Researchers from the Max Planck Institute for Plasma Physics in Garching bei München, led by N. den Harder, have made significant strides in understanding the beam optics of radio frequency (RF) ion sources, which are crucial for the Neutral Beam Injection (NBI) systems in the ITER project. ITER, the world’s largest nuclear fusion experiment, aims to demonstrate the feasibility of fusion as a large-scale and carbon-free source of energy. A critical aspect of this endeavor is achieving a low beamlet divergence, which directly impacts the efficiency of the NBI systems.
The study highlights that the ITER Heating Neutral Beam requires a beam divergence of 7 mrad to ensure effective transmission through the beam duct. However, experiments using RF sources have shown higher divergences of 10 to 15 mrad, particularly at low beam energy. This discrepancy may stem from a broad distribution of perpendicular velocities among the H⁻ and D⁻ particles before they are extracted from the source.
Den Harder and his team investigated this issue further by examining the perpendicular temperatures of H⁻ and D⁻ ions in the RF-driven BATMAN Upgrade test facility. Their findings revealed that these temperatures are influenced by the source filling pressure, decreasing from approximately 4 electron volts (eV) at a pressure of 0.3 Pa to 2 eV at 0.4 Pa. This temperature variation plays a crucial role in the ion-optics calculations for the ITER Heating Neutral Beam grid system.
Interestingly, while the beamline transmission remains relatively unaffected by the perpendicular temperature, the heat loads on the downstream grids increase with higher temperatures. This insight is vital for optimizing the design and operation of the NBI systems, as managing heat loads is essential for maintaining the integrity and longevity of the equipment.
The implications of this research extend beyond the confines of the ITER project. As the energy sector increasingly looks toward fusion as a viable alternative to fossil fuels, advancements in ion beam technology could pave the way for more efficient and effective fusion reactors. The ability to fine-tune beam divergence and manage heat loads could enhance the performance of future fusion facilities, making them more commercially viable.
As den Harder notes, “The beamline transmission is fairly insensitive to the perpendicular temperature, but the heat loads at the downstream grids increase with the perpendicular temperature.” This balance between efficiency and heat management will be critical as the energy sector transitions towards sustainable fusion energy.
This research was published in ‘Nuclear Fusion’, highlighting the ongoing efforts to unlock the potential of fusion power and its implications for a sustainable energy future.
