Preliminary Design of a LH₂ Target Cooling System at LBL

Abstract In order to study nuclear structure at the limits of stability through a program of targeted measurements at the Facility for Rare Isotope Beams (FRIB), a thick liquid-hydrogen target and vertex-tracking detector system specifically optimized to be coupled to the Gamma-Ray Energy Tracking Array (GRETA) for fast-beam measurements at FRIB is under development. The LH 2 target cell and windows will be made of thin Mylar. The target thickness is 5~15 cm with an effective diameter of order 50~60 mm. A cryocooler-based hydrogen cooling system is under design to cool down the target, liquefy gas hydrogen, deliver and maintain liquid hydrogen in the target. The cooling system primarily consists of a cryocooler-cooled cryostat, a gas hydrogen and gas nitrogen storage and handling subsystem including safety devices, as well as instrument rack and PLC control system. The hydrogen is liquefied in the cryostat using a two-stage GM cryocooler at 20 K. The vaporized hydrogen is re-liquefied in the condenser attached to the second stage of the cold head and the liquefied hydrogen flows into the target through its gravity-driven thermos-syphon cooling circuit. A thermal radiation screen at around 30 K is mounted on the first stage of the cold head to protect the cold mass at 20 K from 300K radiation. The safety design of the hydrogen cooling system is carefully taken into account. This paper presents the preliminary design of the LH 2 target cooling system at LBL.