Study on Effective Distance of Supersonic Molecular Beam Injection System for HL-3 Tokamak

Abstract

Supersonic molecular beam injection (SMBI) technology, an indigenous fusion fueling technique in China, have been implemented widespread in international nuclear fusion devices. The effective distance stands out as a pivotal parameter for the system, defined by the span between the injector outlet and the front of the first Mach disk. Notably, the effective distance of SMBI on fusion devices must surpass the expanse from the injector outlet to the plasma boundary. This study conducted an in-depth investigation into the application of the SMBI system within nuclear fusion devices, with a specific focus on analyzing how gas source pressure affects the effective distance of the beam, which is crucial for ensuring the supersonic beam’s efficient penetration to the plasma boundary. To achieve this, a hybrid approach combining computational fluid dynamics (CFD) simulation with experimental testing was employed. The design of the SMBI system was based a modified conical structure developed from the Laval nozzle structure, which was modeled using SolidWorks and then subjected to detailed fluid dynamics simulations within the Fluent software environment. These simulations were aimed at examining the impact of varying cone angles and gas source pressures on the airflow’s velocity and density distribution. The results from the simulations indicate that while the cone angle of the injector has a limited effect on the effective distance, the gas source pressure is a decisive factor. Research is found that the effective distance is directly proportional to the square root of the gas source pressure, a finding that aligns with existing scaling relationships within a certain pressure ratio range. However, beyond the conventional scaling range of 5 to 17 000, the existing relationships are found to be insufficient for accurately predicting the effective distance at higher pressure ratios. In response to this, the study introduced a novel scaling relationship that was applicable to a broader pressure ratio range, specifically from 10 000 to 50 000. This new relationship was validated through experimental testing, confirming the accuracy of the simulation outcomes. Particularly within the HL-3 Tokamak, it is demonstrated that even at the minimum gas source pressure of 9×104 Pa, the effective distance achieved is adequate to fulfill the requirements for fusion fueling, ensuring that the beam maintains supersonic characteristics throughout its journey from the injector exit to the plasma boundary. This research significantly contributes to the field by providing valuable insights for the design of SMBI systems, taking into account the broader pressure ratio range that is often encountered in practical nuclear fusion applications. The findings not only validate the simulation methods used but also offer a scientific foundation for the design and optimization of SMBI systems in other magnetic confinement fusion devices, paving the way for more efficient and effective fueling strategies in nuclear fusion research.

Keywords

• supersonic molecular beam injection
• effective distance
• hl-3 tokamak
• fusion fueling