Speakers
Description
As magnetic confinement fusion advances toward power generation, neutral beam injection (NBI) auxiliary heating faces increasingly stringent demands for higher energy, current density, and pulse duration. Ion beam neutralization—the most critical step in NBI—currently relies on the gas target method, where ions collide with homogeneously injected gas to produce neutrals. While raising the gas target thickness can boost neutralization efficiency to ~60%, this imposes excessive vacuum loads and degrades beam transmission. In contrast, the laser target method introduces no additional gas and achieves theoretical neutralization efficiencies exceeding 90%, representing a key breakthrough for NNBI development. This work addresses critical technical challenges in laser neutralization test platforms for negative ion beams, including: generation and transport of flat rectangular ion beams, stable operation of multi-folded resonant cavities, and predictive analysis of photothermal effects. In accordance with a small-scale laser neutralization test platform, a single-aperture radio frequency (RF) H⁻ ion beam transmission system was built with the integration of relevant NBI functional components, which outputs a Gaussian H⁻ beam with 50 keV@35 A/cm², and the beam cross-section presents as rectangular beam spot with adjustable dimensions. Adopting the resonant folded optical path approach, the configuration parameters of a triangular ring cavity (n=3) and a butterfly ring cavity (n=4) were obtained, which are applicable to full-scale NNBI devices and small-scale test systems. Combined with the Pound-Drever-Hall (PDH) technology, a stably operable butterfly cavity was designed and constructed, with a finesse of approximately 1150 and a laser waist radius of about 130 μm. The beam spot radius w(z) in the remaining optical path ranges from 600 to 700 μm, with an ellipticity of approximately 1.36%. A physical model of the photothermal effects of lenses and high-reflectivity mirrors was established based on the optical-thermal-structural coupling, and the variation law of the thermal damage parameters of the butterfly cavity was calculated, which is consistent with the experimental results of the sample mirror temperature rise. Meanwhile, the curve fitting results can also be used to predict the damage time and damage degree.
| Classification | Beam acceleration and neutralization |
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