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ESS: Opportunities and Challenges

This study explores the opportunities and challenges in optimizing moderator design for the European Spallation Source, focusing on the thermal and cold neutron performance. Various design options are considered, including a single top moderator and an optimized thermal bottom moderator. Results show significant improvements in performance, particularly with an optimized thermal moderator.

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ESS: Opportunities and Challenges

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  1. Moderator optimization studies ESS: Opportunities and Challenges F. Mezei Director, Machine Directorate www.europeanspallationsource.se 2014 September 15 • Luca Zanini- Group leader Neutronics • For the Neutronics team • European Spallation Source • TAC 10, 4 November 2014 • www.europeanspallationsource.se

  2. Moderator design timeline (1) • Started with TDR: two 60°openings per moderator • Flat moderator design • 1 flat moderator enough • Do we need a second moderator for something? • Committee say YES • Suggestion on optimized cold on top, optimized thermal on bottom • Ultimate design for bottom, pancake for top • Instruments design indicates that 3 cm tall is maybe ok for all • That leads to ultimate both on top and bottom: double ultimate • Then, if 3 cm for all is ok, 1 moderator can be really enough IF it delivers high cold AND thermal: • Single top moderator (butterfly)

  3. TDR design

  4. Moderator design timeline (2) • Started with TDR: two 60 openings • Flat moderator design • 1 flat moderator can be viewed by all instruments • Do we need a second moderator for something? • Committee say YES • Suggestion on optimized cold on top, optimized thermal on bottom • Ultimate design for bottom, pancake for top • Instruments design indicates that 3 cm tall is maybe ok for all • That leads to ultimate both on top and bottom: double ultimate • Then, if 3 cm for all is ok, 1 moderator can be really enough IF it delivers high cold AND thermal: • Single top moderator (butterfly)

  5. Top pancake: factor 2.4 on cold (E<5meV), factor 1.5 on thermal with respect to TDR

  6. Moderator design timeline (3) • Started with TDR: two 60 openings • Flat moderator design • 1 flat moderator can be viewed by all instruments • Do we need a second moderator for something? • Yes, if it gives significantly superior performance in something • Suggestion on optimized cold on top, optimized thermal on bottom • Ultimate design for bottom, pancake for top • Instruments design indicates that 3 cm tall is maybe ok for all • That leads to ultimate both on top and bottom: double ultimate • Then, if 3 cm for all is ok, 1 moderator can be really enough IF it delivers high cold AND thermal: • Single top moderator (butterfly) 6

  7. Bottom moderator • Optimization of thermal bottom moderator suggested by Ken Andersen. • Considered 3 options • 3 (H) X 3 (W) cm2 cold, 3 (H) X 6 (W) cm2thermal • 3X6 cold, 3X6 thermal • 6X6 cold, 6X6 thermal • Under study by instrument scientists and compared with TDR-like 6 cm bottom cold moderator

  8. Layout with top pancake and bottom “Optimized-Thermal (OT)”

  9. Results: thermal 20 meV < E < 100 meV • Huge gain due to optimization of thermal • Reason for increase roughly: • ≈50% position optimization • ≈50% water layer effect • Optimum for 3 cm tall moderator Sensitivity to positioning: ≈3%/cm ≈2%/cm

  10. Results: cold E < 20 meV • Strong angular dependence for 3 (H) X 3 (W) tube but large brightness at 0 degrees • 3 X 6 similar to TOP above 15 degrees, higher at 0 degrees; small angular dependence • 6 X 6 lower than top COLD 6X6

  11. OT option: Summary • 3 cm high bottom moderator gives nearly 2 times increase in thermal and equivalent or higher cold neutron performance compared to the top flat moderator • Tube would be attractive but only for few instruments • 6 cm wide moderators allow for high brightness over a 60 degree sector • 6 cm tall gives significant brightness penalty over 3 cm tall

  12. Feedback to top moderator design • Extended premoderator • Thermal moderator different (no Be) • Reducing diameter of cold will improve thermal • Iterations with engineering team

  13. Moderator design timeline (4) • Started with TDR: two 60 openings • Flat moderator design • 1 flat moderator can be viewed by all instruments • Do we need a second moderator for something? • Yes, if it gives significantly superior performance in something • Suggestion on optimized cold on top, optimized thermal OT on bottom • Optimized Thermal for bottom, pancake for top • Instruments design indicates that 3 cm tall is maybe ok for all • That leads to ultimate both on top and bottom: double ultimate • Then, if 3 cm for all is ok, 1 moderator can be really enough IF it delivers high cold AND thermal: • Single top moderator (butterfly) 14

