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Placement optimisation process

The placement optimisation process is an iterative process. It departs from a number of basic requirements and aims at approaching a set of optimisation goals. During this process, the key directives "Avoid", "Reduce" and "Compensate" are followed, to arrive at an optimised placement scenario that meets the requirements of the research infrastructure and the territorial and engineering constraints.

Research infrastructure requirements

These are set initially and are considered during the entire optimisation process to arrive at research infrastructure that is both technically feasible and provides value for a world-wide community of scientific researchers who want to use it for many decades.

• First FCC-ee from ca. 2030 to 2060 and then FCC-hh from ca. 2060-2090 (not only FCC-ee or FCC-hh alone). Therefore the layout and the placement needs to be developed to accommodate both scenarios and will be in use until the end of the century.
• The layout and placement must be compatible with an intensity-frontier lepton collider and with an energy-frontier hadron collider.
• The layout and placement must permit the design, installation and operation of a hadron collider with a centre of mass collision energy that is close to 100 TeV and an integrated luminosity in the order of 1 .
• The layout and placement must permit the design, installation and operation of a lepton collider with collision energy working points Z, WW, ZH and ttbar and integrated luminosities for all these working points that permit carrying out the physics research programme in less than 20 years (not exceeding 50 MW synchrotron radiation power per beam at all energies).
• The layout and placement must permit multiple interaction points.
• To achieve its goals, the layout of the lepton collider must keep a superperiodicity of 2 (i.e. the sequence of arcs and LSS in one half of the ring must repeat in the other (when continuing in the same direction).
• The layout and placement must permit injection either from the LHC or from the SPS.
• A larger number of access points 12 vs. 8 is recommended in order to have more and smaller cryogenic plants to ensure an availability of not less than 95 % (wrong - the baseline foresees less and bigger cryoplants; I would remove this line)
• The layout should remain with FCC-hh experiments at PL and PB to have them closer to the CERN infrastructure and away from mountainous areas, where they would be more difficult to implement.
• Transfer lines from LHC and SPS must be a) technically feasible (ideally use only normal conducting magnets) and b) justifiable with respect to cost (length, inclination).

Flexibility from machine design

• Arcs can only be shortened or lengthened in integer units of FCC-hh cells (current length 213.04 m). A priori, all short arcs shall be changed simultaneously by the same amount; likewise for the long arcs.
• Lengths of long straight sections can be changed monotonously, but should not be decreased below 1300 m (for default 1400 m) resp. 2600 m (for default 2800 m) (minima to check, if necessary) to permit implementing the required functions. A priori, all long straight sections of the same length type shall be changed simultaneously by the same amount;
• The geometrical machine layout must be mirror-symmetric with respect to an axis through the main FCC-hh experiments, PA to PG, which must be located exactly opposite (180 degrees).
• PA and PG can only be displaced simultaneously, in the same direction and by the same amount (which would make the short arcs on one side of the experiments different from those on the other side). The bigger this displacement is, the more tweaking needs to be performed to obtain a geometrically closed ring.
• A priori the points PL and PB, and PF and PH, can be displaced w.r.t. PA and PG, respectively, but a priori only pair-wise by the same amount in opposite directions. The bigger this displacement is, the more tweaking needs to be performed to obtain a geometrically closed ring.
• If absolutely needed, a single point PL, PB, PF or PH could also be displaced alone. The bigger this displacement is, the more tweaking needs to be performed to obtain a geometrically closed ring.
• Access shafts in long straight sections can be displaced along the collider tunnel trace (not pratical, in particular at experiment points with their high affluence).
• Access shafts are per default located inside the tunnel perimeter, for radiation but also for better accessibililty. In specific cases ("no plot found"), the access shaft, but also the whole service cavern, could be placed outside the perimeter. However, this requires a detailed engineering study to ensure good access to the machine for personnel and material.

• Concerning FCC to LHC and SPS transfer lines, a dedicated section on CERN transfer lines provides information with maps.

Flexibility from infrastructure

Questions to infrastructure WPs (VM, 7.10.2020):

• Surface site/acces shaft displacements (configuration A: lateral/sideways w.r.t. the machine, with symmetric connection at the nominal point; configuration B: longitudinally along the machine, with asymmetric connection) - feasible or not, impact (technical, operational, cost-wise), distance limit ? For configurations A and B see scheme.
• Shaft depth - maximum possible, impact ("technology jumps") ?
• Tilt of machine plane - maximum possible, impact ?
• Access to machine tunnel from outside of the ring, technical gallery outside of the ring (configuration C) - feasible ?
• Access to machine through stub tunnel or technical gallery - maximum length ?
• Access through machine tunnel - maximum length ?
• Difference in replies between FCC-ee and FCC-hh ?

