feasible in plasmas featuring electron densities as high as 1.75×10¹⁹ m⁻³. Improved signal clarity enabled the team to observe rapid and subtle temporal shifts in the plasma potential profile during changes in confinement regimes, providing unprecedented insight into plasma behavior.
Specifically, in a series of experiments capturing different heating states within the plasma, the improved diagnostic system revealed a swift decline in overall plasma potential immediately following the cessation of electron cyclotron heating. This was followed by a gradual flattening of the potential gradient, accompanied later by the re-establishment of confinement via neutral beam injection. These detailed temporal potential profiles shed new light on the interplay between heating methods and plasma confinement dynamics, crucial knowledge for optimizing future reactor operation.
The significance of these findings extends far beyond the LHD itself. The novel approach to mitigating space-charge effects in heavy ion beams using multistage acceleration and electrostatic lensing introduces a scalable and adaptable technique that can be employed across many plasma diagnostic platforms. Moreover, similar strategies could benefit a broad range of accelerator applications where high-intensity, tightly focused ion beams are essential.
Ultimately, the ability to produce high-fidelity, reproducible measurements of plasma internal electric potentials is a cornerstone for the next generation of fusion research. These measurements ground theoretical models, guide plasma control strategies, and inform reactor design parameters critical to achieving sustained and economically viable fusion energy production. This research marks a transformative step toward harnessing the Sun’s power here on Earth with precision diagnostics that bring fusion reactors closer to reality.
As fusion science continues its rapid progress, innovations like this provide a roadmap for overcoming the complex physical challenges inherent in managing ultra-hot plasmas and intense charged particle beams. By blending cutting-edge simulation techniques, experimental validation, and smart engineering optimizations, the researchers at the National Institute for Fusion Science have opened new possibilities for plasma diagnostics and fusion reactor development worldwide.
Subject of Research: Experimental study of heavy ion beam transport optimization for fusion plasma diagnostics
Article Title: Enhanced beam transport via space charge mitigation in a multistage accelerator for fusion plasma diagnostics
News Publication Date: 13-Oct-2025
Web References:
10.1088/1741-4326/ae0da1
Image Credits: National Institute for Fusion Science
Keywords
Nuclear fusion, plasma diagnostics, heavy ion beam probe, space-charge effect, multistage accelerator, plasma potential, ion beam transport, tandem accelerator, Large Helical Device, gold ion beams, electrostatic lens, fusion reactor research
