Cryogenic systems of SuperKEKB final focusing superconducting magnets

SuperKEKB is an accelerator to collide electron and positron particles with a nano-beam collision scheme and equipped with two cryostats to accommodate 55 superconducting (SC) magnets for final focusing. Each cryostat is cooled by an independent cryogenic system employing a refrigerator of 250 W at 4.41 K and a subcooler to supply subcooled liquid helium (LHe), which absorbs heat leak by the relatively small sensible heat (-2.4 J/g) with an acceptable temperature rise of 0.35 K from 4.41 K to 4.76 K (saturated temperature at 0.161 MPa). The associated limitation of the permissible heat leaks necessitated a careful cryostat design with comprehensive thermal considerations to reduce heat conduction and radiation and maintain stable cryogenic conditions of SC magnets. This paper presents the designs of the cryostats and cryogenic systems to address the technical challenges to cool SC magnets with the compact spatial constraints. Cryogenic systems were designed to supply a subcooled LHe flow of -20 g/s and to provide a mixed capacity of refrigeration and liquefaction loads. The liquefaction component amounts to 40 L/h (-1.3 g/s), approximately half of the total refrigerator capacity to cool the vapor-cooled current leads of SC magnets, which is also found to significantly influence the total LHe flow (-20 g/s) to cool the cryostats. Dedicated studies were performed to investigate the SC magnet temperature variations with the LHe flow, the system operation modes such as cooldown and quench of SC magnets, and to assess the heat leaks with local and global methods, which facilitate the practical operation of cryogenic systems for the SuperKEKB accelerator and are detailed in this paper.