Thickness-variation measurement at downstream regions for Li target of fusion neutron sources

Abstract

In the developing fusion neutron sources (FNS), a liquid lithium (Li) free-surface jet is adopted as a beam target for neutron field generation. Owing to beam irradiation, Li must flow at a high velocity range to remove the heat load. In addition, Li needs to maintain a stable surface to maintain the objective neutron environment and soundness of the target system. In previous studies on the Li target of a fusion neutron source, experiments attempting to clarify the flow characteristics of free-surface Li were conducted using Li loops. Most of these studies focused on the area from the nozzle exit to the beam irradiation area. Then, in this study, in order to confirm the jet condition in the downstream region, where is a candidate region for monitoring in the actual FNS, the thickness variation was measured at a distance of 10 to 490 mm from the nozzle exit in the Li loop at Osaka University. In that region, it is to be expected that the influence of the well-developed bottom-wall boundary layer and wakes from sidewalls would not be negligible. The laser probe (LP) method was used to apply a noncontact-type optical-comb distance meter. The characteristics of surface fluctuation, such as the average thickness and wave heights, were analyzed from the obtained data. As a result, the increase of the jet thickness was observed in the downstream region, and it was also confirmed that wave heights become larger as flowing downstream. Except for the maximum wave height, the increasing tendencies were not simply linear. However, the complex change of surface-fluctuation characteristics was expected to occur at the region further downstream from the candidate monitoring point in the actual Li target system.