Cryogenics (LT, 7.10.2020)

It is recommended to maintain symmetry to keep the cryoplants and QRL identical; consequently, configuration A is preferred over B (although B is technically not excluded). Configuration A will require longer distribution lines (one per cryoplant) which could be locally (in the stub tunnel) larger than the QRL in the machine tunnel. The cost impact shall be balanced with the advantages from such a displacement.

For memory, the stub tunnel at LHC Pt. 7 is 375 m long. Pt. 7 is used to store He but is not a cryogenic supply point.

Radiation protection (MW, 7.10.2020)

An objective for FCC is to allow access at the shaft bottoms at all times, to enable permanent access for maintenance of the lift and other infrastructure installed underground in radiation-safe areas. Therefore, access control systems to access accelerator areas will only be installed underground.

All configurations (A, B, C) should be feasible. It will be seen in each individual case where the delimitation between the accelerator area and the permanently accessible underground area can be drawn. This will depend on the space needed for the accessible area and the potential integration of chicanes, if required.

Configuration C: for access tunnels passing above or beneath the main accelerator tunnel, a distance of 10 m is considered safe (this shall be confirmed before a detailed underground layout is released for construction design). Specific cases can be studied.

Configuration A: depending on the distance between the shaft bottom and the main tunnel additional chicanes might not be needed. RP does not reuqire to join the main tunnel symmetrically.

Configuration B: displacing an access point longitudinally along the machine is (a priori) not an RP matter. The question is how the shaft bottom is connected to the main tunnel. An offset of the shaft from the main tunnel is always preferred, to have enough space for an adequate chicane, thus permitting access to the shaft bottom during beam operation.

The FCChh is the constraining case; the above therefore applies to both FCC-ee and FCC-hh. If chicanes (where needed) would directly be built for FCC-hh (to be checked that they are also good for FCC-ee), no changes would be needed later. Building only for FCC-ee (in a first step), the minimum distance and the chicane layout would change; in this case modifications will need to be made later.

Magnet powering (JPB, 7.10.2020)

It is recommended to maintain symmetry for identical powering and equal cable lengths; consequently, configuration A is preferred over B (although B is technically not excluded). Power converters could be installed in the stub tunnels.

In LEP and LHC the distance between power converters and loads was/is up to 200 m. The distance has an impact on the powering solution. For FCC, the LEP solution (with PCs in the surface building) won’t be possible with very long cables. A proposal could be to split the PCs in two parts, with the AC/DC part in the surface building and the DC/DC part underground. For high loads (> 1 kA), the PCs need to be close to the loads, which requires underground space. With deep shafts and long cables, the only solution for FCC-ee is to have enough space underground to place at least the DC/DC part of the PC there. This underground space is anyway needed for FCC-hh due to the very high currents.

Electrical distribution (DB, 7.10.2020)

For configurations A and B there are no lengths limits but the following shall be considered: A) if the length is > 1.5 km it could be necessary to have an alcove in the middle to supply equipment (lights, plugs, rails,..) located in the stub tunnel. For B) , depending on the distance from the baseline position, the position of the baseline alcove might need to be adapted.

The functional and technical design of displaced points will have to be customized w.r.t. the baseline. For A), the dimensions and cross sections shall be sufficient to transport and install electrical infrastructure equipment (transmission and distribution power lines, transformers, …).

The different electrical layouts will create deviations from the standard layouts (same as today in LHC Pt. 3 and 7). The sectorisation of the AUG systems shall be customized to reflect displacements and new areas like the stub tunnel (in case of A).

Additional costs will be incurred at the engineering and design phase. Additional material and manpower will be needed for the installation of the electrical infrastructure (cable trays, lights, plugs, AUG, …) in the stub tunnels. Additional costs will be created if any other non-baseline equipment will need to be supplied (example: HVAC for the stub tunnel).

If one considers that 135 kV transmission lines will transit between points through the tunnel (which is the actual baseline) negligible cost differences could be expected due to the different cross sections of the power lines to be installed in the stub tunnel. No significant differences between FCC-ee and FCC-hh are expected, assuming that the position of the underground equipment (converters, RF, CV, cryo) is maintained on for the next use (attention: between FCC-ee and FCC-hh the position of, for instance, cryogenics and RF changes).

Survey (MJ, 7.10.2020)

Before some studies it is currently difficult to give quantitative responses.