  14. “OT (OPTIMIZED THERMAL) 3 X 6” W=8 cm H=3 cm L=24 cm (cold) L=10 cm (thermal)

  15. OT 3 (H) X 6 (W) detailed performance

  16. OT 3 (H) X 6 (W) detailed performance FACTOR 1.8 in THERMAL

  17. OT 3 (H) X 6 (W) detailed performance General gain in cold (still optimizable)

  18. OT 3 (H) X 6 (W) detailed performance [Å]

  19. Moderator design timeline (5) • Started with TDR: two 60 openings • Flat moderator design • 1 flat moderator can be viewed by all instruments • Do we need a second moderator for something? • Yes, if it gives significantly superior performance in something • Suggestion on optimized cold on top, optimized thermal OT on bottom • Optimized Thermal for bottom, pancake for top • From a pure brightness viewpoint the best would be using the ultimate on top and bottom • double OT • Then, if 3 cm for all is ok, 1 moderator can be really enough IF it delivers high cold AND thermal: • Single top moderator (butterfly) IMPROVED THERMAL AND COLD FOR ALL INSTRUMENTS 20

  20. Key ingredients for high brightness moderators • Low-dimensionality is critical for high brightness moderators • flat moderators : increase cold Brightness • tube moderators : further increase cold Brightness, but for a limited angular span • The size of the top pancake has one big drawback: the thermal is rather penalized because it is far from the hotspot • Optimized water layer: increase thermal Brightness • Two possible optimized solutions are • Optimized Thermal: more compact thermal and cold, where thermal is optimized and the cold provides excellent brightness for angle < 60 • Additional solution, combining flat/tube concepts while keeping the thermal moderator close to the hotspot.

  21. Butterfly: sum of several ideas The top pancake works great for the cold, less so for the thermal due to distance from the hotspot

  22. Butterfly: sum of several ideas The top pancake works great for the cold, less so for the thermal due to distance from the hotspot 22 cm 24 cm

  23. Butterfly: sum of several ideasoptimization of thermal source Bring the thermal neutron source close to hotspot, with additional grooving effect

  24. Butterfly: sum of several ideasflat moderator with tube-like enhancements For intermediate angles the flat moderator gives a high brightness gain

  25. Butterfly: sum of several ideasflat moderator with tube-like enhancements tube-like effect Exploits not only the flat moderator concept, but (for several beam lines) tube-like behaviour for maximum gain, still preserving overall a 3X6 cm2 view surface

  26. Butterfly: sum of several ideasflat moderator with tube-like enhancements tube-like effect tube-like effect

  27. Performance comparison 1E14 9E13 8E13 7E13 6E13 5E13 4E13 Integral brightness [n/cm2/s/sr] 3E13 2E13 1E13 [Å] angle

  28. Moderator design timeline (6) • Started with TDR: two 60 openings • Flat moderator design • 1 flat moderator can be viewed by all instruments • Do we need a second moderator for something? • Yes, if it gives significantly superior performance in something • Suggestion on optimized cold on top, optimized thermal OT on bottom • Optimized Thermal for bottom, pancake for top • From a pure brightness viewpoint the best would be using the ultimate on top and bottom • double OT • Again from pure Brightness viewpoint, 1 moderator can be really enough IF it delivers high cold AND thermal: • Single moderator (butterfly)

  29. Concluding remarks • New proposal for consideration: A single butterfly moderator, located in the bottom position and viewable by all beam lines, is proposed because it offers the greatest flexibility of all concepts studied • All instruments would be pointed at the butterfly, and selection of the top moderator, which will also have the potential for ~ 120° viewing, would be settled later (next generation moderator/reflector plug) • Future instruments (or instruments moving from bottom to top) will be able to gain from a further improved source • Extended vertical beam inserts could be provided to allow viewing of both the top and bottom • Plugs need not be much bigger than originally planned for ESS, thanks also to the reduced size of the moderators. • This concept indicates that the second moderator should be built at the start only if really needed; if it provides only marginal improvement over the butterfly for just a few instruments, it would go at the cost of this great flexibility of ESS. • We welcome TAC comments on this proposal

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