ESS moderator configuration Luca Zanini ESS K Batkov

ESS moderator configuration Luca Zanini ESS K. Batkov, E. Klinkby, T. Schönfeldt, A. Takibayev, F. Mezei www. europeanspallationsource. se nnbar workshop, Lund, February 19, 2015

ESS Target-moderator-reflector assembly protons 2 Ge. V, 2. 5 m. A, 5 MW proton beam 14 Hz, 2. 87 ms long pulse (Bengt Jönsson) 2

(Bengt Jönsson) 3

Proton beam 4

Moderator Design Principles q Low-dimensional moderators q Importance of positioning q Importance of premoderator q Beam extraction q. Importance of reflector q. Try to open to 2 × 120° sector 5

Why flat moderators work the physics Reflector side ortho Quiet spot para Target side 6 qthermal neutrons arriving from the surroundings are transformed into cold ones within about 1 cm of the walls of the moderator vessel 6

Why flat moderators work the physics (Kai, 2004) Reflector side Target side behaviour used at JPARC, reflectometer points at the bottom qthermal neutrons arriving from the surroundings are transformed into cold ones within about 1 cm of the walls of the moderator vessel 7

Why flat moderators work the physics qthermal neutrons arriving from the surroundings are transformed into cold ones within about 1 cm of the walls of the moderator vessel q along the direction of these walls this intense layer of cold neutrons can be seen from the outside into depths comparable to 10 cm. 8

Single collision model for low-dimensional moderators 9

The premoderator shapes the spectrum

Flat moderator reference configuration the “pancake” q. Optimized for cold (2. 5 × TDR) q. Thermal extraction from the side (not so good) q. Current choice for top moderator

Qualitatively new level of performance

Higher brightness, lower heat load Moderator height [cm] 13

Correct positioning can improve brightness by 30 -50% Sensitivity to positioning: ≈3%/cm (elevation with respect to target) Thermal source some 15 cm from hot spot ≈2%/cm (offset with respect to target) 14 14

Importance of premoderator, also for water moderator 15

Optimized Thermal (with optional grooving) q Optimized for thermal q Higher brightness than pancake on a double emission surface q Cold extraction from “tube” moderator q Serving 2 × 60 -70° maximum q Good for second (bottom) moderator q Current choice for bottom moderator q. Grooving or reentrant holes look at maximum and more thermalized flux q. Does not work on pure para-H 2. IBR-II Dubna THERMAL FRM-II cold 16

Importance of reflector q 2 × 60° was mandatory for the TDR q 2 × 120° is possible for flat moderator 17

Combining all the design principles q. Optimized for thermal and cold q. Exploits all design principles (low. D, positioning, premoderator, beam extraction, grooving) q. Serving the full 2 × 120° 18 (Troels Schönfeldt)

Proton beam 19

Engineering model and neutronic optimization 20

Moderator configuration choice q Final decision imminent. Criteria: q Performance q Engineering feasibility q Beam extraction q Two moderators: 3 cm flat on top, 6 cm tall on bottom q Top: high-brightness q 3 cm PC or q 3 cm BF q Bottom: 6 cm tall either q OT - 2 × 70° q BF - 2 × 120° 21

Summary results THERMAL COLD *+- 10% variation in thermal and cold from engineering * 22

Summary results THERMAL COLD Factor 1. 56 *+- 10% variation in thermal and cold from engineering * 23

n-nbar beam extraction • 10° opening will require 2 -3 beamlines (can be retrofitted) • Can use top or bottom moderator (slightly better) • Butterfly design favorable (can extract cold neutrons from both “wings”) (R. Linander) 24