Efficiency of neutral beam neutralizers in JET and ITER

Abstract

Neutral beam neutralizer efficiency plays a key role in determining the overall efficiency of neutral beam systems. Understanding the shortfall in neutralization efficiency encountered in positive ion neutral beam systems at JET is therefore of importance in ensuring the adequacy of the ITER design and in formulating beam-line designs for DEMO. Experimentation has previously demonstrated both the presence of background plasma and elevated gas temperatures, suggesting that the reduced efficiency may be due to a reduction in gas density. However, historical modifications to the neutralizer design at JET in accordance with observations from models produced little improvement in the neutral beam power delivered to the tokamak. This paper describes the development of the neutralizer models from an initial global heating balance for the gas alone, through to the recent application of computational fluid dynamic (CFD) to provide a consistent beam–plasma–gas system able to capture details of the neutral gas flow within the neutralizer. It is demonstrated that for the JET neutralizer a full 3D computation is necessary to correctly capture the behavior of the beam–plasma–gas system. The analysis is also extended to the ITER neutralizer. Overall, the importance of capturing the full complexity of neutral beam neutralizers is highlighted. The necessity of developing 3D modeling capability to support the design of future DEMO systems is demonstrated not only for beam neutralizers but for other beam components that include a fluid element, such as the duct.