Speaker
Description
The negative ion source currently being developed for the ITER neutral beam injector (NBI) suffers from two fundamental design shortcomings: (i) a plasma asymmetry along the direction parallel to the plasma grid, and (ii) an excessively elevated plasma potential that may degrade beam divergence within the accelerator. To overcome these issues, we propose a novel source architecture designated ANIS (Advanced Negative Ion Source).
Negative ions for fusion applications are produced in a two-stage system comprising an ionization chamber followed by a magnetized expansion region, the latter serving to reduce electron temperature and flux at the extraction zone. In current RF-driven sources, a dipolar magnetic field topology drives an electron Hall drift toward one of the lateral walls, polarizing the plasma and generating a transverse electric field that results in asymmetric distributions of plasma density, temperature, and potential [Fan14, Zie21]. Additionally, at the low operating pressures anticipated for ITER (~0.3 Pa), the plasma potential can reach up to 70 V, promoting the formation of energetic neutrals and negative ions that broaden beam divergence [Wim24].
The ANIS concept addresses these limitations through two principal design innovations: (i) a closed electron drift magnetic configuration that suppresses the Hall effect and enforces axisymmetric plasma profiles; and (ii) electrodes placed at the magnetic mirrors within the discharge region to independently control both the magnitude and spatial gradient of the plasma potential, thereby shaping the energy distribution function of neutrals (which are precursors to the production of negative ions on cesiated surfaces), and improving beam optics.
Three-dimensional Particle-In-Cell Monte Carlo Collision (PIC/MCC) simulations [Fub17, Fub14] confirm that the ANIS magnetic geometry produces axisymmetric plasma profiles and a markedly reduced plasma potential. The magnetic topology is also shown to be tunable via concentric permanent magnets or coils, providing a versatile platform for systematic parametric studies of plasma transport.
This work presents both the numerical characterization of the plasma behaviour and the associated experimental program.
References
[Fan14] U. Fantz et al., Plasma Sources Sci. Technol. 23 (2014) 044002
[Fub17] G. Fubiani et al., New Journal of Physics 19 (2017) 015002
[Fub14] G. Fubiani et al., Phys. Plasmas 21 (2014) 073512
[Wim24] C. Wimmer et al., Journal of Physics: Conference Series 2743 (2024) 012033
[Zie21] D. Zielke et al., J. Phys. D: Appl. Phys. 54 (2021) 155202
| Email address | gwenael.fubiani@cnrs.fr |
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| Funding Agency | EUROfusion |
| Visitor's Visa | no |
| Classification | H- and D- sources for fusion, accelerators and other applications |