The Large Helical Device (LHD) is the world’s largest superconducting plasma confinement device. It utilizes a heliotron magnetic configuration.
Researchers from the National Institute of Fusion Science in Japan developed the LHD for nuclear fusion research. They have successfully tripled the efficiency of a critical diagnostic tool by implementing an innovative “electrostatic lens” technology.
This breakthrough solves a long-standing challenge related to limitations of beam-transport, allowing significantly more precise and detailed measurements of electric potential within high-density plasma.
This enhancement to the Heavy Ion Beam Probe (HIBP) technology has a direct impact on the search for sustainable fusion energy.
The researchers said in a new report, “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.”
The LHD is the “world’s largest superconducting plasma confinement device,” and employs a heliotron magnetic configuration.
In the pursuit of fusion energy scientists must confine plasma at temperatures exceeding one hundred million degrees. The report added that “Therefore, accurately measuring the internal plasma potential is essential for improving the performance of future fusion reactors.”
To measure this potential, the LHD uses an HIBP system that injects a high-energy beam of gold ions (Au⁺) into the plasma. A clean, precise signal requires a very high-current beam.
However, there was a significant bottleneck. While their ion source could produce a strong beam of negative gold ions (Au⁻), the beam would expand because of its own “space-charge effect” before it could be properly injected into the main accelerator.
The report mentioned, “At higher beam currents, the beam expands due to the space-charge effect, resulting in significant beam loss before entering the tandem accelerator.”
Instead of a costly and/or complex hardware overhaul, the research team created a practical and compact solution. IGUN is a specialized program used in the design and optimization of particle optics devices, including electron and ion guns, beam transport sections and collectors. Using the ion-beam transport simulation code IGUN, they identified the exact cause of the beam expansion.
They then suggested reconfiguring the existing multistage accelerator, which sits between the ion source and the main tandem accelerator. By carefully optimizing the voltage distribution (voltage allocation) of the electrodes, they transformed the component into an electrostatic lens.
This new lens effectively focuses the high-current ion beam, preventing it from expanding and guiding it efficiently into the accelerator’s entrance.
Numerical simulations predicted that the new voltage configuration could reach a beam transmission efficiency exceeding ninety five percent
Subsequent plasma experiments confirmed the success of this approach, proving 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 one and three quarters times ten to the nineteenth per cubic meter.
The enhanced signal clarity allowed the detection of rapid, time-sensitive changes (temporal transitions) in the internal plasma potential as different heating systems were turned on and off.
the researchers concluded that “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.”