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Neutral beam injection (NBI) is an established heating method on many existing magnetic confinement fusion devices. Most currently operated NBI systems create neutral Hydrogen, Deuterium, or even Tritium [1] by neutralizing positive or negative ion beams in a gas neutralizer (GN). For ITER a beam energy of 1MeV Deuterium was chosen. It was found that the neutralization yield of positive ions in the GN decreases with energy and becomes vanishingly small above 200 keV/amu, while neutralization yield of negative ion remains approximately constant at around 55% above this energy. However, there are several key constrains for use of positive or negative ion beam systems: the creation of negative ions with energies larger than 1 MeV presents significant technological challenges, and the ion current density is about a factor of ten lower than that of positive ion sources.
This paper investigates the feasibility of using a liquid spray in an NBI system to neutralize a positive ion beam or convert it into a beam of negative ions based on experimental results [2]. A large number of negative ions and neutral atoms at a few hundred keV energies were obtained from the interaction of positive ions with a water spray containing sub-micron-sized droplets. The efficiency enhancement has been attributed to the resonant charge exchange, where the projectile velocity is comparable to the orbital velocity of an electron in an atom at rest, called the Massey criterion [2]. The interaction is elastic; it proceeds without energy exchange, and angular spread is very narrow (~ 1°) [3], which means the beams of negative ions and neutral atoms retain the property of positive ion beam, namely, have the same emittance and similar energies. This phenomenon offers an opportunity to generate high current, high-energy negative ion, and neutral atom beams in a wide range of parameters.
In the proof of principle experiment it is shown the converttion of positive ions into neutral atoms and negative ions with high efficiency over a wide range of energies. The energy of positive ion beam conversion efficiency of 38% for H^+→ H^0 and 50% for C^(4+)→ C^0 was estimated for energy range 80-500 keV and 270-1440 keV, respectively. These estimates are conservative, and the beam-target interaction conditions can be optimized by adjusting the spray density and droplet size for a given positive ion beam.
The spray converter removes engineering challenges associated with a large-scale negative ion accelerator. It has no restrictions on ion species. It promises increased neutralization efficiency of positive ion beams in neutral beam injection beamlines. We compare the required specifications for NBI systems and assess how they can be met.
[1] R. J. Hawryluk, Rev. Mod. Phys. 70 537 (1998)
[2] S. Ter-Avetisyan, M. Schnürer and V. Tikhonchuk, J. Appl. Phys. 134, 063302 (2023).
[3] B. G. Lindsay, W. S. Yu, K. F. McDonald and R. F. Stebbings, Phys. Rev. A 70, 042701 (2004).
