Researchers from the National Institute of Fusion Science in Japan, developing the Large Helical Device (LHD) for nuclear fusion, have successfully doubled or tripled the efficiency of a critical diagnostic tool by implementing a novel “electrostatic lens” technique. 

This advancement solves a long-standing challenge related to beam-transport limitations, enabling significantly more precise and detailed measurements of electric potential within high-density plasma.

This enhancement to the Heavy Ion Beam Probe (HIBP) system has a direct impact on the quest for sustainable fusion energy.

“Achieving high-precision and reproducible measurements of the internal potential structure in reactor-grade fusion plasmas is extremely important as a fundamental database for future research on plasma control and reactor design,” said the researchers in a new report.

Notably, LHD is the “world’s largest superconducting plasma confinement device,” and employs a heliotron magnetic configuration.

A traffic jam for ions

In the pursuit of fusion energy—the same process that powers the Sun—scientists must confine plasma at temperatures exceeding 100 million degrees. 

“Therefore, accurately measuring the internal plasma potential is essential for improving the performance of future fusion reactors,” added the report. 

To measure this potential, the LHD uses an HIBP system that fires a high-energy beam of gold ions (Au⁺) into the plasma. A clear, precise signal requires a very high-current beam. 

However, researchers faced a significant bottleneck: while their ion source could produce a strong beam of negative gold ions (Au⁻), the beam would expand due to its own “space-charge effect” before it could be properly injected into the main accelerator. 

“At higher beam currents, the beam expands due to the space-charge effect, resulting in significant beam loss before entering the tandem accelerator,” noted the researchers.

Optimizing voltage as a lens

Instead of a costly or complex hardware overhaul, the research team developed a practical and compact solution. Using the ion-beam transport simulation code IGUN, they identified the exact cause of the beam expansion.

They then proposed reconfiguring the existing multistage accelerator, which sits between the ion source and the main tandem accelerator. By meticulously optimizing the voltage distribution (voltage allocation) of the electrodes, they transformed the component into an electrostatic lens. 

This lens effectively focuses the high-current ion beam, preventing it from expanding and guiding it efficiently into the accelerator’s entrance.

Clearer view of fusion

Numerical simulations predicted the new voltage configuration could achieve a beam transmission efficiency exceeding 95%. 

Subsequent plasma experiments confirmed the success of this approach, showing that the Au⁻ beam current successfully injected into the accelerator increased by a factor of two to three.

As a result, the high-energy Au⁺ beam injected into the plasma also increased, thereby expanding the HIBP’s measurable range up to a line-averaged electron density of 1.75×10¹⁹ m⁻³. 

The enhanced signal clarity enabled the detection of rapid, time-sensitive changes (temporal transitions) in the internal plasma potential as different heating systems were turned on and off.

“The method developed in this study provides a practical and compact solution for optimizing heavy ion beam transport and can be extended to other diagnostic systems and accelerator applications that require high-intensity beams,” concluded the researchers.