The Mainz Energy-recovering Superconducting Accelerator (MESA) is an electron accelerator currently under construction at the Institute of Nuclear Physics, Johannes Gutenberg University Mainz, Germany. MESA is designed as a superconducting multi-turn energy recovery linac (ERL) to provide high intensity, low-energy electron beams for precision electron scattering experiments testing the limits of the Standard Model of particle physics.
 The accelerator complex comprises two superconducting radiofrequency cryomodules and the P2 experiment, the latter aimed at a high-precision determination of the weak mixing angle. These subsystems are subject to specific cryogenic cooling requirements. The two cryomodules operate with helium at 1.8 K, 16 mbar and a total flow of 8 g/s. The P2 experiment contains a superconducting solenoid that operates at 4 K and a target within which a volume of liquid hydrogen must be maintained  at 15 K. The required cooling power of 4 kW for the target is provided by a helium refrigeration loop.  The solenoid and cryomodules are actively thermally shielded by using liquid nitrogen.
A cryogenic infrastructure was designed and manufactured to provide and distribute helium and nitrogen throughout the accelerator complex. An above-ground helium liquefier including storage dewar supplies liquid helium to the underground cryomodules and P2 solenoid. An additional, dedicated helium refrigerator supplies cooling power to the P2 target. The cryogenic distribution system comprises a range of major equipment assemblies. Central to this infrastructure are three distribution valve boxes and several multi transfer lines, which provide the interface between the liquefier, refrigerator, cryomodules and the P2 experiment. One of the valve boxes, as well as a significant part of the cryogenic transfer lines, are located within the underground accelerator’s radiation protection area. Furthermore, the system includes an outdoor sub-atmospheric electric heater and a sub-atmospheric compressor station to enable reintegration of the 16 mbar helium return stream into the liquefier circuit.
At present, the majority of the cryogenic infrastructure, including the liquefier, cryomodules, and cryogenic distribution system, has been installed and commissioned, while finalization of parts of the P2 experiment and the liquid nitrogen distribution remains scheduled for 2026.
This presentation introduces briefly the MESA accelerator including the P2 experiment, followed by a detailed description of the MESA cryogenic infrastructure. It highlights, in particular, the design considerations for the cryogenic distribution system including the sub-atmospheric heater, providing insights in dealing with confined spaces and radiation environment. It further discusses installation challenges and closes with lessons learned.

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