Displacement will certainly cause some loss of precision. Whether it will be significant will depend on the position and orientation tolerances for the machines, and the configuration of the connection from the surface, down the shaft, and into the tunnel.

Without specifications and network simulations, the length limits are difficult to define. Configuration B) would probably have a lower impact, unless the shaft ends up coming down in the LSS.

Basically, the deeper the shaft the lower the precision in the position transfer from the surface (a Postdoc will start this study with an analysis of the ways to make this transfer, deliverable: T0 + 12 months). NB: it is not evident that, with current instruments, one can achieve the precision achieved for LEP.

Re FCC-ee and FCC-hh, the significance of the impact is tied to the position and orientation specifications for the machines.

Another area to look at, in order to assess the overall impact, is the impact of the configurations of the surface sites, and the achievable precision in the transfer of position and orientation from the Geodetic Surface Reference Network to the reference points at the top of each shaft.

Ventilation (GP, 9.10.2020)

For configuration A, for fresh air supply a bigger duct than in the machine is needed in the stub tunnel (bigger with increasing length of the stub tunnel). Custom-built fans might be needed to ensure the required pressure. For extraction, the duct diameter and the suction pressure at the extraction unit could be increased. For emergency extraction (smoke and helium) see fresh air supply. For local ventilation, the stub tunnel would need a separate ventilation entailing cost and space for air supply, extraction and emergency extraction (unless its volume is connected to the tunnel, in which case the total air flow would need to be increased).

Operationally, longer distances to cover increase the travel time to the installation. New ventilation systems need to be operated.

For configuration B, if the longitudinal displacement increases the maximum length so far taken into account, one would have to deal with more compartments. This could imply the increase of the equivalent diameter for the fresh air and the emergency extraction passages.

The principles laid down above have to be applied for FCC-ee and FCC-hh and for each point of the ring.

Cooling (GP, 9.10.2020)

The length limits depend on the point where the change from the baseline happens (in a point where no tunnel cooling is needed, or an experiment, or an RF point).

In configuration A, if the stub tunnel is very long, one will have to play with the diameter for the cooling pipes of the tunnel circuit to keep the pressure rating in PN16(since one does not want to change to PN25). Another parameter (for the time being unknown) is the pressure drop across the magnets, cables and other consumers of cooling water.

Configuration B could imply an increase of pipe diameters. The degree of that depends on the heat load to remove in the part of the sector that is added and the number of connections. In case of high load, the increase of diameter and the balancing of the circuit can represent an important issue.

Smaller shaft depths are preferred for open circuits; greater depths imply higher PN. Higher PN involve higher cost and can cause longer installation time. If one has to send make-up water from one point to another (in case it is not available at a given point) one must either work with a high PN in the tunnel or break the pressure, but then one needs a more powerful pumping station to pump the water to the surface. Likewise for sump water, the pumps will need to be more powerful and the PN rating can also be higher.

Reasonable tilts of the tunnel place will cause not particular problems. The high and low points should be located in an access point and not in a sector, to avoid long pipes for the sumps (low point) and to avoid helium/smoke pockets and ODH related issues (high point).

The principles laid down above have to be applied for FCC-ee and FCC-hh and for each point of the ring.

Questions to infrastructure WPs on scenario PB17-0.8 (VM, 18.1.2021):

Scenario PB17-0.8 might require to displace the access (surface site and access shaft, not LSS and not machine function) at PF by around 1200-1300 m and at PH by around 850 feasible. In PH access could be to the end of the technical gallery, in PF to the machine tunnel or a connection tunnel (extension of technical gallery).

• Are such displacements of the access feasible ?
• Which impact will that have (on operation/maintenance, safety, cost, ...) ?

Radiofrequency (OB, 18.10.2020)

For the RF system, in particular the LLRF system, there is no strict distance limitation between the surface building and the galleries. As in LHC, the FCC-hh cavity controllers will be installed in Faraday cages located in the tunnel, close to the equipment.

Cryogenics (LT, 20.10.2020)

The PF and PH sites are not identified as sites with cryogenic plants. Consequently, the proposed displacements have minor impacts on cryogenics.

