Liquid Lithium Flow Demonstration and Vacuum Characteristics Study for Design of a LINAC Based Neutron Source with Free Surface Liquid Li Target System

Abstract

High intensity accelerator-driven neutron sources with a free surface liquid lithium (Li) target system are widely being designed and developed because heat dissipation under high beam power is easier compared to a solid target and there is no need to periodically replace the target. However, a linear accelerator must be operated under ultra-high vacuum (<10⁻⁵ Pa) to avoid beam loss with the residual gas and keep the designed performance. On the other hand, a liquid Li target system should be handled under much lower vacuum (>10⁻³ Pa) to avoid Li boiling. Therefore, the experimental verification of outgas rate, residual gas type and Li vapor behavior are required for individual condition of operational temperature and flow velocity of the liquid Li. We assembled a 1/10 scale experimental setup of the high energy beam transport section of the advanced fusion neutron source accelerator and demonstrated the design with a liquid Li loop system. Although the outgassing was higher than expected, differential pumping of 10⁻⁴ and 10⁻² Pa was experimentally demonstrated under Li flow. The total Li vapor contamination in this experimental setup is estimated to be a few grams a year. The results will be useful for designing the high intensity accelerator-driven neutron sources with a windowless liquid Li target system.