1. FCC-hh Detector Magnets Design Study Herman ten Kate C.Berriaud, B.Cure, A.Dudarev, A.Gaddi, H.Gerwig, M.Mentink, G.Rolando, H.Silva, U.Wagner, S.Klyukhin 3 rd Annual FCC Collaboration Meeting Rome April 11, 2016 1.History, initial specs and magnet options 2.Present systems being studied 3.Today’s Default Twin Solenoid & Dipoles 4.Magnet Services, Cryogenics and Current 5.Installation in the cavern 6.Ways to reduce cost 7.Conclusion 1

2. 1st discussion regarding magnets in winter 2013-2014 for the FCC start-up meeting. Initial requirements defined in Febr 2014 based on 100 TeV collision energy: Study scaling up while maintaining resolution starting from LHC detectors; CMS+, ATLAS+ Design two general purpose detectors & develop options Cover in particular the low angle forward direction. Based on this, 1 st options looked at in 2014 were: CMS+, a 6 T solenoid with iron yoke ATLAS+, a 3.5 T ID solenoid with 2 T muon toroids. 1. Introduction – history of last 2 year Initial sketches for a fully shielding solenoid/yoke and a solenoid/toroid combination, Febr 2014. 2

3. Bending power: FCC collision energy 100 TeV, 7x the 14 TeV of LHC When same tracking resolution, so BL 2 /σ has to be increased by factor 7!  ID granularity assumed as in ATLAS/CMS, ≈20 µm, σ unchanged (at this moment)  Scale up tracker radius to 2.5 m and solenoid field to 6 T, considered maximum for NbTi.  Scale up field in solenoid-toroid system, field/track length combination in solenoid around the ID of 3.5T/3m and a toroid of with ≈2 T + increase of radius. Low angle coverage in forward direction:  Again based on 20-30 µm tracker granularity ---> add a dipole providing 10 Tm! E&HCAL depth increase from 10 λ to 12λ (using iron) radial thickness some 3.5 m!  Thus bore of solenoid/yoke or toroid has to increase up to 12 m, length scales accordingly. E&HCAL to cover low angles, move unit out, from 5 to 15 m, system gets longer. Coverage in forward direction, in central magnets up to eta≈1.5 (25°)  Length solenoid is ≈4x, so 24 m for 6m radius! Higher field, larger bore, longer system. 3 Understanding design drivers for the magnet

4. Option Solenoid/Yoke + Dipoles (CMS+) 4 Example CMS+: 6 T, 12 m bore, 23 m long, fully shielding, 28 m outer diameter Stored energy ≈50 GJ, 6.3 T peak field. Yoke? Thickness depends on how yoke is used. 100% shielding requires ≈6.3 m iron for 10 mT stray field at 22 m, mass ≈ 120 kt (600 M€). Huge mass, serious consequences for cavern floor, installation, opening & closing system. Partly shielding, iron for measuring muon exit angle only, yoke thickness may be limited to 1-2 m, some 20-40 kt, still some (100-200 M€). When not fully shielding the fringe field has to be accepted, or locally reduced by active compensation (later more on this). Conclusion: Fully shielding yoke not feasible, do different shielding, partly shielding or no shielding at all.

5. Option Toroid/Solenoid + Dipoles (ATLAS+) 5 Central Solenoid for the inner detector trackers. 1 air core Barrel Toroid with 7x BL 2, 2 End Cap Toroids to cover forward direction. 2 internal Dipoles to cover low-angle forward direction. Example, size: 30 m diameter x 52 m length (36,000 m 3 ), 10 coils in BT and ECTs. 3.5 T solenoid; 2 T /10 Tm in dipoles and ≈1.7 T in toroid. 55 GJ stored energy (for 16 Tm; 130 Tm 2 )! Various variations studied Crucial Question: do we really need a stand alone muon tracking detector for FCC ? No! 6 T in ID + muon angle outside solenoid is enough! Thus toroids were put aside!

6. Solenoid/Yoke + Dipoles 6 Requirement: 2 dipoles, each generating ≈10 Tm. Initially 2 designs briefly investigated, inclined saddle coils or inclined racetrack Some 2 T in the bore and 5.5-6 T in the windings. Again heavy yoke, are there other solutions? When Solenoid is not fully shielding as we assume, huge forces and torques are present between the systems and cross talk in field, very non-linear. Need to find an elegant solution with minimum forces and torques. Example of 2 dipole designs, saddle coils (left) or flat racetracks (right).

7. Actively shielded Solenoid: Twin Solenoid + Dipoles 7 Twin Solenoid: 6 T, 12 m free bore, ≈20 m long main solenoid + shielding coil 54 GJ, system size D27xL21, stray field 5 mT at 28-30m. Muon tracking space: gap with ≈2-3 T for muon angle or sagitta measurement Light: system mass ≈6-8 kt, much lighter than any solenoid-yoke option Gap between coils can be tweaked in size and field Few variants developed to make the system more compact, allowing smaller shaft size. Novel concept for air-core dipoles developed as well.

