Enhanced volume production of negative hydrogen ion in pulsed modulated ion source for fusion

Abstract Neutral beam injection for fusion requires a negative hydrogen ion source (NHIS) capable of delivering high-quality H - beams, and pulsed modulated discharges are one potential method of increasing H - density volume generation. A two-dimensional fluid model is developed to investigate a NHIS operating under pulsed modulated discharge. The vibrationally excited states of hydrogen molecules are grouped to save computational time and improve model stability. The results reveal that with the increase of pulse duty cycle and pulse frequency, the cycle-averaged H - density exhibits a non-monotonic variation. The optimal H - ions enhancement occurs at pulse frequency of 5 kHz ~ 8 kHz and duty cycle of 30% ~ 50%. The enhancement is also pressure-dependent and is more pronounced at low pressures. Notably, the cycle-averaged H - density from pulsed modulated discharge at 0.4 Pa even surpasses that from CW discharge at 0.8 Pa. This demonstrates the feasibility of operating a NHIS at lower pressures, potentially reducing co-extracted electrons and minimizing H - ions destruction during extraction. Additionally, an alternative dual-pulse ion source is designed to address the temporal inhomogeneity of the plasma parameters in pulsed modulated discharges, and promising simulation results are achieved. Finally, the model is compared with experimental measurements to verify the accuracy.