At the European Spallation Source (ESS), a 5 MW beam of 2 GeV proton with a normal current of
62.5 mA is accelerated by a linear accelerator and is directed onto a rotating tungsten target at a
repetition of 14 Hz and a pulse length of 2.86 ms. High-energy spallation neutrons are subsequently
moderated to cold and thermal neutrons using a combination of two hydrogen moderators and a
thermal water premoderator, resulting in the production of a cold neutron beam. The nuclear
heating for the two-moderator configuration is estimated to be 6.7 kW at the proton beam power of
5 MW.
The ESS Cryogenic Moderator System (CMS) is designed to supply subcooled liquid hydrogen (18 K
and 1.0 MPa) with a parahydrogen fraction exceeding 99.5% to the two hydrogen moderators,
maintaining the temperature rise across the moderators within 3 K during the beam injection . Two
ball-bearing pumps arranged in series deliver liquid hydrogen to each moderator at a flow rate of
240 g/s. The CMS is cooled by the Target Moderator CryoPlant (TMCP), A 20 K helium refrigerator
with cooling power of 30,3 kW at 15 K, which removes static and dynamic heat loads via a plate-fin
heat exchanger and maintain the hydrogen supply temperature at 18 K. An ortho-to-parahydrogen
catalyst vessel is installed to ensure the required parahydrogen friction. In addition, the
parahydrogen fraction of supply and return liquid hydrogen is measured using a newly developed
an in-situ ortho-to-parahydrogen friction measurement system (OPMS) based on a Raman
spectroscopy. The temperature distribution across the CMS varies due to dynamic heat loads at
proton beam injection or trip, resulting in pressure fluctuations because the CMS forms a closed
loop. To mitigate these fluctuations, the CMS is equipped with a 71 L buffer tank. Furthermore, the
TMCP includes a valve box to compensate for dynamic heat loads by adjusting the feed helium flow
rate, thereby controlling the effective cooling capacity.
Commissioning of the TMCP was completed independently, without connection to the CMS, in
December 2022. Installation of the CMS began in 2021 and completed in May 2024. Preliminary
commissioning of the integrated CMS and TMCP was subsequently conducted using helium prior to
hydrogen operation in 2024. The first hydrogen cooldown of the CMS was successfully achieved in
March 2025 using hydrogen. However, thermal oscillations were observed in several capillary
pipes. Performance tests are currently being conducted to optimize operating conditions and to
establish appropriate operating procedure for the development of an automated control system. At
present, the CMS is operated in semi-automatic mode. Prior to commissioning, a failure protection
system was developed to automatically and safely shut down the CMS in the event of abnormal
conditions. During commissioning period, the CMS safely shut down several times in response to
unexpected events, such as a vacuum pump failure and issues in the ESS water cooling system,
demonstrating the proper functioning of the protection system.

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