Review of FCC tunnel footprint & implementationOutcome & conclusions

Austin Ball, Paul Collier, Massimo GiovannozziPhilippe Lebrun (chair), Lluis Miralles, Ralf Trant

CERN, 23 June 2015

Review of FCC Footprint 11 June 2015

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Scope

• The FCC Conceptual Design Report (CDR) aims at– providing a consistent description of a future FCC complex,– addressing main design and construction challenges,– presenting possible – if not optimized – configurations, with a level of

detail sufficient to perform quantity, schedule and cost estimates• In order to proceed towards the writing of the CDR, it is necessary to

converge on the technical baseline of the report, starting with the tunnel footprint and implementation: this is the purpose of the review

• Outside the scope of the review– Transverse cross-section of tunnel (single vs. twin tunnels, safety tunnel,

safety area in single tunnel)– Local tunnel singularities, e.g. stub tunnels for housing equipment– Number, size & configuration of experimental and technical caverns– Number, size & configuration of shafts and/or access ramps– Number, size & configuration of technical areas at ground level

Ph. Lebrun

Review of FCC Footprint 11 June 2015 3

Preliminary LayoutFirst layout developed

• Two high-luminosity experiments (A and G)

• Two other experiments (F and H)

• Two collimation/extraction insertions

• Two injection insertions, should include RF

• Insertion lengths are based on first order designs or even estimates, will be reviewed as optics designs are made

Presented by D. Schulte

Review of FCC Footprint 11 June 2015

4Ph. Lebrun

Presented by K. Oide

Review of FCC Footprint 11 June 2015

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General observations

• The review panel thanks the speakers for the comprehensive and detailed material presented, recalling the high-level parameters of the machines studied and summarizing the state of advancement of the FCC study– INDICO site https://

indico.cern.ch/event/400632/other-view?view=standard• The review panel notes that the presently proposed machine

configurations and layouts appear– well developed and substantiated for FCC-hh– much less for FCC-ee, in particular on the following points

• No coherent and accepted solution for IR optics and beam separation close to the interaction points

• No complete staging scenario for RF (common/shared between rings, voltage and power, need for RF sections at points C, E, I, K)

• No design of topping-up ring and layout (RF, experiment bypasses, top-up lines)

• This difference of approach is however not a showstopper for the review, since the FCC mandate specifies that the main emphasis of the conceptual design study shall be the long-term goal of the hadron collider

• Still, the outcome of the review should aim at preserving the possibility of presently open options for FCC-ee, pending better definition of the corresponding accelerator, collider and experiments

Ph. Lebrun

Review of FCC Footprint 11 June 2015

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Tunnel perimeter [1/2]Facts & findings

• FCC-hh– The FCC-hh perimeter must be a simple rational multiple of that of the

LHC, used as a high-energy booster, for filling the rings– To reach 100 TeV collision energy, about 82 km of arcs are needed with

dipoles operating at 16 T with a filling factor of 80 %– Considering the present number and length of LSS and ESS, this requires

a total perimeter of 100 km– Geometrical solutions exist for injecting into a 100 km perimeter FCC

from the LHC • FCC-ee

– Limitation of synchrotron power calls for maximum possible radius of curvature in the arcs

– Maximum accelerating gradient and RF power for the collider rings requires up to 2.4 km of RF straight sections, distributed symmetrically around the tunnel perimeter

Ph. Lebrun

Review of FCC Footprint 11 June 2015

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Tunnel perimeter [2/2]Facts & findings

• Geology and civil engineering– With the present symmetric racetrack shape, machines with perimeters of

80 km, 87 km, 93 km and 100 km (i.e. 3.00, 3.25, 3.50 and 3.75 times LHC) can be accommodated at about the same cost/risk per unit length

– Going below 80 km, or above 100 km increases the cost/risk per unit length

– With optimized location, the maximum depth of tunnel & caverns, and thus the risk of rock convergence, is about the same for the 93 km and 100 km options

• Infrastructure & safety– Sector lengths resulting from a 100 km perimeter tunnel with the

proposed layout look acceptable from the point of view of utility distribution, access and safety

Ph. Lebrun

Review of FCC Footprint 11 June 2015

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Tunnel layout & implantation [1/3]Facts & findings

• FCC-hh– The proposed geometry and insertion configuration appears well

substantiated, following clear principles• Symmetry of the layout: the two main experiments are on opposite sides for all

bunches to collide with maximize luminosity, additional experiments are symmetrically located to ensure same path length for both beams

• Additional experiments are clustered close to one of the main experiments, separated by at least 1.6 km arc to avoid cross-talk

• Injection LSS are close to the LHC, with FCC and LHC tunnels either tangent or secant

• Injection and collimation/extraction insertions are far from experiments• RF is on the second beam lines of the injection LSS; there is no dedicated

insertion, and the beam separation needs to be increased from 0.25 to 0.40 m• Collimation and extraction are located in the ESS; the design is flexible and the

final geometry remains to be optimized, which could eventually result in shorter ESS

– There is no fundamental obstacle to a non-planar design; this would however require a non-dispersive bending system taking substantial longitudinal space and thus reducing the dipole filling factor, and hence the maximum beam energy

– Electrical and cryogenic end-of-sector feed/return boxes will need some longitudinal space at end of sectors, including at points C, E, I, K: this must be accounted for in the optics design

Ph. Lebrun

Review of FCC Footprint 11 June 2015

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Tunnel layout & implantation [2/3]Facts & findings

• FCC-ee– With the present status of lattice and insertion design, it appears that

FCC-ee could probably fit into the tunnel layout proposed for FCC-hh, with several additional features

