New plasma discharge patterns of microwave-driven hydrogen ion sources: Transient global model and experiment

Microwave-driven ion sources (MDIS) efficiently generate high intensity H + , H 2 + , and H 3 + ion beams, while precise control of species composition remains challenging. This work combines transient global model (TGM) with comprehensive experimental characterization of hydrogen plasma discharge in a 2.45 GHz MDIS developed in Peking University (PKU). Based on number density and energy conservation equations for main particles, TGM reveals that H 2 + transiently dominates plasma ignition and serves as an efficient energy transfer medium, exhibiting an equilibrium temperature of 0.63 eV that exceeds temperature of H + (∼0.4 eV). Experimentally, the H 2 + fraction is found to initially increase with microwave power before declining. The H + fraction exhibits a double-peak structure with pressure, and a low-to-high density mode transition is identified. The experimental results are in good agreement with the simulation results. Based on these regular patterns, a ternary phase diagram was constructed to guide source design and operation. These findings expand the understanding of low-temperature hydrogen plasma discharge mechanisms while advancing hydrogen plasma kinetics, offering quantitative guidance for MDIS optimization.