8. 2. Solenoid with ‘light’ Yoke + Dipoles 8 Solenoid with light yoke, no shielding, minimum iron layer, just for muon measurements. Yoke allows muon tracking with 1 Tm in barrel and 2.4 Tm in end caps. Stray field with 2 (1 m, 21 kt) and 3 (1.5m, 28 kt) layers of iron in barrel; we see 14 and 13 mT at 50 m radial distance, and some 200-300 mT at 30 m, high numbers! Invent additional local active shielding for side cavern (doable). Further solenoid-yoke optimization and options are looked at, work in progress…. Need clear physics requirement on muon system in yoke area, minimum Tm and sizing. Also need to define options for tracking and calorimetry in the forward direction. Problem: no shielding in solenoid and dipole, causing huge forces, need to minimize this. Few numbers Solenoid: 6 T in 12 m bore, 25m long 44 GJ 2x0.5 = 1.0 m barrel iron 4x0.6 = 2.4 m end cap iron system size: ≈ D19 m x L32 m 22 kt ‘light’ yoke, total ≈ 30 kt

9. Not shielded: Bare Solenoid (no yoke) + Dipoles 9 Bare Solenoid, no yoke, no shielding, for muon angle measurements only. Stray field is 0.1-0.4 T within 20m radius, but less that 0.5 mT (5 Gauss pacemaker limit) for R>150m, and 350m, thus safe in a 300-400m deep cavern! Invent additional local active shielding for susceptible equipment. Clarify physics requirement on muon system outside solenoid, minimum Tm and sizing. Define options for tracking and calorimetry in the forward direction. Stray field must be made acceptable in the periphery of the solenoid and far away ---> study local shielding options and get far-away field approved (seems OK). Nice, no iron, definitely the lowest cost solution and minimum size as well. Few numbers Solenoid: 6 T in 12 m bore, 25m long 44 GJ 2x0.5 = 1.0 m barrel iron 4x0.6 = 2.4 m end cap iron system size: ≈ D18 m x L32 m total ≈ 5-6 kt

10. Shielding effect of all designs Solenoid and MYS do practically the same, no shielding effect. Fully optimized TS works well; the “short version” less but right at 20-100 m. 10 General public safe zone Pacemaker limit For magnetic shielding to the general public there is no need for using iron yoke

11. 3. Today’s Default: Twin Solenoid & Dipoles Twin SolenoidDipole Stored energy 53 GJ 2 x 1.5 GJ Total mass 7 kt 0.5 kt Peak field 6.5 T 6.0 T Current 80 kA 20 kA Conductor 102 km 2 x 37 km Bore x Length 12 m x 20 m 6 m x 6 m FCC Air core Twin Solenoid and Dipoles State of the art high stress / low mass design. 11

12. PropertyValue TS cold mass3.2 kt TS vacuum vessel mass2.4 kt TS stored energy53 GJ Dipoles cold mass2x 380 t Dipoles vessel massTo be det. Dipoles stored energy2x 1.5 GJ Free bore12 m Outer diameter27 m System length42 m Total stored energy56 GJ Full pseudo-rapidity coverage: TS for low-η particles, dipoles for high-η particles. Force & Torque neutral twin solenoid through cold spokes, force & torque neutral dipoles through lateral compensation coils. Shielding: ‘peanut-shaped’ 5 mT boundary: R Z=0 = 32 m, R Widest = 35 m, Z R=0 = 57 m. Twin Solenoid & Dipole system – bare coils Twin Solenoid: Shielding outer solenoid Dipole main coils Dipole lateral coils Twin Solenoid: Spokes Force and torque neutral dipole Twin Solenoid: Inner solenoid 12

13. Field integrals: Twin Solenoid + Dipoles Twin solenoid field integrals: 36 Tm in 12m bore at η = 0 15 Tm in gap at η = 0 0 Tm at η = ∞ + = Magnetic field lines in vertical yz plane Dipole polarity: right-up & left-down configuration Dipole field integrals: 10 Tm for η > 2.5 Complex field map for η < 2.5 Peak field on conductor: 6.0 T 13

14. New option: a Balanced Conical Solenoid (BCS) magnet PropertyValue B center [T]5.5 Mean free bore [m]5.6 Cold mass length [m]7 Axial assembly length (TS + 2x BCS) [m] 40 η coverage2.5 Stored energy [GJ]3.2 Cold mass [t]350 Peak stress [MPa]100 Balanced Conical Solenoid Conical solenoid augments bending power of twin solenoid for pseudo-rapidity η ≥ 2.5 Just conical solenoid, axial force: 280 MN toward TS Addition of balancing coil: Makes not just cold mass, but each individual coil force and torque neutral. Mechanical stability: Stable in axial direction, unstable in off-axis directions: dT/d o =-2.9 MNm/ o, dF z /dz=-1.9 MN/m, dF x /dx=+1.1 MN/m Supports needed, in particular in off-axis direction. Radial position R [m] 20 15 10 5 0 Axial position Z [m] -25 -20 -15 -10 -5 0 5 10 15 20 25 Balancing coil (Counter clockwise) Conical coil (Clockwise) B [T] 0.1 T 0.2 T 0.3 T 0.5 T 14

15. Cross-over Field integral ‘seen’ by tracker: TS, TS+BCS, dipole Inner tracker boundary, r = 2.5 m, z = 8 m Forward tracker boundary, z = 23 m Comparison: Addition of BCS: ≈2x field integral enhancement with respect to just TS. TS + BCS versus dipole: Field integral of dipole better for η > 3.1, double field integral better for η > 4. For η > 4, balanced conical solenoid may be augmented with low-angle dipole weighing tons, not hundreds of tons. 15 Is the balanced conical solenoid a valid alternative for the dipole?

