The European Spallation Source (ESS) is a neutron-scattering facility which will utilize a pulsed 2.0 GeV proton beam generated in the linear accelerator (LINAC) for releasing high-energy neutrons in the ESS target station. The 2K superconducting LINAC is designed to comprise 43 cryomodules. The cryogenic distribution system (CDS) connects the cryogenic plant to these cryomodules via a 400 meters long cryogenic multi-transfer line, 43 valve boxes and an endbox.
The Accelerator Cryoplant (ACCP) consists of a 4K helium refrigeration and a 2 K sub-atmospheric section within the same cold box. To enable greater flexibility during 2 K operation, ESS chose a cold compressor string with three serial cold compressors and one sub-atmospheric warm compressor, which pumps down helium to the saturation pressure at 2 K. Following site acceptance test in mid-2020, the ACCP was commissioned with CDS during 2022 and 2023. The first ACCP-CDS and Superconducting Linac cooldown with 27 cryomodules was achieved in December 2024. After the installation of 6 additional cryomodules, a total of 33 cryomodules have been successfully commissioned to 2K since November 2025, in preparation for beam-on-target.
The presentation will present the superconducting LINAC cooldown, 2K pumpdown and 2K stable Operation. Operational experience, as well as the challenges and lessons learnt will also be covered.

Keywords
Helium liquefaction|Accelerator Cryoplant|2K operation|Operational experience|Cold compressors