• Separate tunnels are needed for the electron and positron storage rings (24 m transverse separation) over ~3 km on either side of the collision points

• Bypass tunnels are needed for the accelerator (topping-up) ring at the collider experiment locations

• Klystron galleries parallelling the accelerator and collider tunnels are needed in the LSS

• There is possible need for additional LSS and klystron galleries at points C, E, I, K

– A non-planar geometry could seriously affect the performance of the machine

• To limit the increase in vertical emittance, a series of weak bends would be required, resulting in several km of additional magnets and loss of horizontal bending

• A bend in the plane of the machine would induce large spin rotations which would need to be corrected locally with additional equipment (spin rotators) to preserve transverse polarization – essential for energy calibration. No single solution presently exists for spin rotators at the W and Z energies

– Post review: it seems that “the issue of spin depolarization and generation of vertical emittance by kinks has been solved” (K. Oide)

Ph. Lebrun

Review of FCC Footprint 11 June 2015

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Tunnel layout & implantation [3/3]Facts & findings

• Civil engineering & infrastructure– Favor a tunnel position as southerly as possible within the constraints of

• Maintaining 12 plausible surface sites• Avoiding the highly fractured limestone in Vuache, as well as thrusted molasse

and high overburden under Prealps• Containing shaft depths• Enabling injection from LHC (or SPS)

– A non-planar layout would bring technical and cost benefits by• reducing the maximum depth of the tunnel and experimental caverns –

potentially problematic – by 50 to 100 m• Reducing the total depth of shafts by up to 20 %, with easier access and

corresponding reduction in cost and excavated volume– Overall depth of tunnel and caverns could also be reduced by crossing

Petit Lac in the (water-bearing) moraines rather than molasse; this would require

• Special tunnelling techniques (slurry TBM), with additional risk• Preservation of the exploited aquifers

Ph. Lebrun

Review of FCC Footprint 11 June 2015

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Recommendations [1/2]

• The review panel recommends– choosing a 100 km perimeter tunnel for the CDR– continuing to investigate issues and limits of tunnel and cavern

excavation at large depths in sedimentary rock, based on• survey of existing cases• refined geological studies of the large-depth areas concerned

– keeping the present, symmetrical and planar tunnel layout– keeping the reference solution of crossing the Petit Lac in the molasse

layer– For FCC-hh

• optimizing the design of the collimation and extraction systems with the aim of reducing the length of the ESS and allocating the saved length to the arcs for building up operational margin on the bending field

• making use of the arc bending to separate coasting beams from extracted beams

• reviewing the length of the SARCs separating the LSS, considering the optimisation of warm/cold injection lines with variable curvature and the minimum spacing needed between experiments

• allowing for end-of-sector electrical and cryogenic feed/return boxes in the optics studied

– For FCC-ee• continuing design work and agreeing on a reference lattice and interaction

region layout• clarifying the requirements for staged RF systems for the collider ring(s) –

common or separate – and for the accelerator (topping-up) ring

Ph. Lebrun

Review of FCC Footprint 11 June 2015

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Recommendations [2/2]

• The review panel recommends (continued)– Having another round at optimizing the 100 km tunnel position, with the

above mentioned constraints– Producing a single map showing the footprints of both FCC-hh and FCC-ee

reference layouts

Ph. Lebrun

Review of FCC Footprint 11 June 2015

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Appendix AQuestions for the review [1/2]

• 1. What are the high-level performance parameters to be achieved by FCC-hh and FCC-ee for the CDR?

• 2. What is the perimeter and configuration chosen for the tunnel and are these choices appropriate? (arc bending radius and filling factor, number and lengths of Long Straight Sections (LSS) and Extended Straight Sections (ESS), limits set by geographical and geological features).

• 3. To which functions are the LSS and ESS allocated? Where are the physics experiments? Are these choices optimum or could they be improved?

• 4. The minimum distance between clustered collision points is given by the minimum angle between the IPs to avoid crosstalk. What closest spacing of IPs can be considered?

• 5. There seems to be a preference from experiments to have all IPs as close as possible to the main CERN site. Is this feasible and how does it impact on layout?

• 6. Is there a need for separate accelerator tunnels bypassing the experiments? Can the injector machine(s) also pass through the detectors/experimental caverns?

Ph. Lebrun

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Appendix AQuestions for the review [2/2]

• 7. Specifically for FCC-hh, what is the length of the short Technical Straight Sections needed to accommodate the end-of-sector cryogenic and electrical equipment?

• 8. Specifically for FCC-ee, how many LSS/ESS need to house SCRF cryomodules? Over what length(s)? What is the staging model for SCRF cryomodules and for the cryogenic plants serving them?

• 9. What are the constraints/drawbacks/benefits on tunnel depth, slope, non-planarity coming from a) geography, geology and civil engineering, b) accelerator physics, c) accelerator systems, d) utilities, e) safety, f) environment?

• 10. What injection options are considered? How do they impact the configuration, slope, non-planarity, location of the tunnel?

• 11. How are the beams disposed of? What are the corresponding requirements on tunnel configuration?

• 12. Given the answers to the preceding questions, what appears to be the optimal position of the tunnel?

Ph. Lebrun

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Appendix BTimeline of the FCC study

Ph. Lebrun

2014 2015 2016 2017 2018Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4 Q1 Q2 Q3 Q4

Explore options“weak

interaction”

Report

Study plan, scope definition

FCC Week 2018 contents of

CDR

CDR ready

FCC Week 2015: work towards

baseline

conceptual study of baseline “strong

interact.” FCC Week 17 & ReviewCost model, LHC results study re-scoping?Elaboration,

consolidation

FCC Week 2016

Progress review

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Appendix CProgram of the Review

Ph. Lebrun