16. Apply PIPOS-principle: put all maintenance-intensive equipment, on surface obvious gain: m 3 on surface cheaper, access time, access availability, radiation. Proximity cryogenics in main cavern (to be minimized). Shield refrigerator on surface for cooling down & keeping system at <100 K (forced 20 b gas flow at 50 K level over 400 m height not an issue). Main refrigerator on surface for liquid He, enabling thermal siphon cooling. Option: MR on surface LHC-solution (liquid transfer 400m down), costs +20% in power and +15%, 500k/y over 30y = 15M, cheaper than m3 in side cavern. Power Converters/Breakers/RDU on surface. No use of m 3 in side cavern! No water cooled bus bars (1.5-2 MW saving & no water). It requires 300 - 400 m long superconducting transfer lines. Thus: build together in the surface hall Electrical Circuit and Cryoplant with current leads cryostat in between, gives the shortest connections and minimum loss. 16 4. Helium cryogenics and Current supply

17. 5. Cavern – magnet installation challenges 17 Depth of shafts? Main detectors in points A & G: 300 - 400 m below surface! Installation scenario’s needed, a 300 m deep shaft is not easy! Light: partitioning, 2-3 kt units, descending through a 28 m shaft? Heavy: assembly TS system on surface and crane down 7-8 kt ? Many questions! ≈28m ≈ 67x35x38 ≈ 90000 m 3

18. Installation scenario (10 s) 18 Installation, short & long opening scenario’s developed (to be adjusted for design updates)

19. Shielding options for side cavern, doable! 19 Iron requirement on side cavern was checked for the 3 solenoid options, to achieve < 5mT for pumps operations in side cavern: Assuming 25 m gap we need 0 for TS, 10 kt for MYS and 25 kt for bare Solenoid. Some reduction possible when not shielding entire side cavern.

20. Reminder bending power requirements: FCC 100 TeV, when same tracking resolution, so BL 2 /σ has to be increased by factor 7  Present ≈20 µm tracker granularity leads to 6T/12m system and 10Tm in forward dipole Pushing hard for, investing in, higher tracker resolution, say 10 or even 5 µm, then a 1.5 m ID tracker is sufficient, as well as some 4 Tm in forward direction! Then 4 T in a bore of 10 m, and half the field in the forward dipole would be sufficient Even further, a second option is to reduce the depth of the calorimeter, accept 10 or 11λ in steel or 10λ in tungsten, leading to a 4T and 9m bore system. This has a huge impact on size and construction cost! Investing in large scale feasible and affordable tracker point resolution pays off, thereby reducing technical risks on the magnets as well as makes the detector affordable. And it reduces cost of detector infrastructure, cavern, shafts, cranes… 20 6. How to reduce size and cost….. Size / Cost (rough estimate) Magnet Cost [B€] (25±5%) Detector Cost [B€] (75±5%) 6T/12m + 10Tm0.70 – 0.902.8 – 3.6 4T/10m + 4Tm0.35 – 0.451.4 – 1.8 A factor ≈ 2 !

21.  1st look at magnet systems probing 100 TeV p-p collisions completed.  Magnet size and field grow with the collision energy.  Based on present tracker resolution of 20µm, 6T/12m bore magnet systems are “huge”: 20-30 m diameter, 30-50 m long, 50-60 GJ.  Toroid based systems abandoned since stand-alone muon tracker not needed anymore.  Evolved to three 6T/12m straw-man designs (worst case, most challenging, and costly) ‒Twin Solenoid, nice features: light, elegant, allowing high-quality muon tracking ‒Solenoid + Minimum Yoke, heavy, even with minimized yoke, partly shielded. ‒Bare Solenoid (no shielding): lightest, cost effective, solve local shielding.  And developed innovative 10 Tm dipoles and solenoids for covering forward physics  Cost reduction possible (some factor 2) by pushing up tracker resolution.  Trend is towards an ‘affordable’ 4T/10m-20 GJ design, also reduced forward direction.  Stay open minded: continue to include and test new ideas and configurations. 7. Conclusion 21 More presentations on detector magnets: Matthias Mentink e.a. – oral Wednesday 11:10-11:40 in FCC-hh detector session Matthias Mentink e.a. – poster Wednesday 17:30-19:30 on Solenoid forward magnet options