The HARP liquid hydrogen system

A liquid hydrogen system has been developed that serves as a scatterer for the High-Acceptance Recoil Polarimeter (HARP). In order to satisfy the conflicting design requirements regarding safety, the volume of the system, the energy loss of the recoiling protons, and the location of the cryogenic unit, a self-supporting thin-walled modular cryogenic system has been developed. The system was successfully operated during two commissioning runs of HARP in a high-luminosity electron scattering environment. The operational parameters of the system have been continuously monitored and are quantitatively understood.