Investigation of applicability of long-distance LP system for Li target diagnostics in Fusion Neutron Source

Abstract

The beam target for generating a fusion neutron field in the Advanced-Fusion Neutron Source (A-FNS) is currently designed as a one-side free-surface liquid lithium (Li) jet flowing along a vertical concave wall. Characteristics of its thickness fluctuation were diagnosed in the EVEDA (Engineering Validation and Engineering Design Activity) Lithium Test Loop (ELTL), which has almost the same scale as the actual target excepting its channel width. In the diagnostics, the Laser Probe (LP) system was used. However, in an actual fusion neutron source, the diagnostics need to be operated from over 10 m away with no condenser lens at a downstream point from the final mirror for directing the incident laser to the Li surface from the perspective of Li vapor and radio activation. Whereat, the long-distance LP system for the actual Li target was conceptually designed for the IFMIF, and its precision error was also verified using a specular-reflection object and a water loop. In this previous study, the precision of measurement was evaluated as sufficiently high compared with the required limitation of surface variation (±1 mm). Then, in this study, we aim to verify the possibility of long-distance measurement of thickness variation for the free-surface Li flow. In the measurement, the incident laser head was set at a distance of about 10 m from the Li surface. And, while the previous measurement setup employed the focusing lens system with a focal length of 300 mm, some focusing lenses with longer focal length were used in this study. As a consequence, the Li surface variation was successfully detected and measured though the signal intensity of the reflected laser became low. Furthermore, it was confirmed that the impact of focal lengths on the detected signal was small.