Negative Hydrogen Laser-stripping Injection Method for China Spallation Neutron Source

To circumvent Liouville’s theorem via non-Hamiltonian processes and enable higher beam power and density accumulation in the China Spallation Neutron Source Phase Ⅱ (CSNS-Ⅱ), a novel injection technique is urgently required to replace the conventional carbon-foil stripping method, which suffers from short lifetime, excessive beam loss, high radiation dose, and electron-cloud effects under high-intensity operation. A comprehensive study of a three-step laser-stripping injection scheme for negative hydrogen beams was presented. Firstly, magnetic stripping of the loosely bound outer electron using a high-gradient dipole magnet to produce neutral hydrogen atoms. Secondly, sequential laser excitation via two photons (266 nm and 532 nm) to promote atoms to the second excited state (n=3). Thirdly, photoionization of the excited atoms into protons using an infrared laser (1 064 nm). A dedicated simulation suite was independently developed to model each physical process with high fidelity, enabling systematic parameter optimization. Based on the CSNS-Ⅱ injection parameters, a feasible scheme was proposed that minimizes emittance growth. Simulation results show that a longitudinally tapered dipole magnet with a peak field of 2.35 T and edge gradient exceeding 60 T/m achieves efficient stripping while limiting emittance blow-up to a factor of 5.6. The two-step laser excitation reduces the required peak power by over 80% compared to single-photon excitation, with optimized laser parameters of 311.1 kW at 266 nm and 70.1 kW at 532 nm. Final ionization was accomplished using a 1.25 MW, 1 064 nm laser, ensuring a total stripping efficiency above 99.7%, as mandated by the beam dump design. In terms of engineering implementation, the proposed scheme boasts multiple adaptability advantages: The selected lasers are all mature industrial-grade light sources. Relying on existing solid-state laser frequency doubling and amplification technologies, they can achieve long-term stable output, substantially reducing equipment research and development as well as maintenance costs. The design of the longitudinally high gradient dipole magnet has undergone multiple rounds of electromagnetic simulation and structural optimization. Key indicators such as its peak magnetic field and edge gradient can be precisely achieved by adjusting the Halbach segmentation strategy. Moreover, the overall size of the magnet is highly compatible with the existing installation space in the CSNS-Ⅱ injection area, eliminating the need for large-scale modifications to the main structure of the accelerator. Furthermore, compared with traditional foil stripping technology, this scheme achieves significant improvements in operational safety and economy. The laser stripping process involves no solid material consumption, completely avoiding equipment downtime and replacement costs caused by stripping foil evaporation, thus remarkably extending the annual effective operation time of the accelerator. Meanwhile, it can effectively predict the trajectories of stripped electrons, significantly reducing the beam loss rate, greatly minimizing radiation damage to surrounding equipment.