Stability of beam positioning in ShenGuangIII

This paper describes how major beam positioning stability challenges faced by ShenGuangIII (SGIII) facility were met and solved by support system designs. The facility must provide vibrational and thermal stability conditions for alignment-sensitive components. The positioning stability error budget is developed for the alignment-sensitive components to provide connections between mechanical designs and performances of the support systems when they are subjected to multiple input sources that can cause structural vibration and drift. The largest allocation for vibration sources is broadband ambient vibration that is specified as acceleration power spectral density (PSD) at top-of-foundation. The components are supported in the laser bay primarily by relatively large and massive hybrid support structures of reinforced concrete (RC) pedestals and steel frames or vessels and in the target bay/switchyard primarily by building and large steel space frames. The support systems are required to afford the fundamental frequencies as best as they could. Finite element models of key modules in the laser bay and the global target bay/switchyard structures have been used to guide the designs of the combined conventional materials and optical system structures. Finite element analyses of broadband ambient vibration have demonstrated that the designs satisfy the positioning error budget. SGIII construction was completed in 2015. Ambient vibration measurements were performed and demonstrated that the acceleration PSDs remaining within guidelines. The dedicated shots to flat gold-plated targets showed that SGIII met its design requirement of ±30μm rms beam pointing to target.