Abstract
A global approach coupling the moderator to the beam extraction system has been applied for the design optimization of the thermal and cold moderators of the European Spallation Source (ESS), which will be the brightest neutron source in the world for condensed-matter studies. The design is based on the recently developed high-brightness low-dimensional moderator concepts. Para-hydrogen is used for the cold neutron source, while thermal neutrons are provided by moderation in water. The overall moderation configuration was chosen in order to satisfy a range of requirements on bispectral extraction, beamport configuration and instrument performance. All instruments are served by a single moderator assembly above the target, arranged in a 'butterfly' geometry with a height of 3 cm. This was determined to be the optimal height for trade-off between high brightness and efficient guide illumination, by analysis of the performance of 23 instruments, based on the reference suite of the ESS Technical Design Report. The concept of 'brilliance transfer' is introduced to quantify the performance of the neutron optical system from the source to the sample. The target monolith incorporates a grid of 42 neutron beamports with an average separation of 6°, allowing a free choice between cold and thermal neutron sources at all instrument positions. With the large number of beamports and the space below the target available for future moderators, ample opportunities are available for future upgrades.
Highlights
The European Spallation Source (ESS) is currently under construction in Lund, Sweden, and is expected to come on-line in the early 2020s, starting its user program in 2023
The issue of beam extraction is considered for the pancake moderator geometry in x6, while in x7 the ‘butterfly’ moderator geometry is introduced as a solution to the beam extraction problem. x8 concludes the paper with a short summary and the outlook for further upgrades
The pancake moderator provides a large increase in both thermal and cold brightness compared with the Technical Design Report (TDR) geometry
Summary
The European Spallation Source (ESS) is currently under construction in Lund, Sweden, and is expected to come on-line in the early 2020s, starting its user program in 2023. Instruments to access the spectrum they need by tilting their neutron optics to point at the desired source and, if required, employing a bispectral switch system (Mezei & Russina, 2003) to stitch the cold and thermal spectra together and increase the available bandwidth This became a design requirement on the moderator and beam extraction system from a very early stage in the project – to allow for each beamport a free choice of neutron spectrum, whether cold, thermal or bispectral – maximizing the flexibility available to instrument designers and the instrument performance. This results in a trade-off when reducing the source size, between the resultant brightness increase and the loss of beam extraction efficiency This paper considers those trade-offs for a full suite of instruments, based on the TDR reference suite. The issue of beam extraction is considered for the pancake moderator geometry in x6, while in x7 the ‘butterfly’ moderator geometry is introduced as a solution to the beam extraction problem. x8 concludes the paper with a short summary and the outlook for further upgrades
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
