First high-precision measurement of potential dynamics inside reactor-grade fusion plasma

Nuclear fusion, which operates on the same principle that powers the Sun, is expected to become a sustainable energy source for the future. To achieve fusion power generation, it is essential to confine plasma at temperatures exceeding one hundred million degrees using a magnetic field and to maintain this high-energy state stably.  A key factor in accomplishing this is the electric potential inside the plasma. This potential governs the transport of particles and energy within the plasma and plays a crucial role in establishing a state in which energy is effectively confined and prevented from escaping. Therefore, accurately measuring the internal plasma potential is essential for improving the performance of future fusion reactors.

A non-contact diagnostic technique called the heavy ion beam probe (HIBP) is used to measure plasma potential directly. In this method, negatively charged gold ions (Au⁻) are accelerated and injected into the plasma. By detecting how their charge state changes through interactions with the plasma, the electric potential inside the plasma can be inferred with high sensitivity. However, obtaining high-precision signals requires a strong and stable ion beam. Although advances in negative ion sources have increased the available beam current, efficiently transporting and injecting high-current beams into the accelerator has remained difficult, limiting the achievable diagnostic precision.

In the Large Helical Device (LHD), the HIBP system has been developed to measure electric potential in plasmas. In this system, a gold negative ion (Au⁻) beam is injected into a tandem accelerator, converted into a gold positive ion (Au⁺) beam, and further accelerated up to 6 mega–electron volts (MeV) at the accelerator’s exit before being injected into the plasma. The beam that becomes Au²⁺ through collisions with the plasma passes through the magnetized plasma, and by measuring the energy difference between the incident Au⁺ beam and the Au²⁺ beam after it traverses the plasma, the electric potential at the position where Au²⁺ was produced can be determined. To obtain a clear and precise potential signal, a higher injection current into the plasma is required. Although the output current of the Au⁻ ion source had been successfully increased, the injection beam current into the tandem accelerator could not be increased in direct proportion, which remained a significant challenge.

To identify the cause of this limitation, the research team analyzed the heavy-ion beam transport efficiency on the low-energy side — from the negative ion source to the entrance of the tandem accelerator — using the ion-beam transport simulation code IGUN. The simulations revealed that when the Au⁻ beam current is below 10 microamperes (µA), the beam can pass through the entrance slit during the acceleration, as shown in Fig. 2(a). However, at higher beam currents, the beam expands due to the space-charge effect, resulting in significant beam loss before entering the tandem accelerator, as shown in Fig. 2(b). For heavy-ion beams such as gold, this space-charge-induced limitation becomes particularly pronounced even if the output current from the negative ion source is increased. To improve beam transport efficiency, the team proposed using the multistage accelerator located between the ion source and the tandem accelerator, not only for acceleration but also as an electrostatic lens by optimizing its voltage distribution, as illustrated in Fig. 2(c). Numerical simulations demonstrated that by optimizing the voltage allocation of the multistage electrodes, a high-transmission region exceeding 95 % could be achieved, as shown in Fig. 3, significantly enhancing the beam transport efficiency compared with the conventional voltage configuration (blue circle). Subsequent plasma experiments confirmed the validity of this approach, showing that the Au⁻ beam current injected into the accelerator increased by a factor of two to three.

As the Au⁻ beam current increased, the corresponding Au⁺ beam injected into the plasma also increased, thereby expanding the measurable range of plasma potential in the LHD up to a line-averaged electron density of 1.75×10¹⁹ m⁻³. The enhanced signal clarity enabled the detection of temporal transitions in the internal plasma potential distribution associated with changes in the plasma confinement state (Fig. 4). At t = 4.0 s, the plasma was sustained by electron cyclotron heating; at t = 6.1 s, 0.1 s after the heating was turned off; and at t = 7.0 s, by 180 keV neutral beam injection. The results revealed a rapid overall decrease in plasma potential immediately after the termination of electron heating, followed by a gradual flattening of the potential profile. Because variations in plasma potential strongly influence plasma confinement performance, these experimental data are indispensable for improving predictive models of plasma behavior and for establishing new confinement frameworks in fusion research.

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. Furthermore, 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.