Optimisation goals

• The total circumference of the collider tunnel should not be shorter than 92 km.
• The total circumference of the collider tunnel should not be longer than 100 km.
• The layout and placement optimisation considers 2 IPs for FCC-ee at PA and PG and 4 IPs for FCC-hh at PA, PB, PG, PL.
• The shafts to access the experiment caverns at PA and PG must be located at either side of the detectors, within the total length of the caverns, in order to permit lowering detector elements.
• Technical galleries and service caverns must be preferably located at the inside of the ring to avoid exposure of equipment to stray radiation and for easier access to the transport side of the machine tunnel.
• The connection tunnels from the experiment caverns to the service caverns are approximately 50 metres long.
• The depth of shafts should be kept below 300 m. 400 m are considered the maximum tolerable shaft depth.
• Shaft depths can be reduced by placing sites at locations with lower altitudes and by tilting the ring. However, the tilt must not exceed 0.5 % (to check).
• Mid-site access shafts (PC, PE, PI, PK) can be displaced along the collider trace up to the end of the straight section at this point (there are no straight sections at the mid-arc points).
• Access shafts for PF and PH can be displaced along the collider trace up to the end of the straight section at this point (or more, with the consequence that one has to go through the machine tunnel to reach the centre of the straight section, or extend the technical gallery next to it until the bottom of the shaft)
• Surface sites for experiment points should not be smaller than 5 ha to permit hosting all necessary functions and to be able to foresee buffer spaces and transport facilities.
• Surface sites for non-experiment points should not be smaller than 3 ha to permit hosting all necessary functions and to be able to foresee buffer spaces and transport facilities.
• The collider tunnel should be lower than 35 m under the bed of Lake Geneva.
• Sites should be closed to transport infrastructures (road, railroad) to ease the access of persons and materials and to permit implementing efficient evacuation of excavation materials.
• In urbanised zones, buildings and technical infrastructures at sites should be at least 50 m away from residential buildings to avoid nuisances and to permit visual shielding. In any case, in France the Code de l'urbanisme applies.
• In non-urbanised zones, buildings and technical infrastructures at sites should be at least 75 m away from residential buildings to avoid nuisances and to permit visual shielding. In any case, in France the Code de l'urbanisme applies.
• The process aims at locating one surface point on existing CERN territory.
• The process aims at locating surface sites on land plots which are owned by the governments of by public entities.
• Unconstructed land is preferred over constructed land, if constructed land is occupied by residential buildings.
• Forests should be preserved.
• Wetlands should be preserved.
• High-value agricultural exploitations should be preserved.
• Biological corridors should be preserved.
• Low and medium ecological quality zones are preferred over high ecological quality zones.
• The placement aims at maximising the percentage of tunnel length in molasse rock, and minimise its percentage in limestone.
• Potential synergies with neighbouring residential areas, public infrastructures or industrial installations should be considered:
  • waste heat use by swimming pools, indoor sport facilities, schools, public buildings
  • waste heat use by agricultural facilities (glasshouses or piscicultures)
  • extension or shared use of electrical infrastructures (substations)
  • extension of existing electricity lines
  • extension of existing roads
  • shared use of computing facilities with technological and business parks (due to fast fibre technology for data links, the location of data centres should matter only little)
  • impact potentials for regional tourism
  • possibility to create new natural reservations, protected zones, re-creational areas and biological corridors

A 4 IP alternative layout

* A 4 IP alternative layout for FCC-ee requires a different layout (superperiodicity of 4, with experiments always at 90 degrees) * With a 45-45 degree layout (and keeping one point at the baseline position of PB) a point would fall into the Mandement region, which is not feasible * With a 60-30 degree layout one experiment would be close to the lake (strategic site close to highway in Bellevue), which is challenging from an engineering point of few (unstable underground) (to avoid this, the sequence of points should be rotated by one point, as explained).

Iterative process

Avoid

The iterative process to develop feasible placement scenarios starts with the establishment of zones that should be avoided. These zones have been established as a common activity of experts in France and Switzerland. The resulting "territorial sensibility grids" have consequently been reviewed and approved in the frame of concertation groups with the two host states.

The grids define four different levels of constraints: Intolerable, strong constraints, tolerable constraints and negligible constraints. Each constraint level is colour coded so that the affected zones can be easily identified on geographical maps. The following table explains the meaning of the different levels.

France: For the establishment of the territorial sensibility grid, the public entity Cerema was the expert partner of CERN. An initial grid has been established in a process to establish a baseline scenario for the FCC conceptual design report in the period 2017/18.

Switzerland: For the establishment of the territorial sensibility grid, the company ECOTEC was the expert partner of CERN. An initial grid has been established in a process to establish a baseline scenario for the FCC conceptual design report in the period 2017/18. Since the grid was developed by a non-authorative partner, the grid has been reviewed and approved by the "Structure de Concertation Permanente avec l'état hôte Suisse".

Once the territorial constraint maps for the zones of interest are created, an iterative search for suitable particle collider layouts and placement starts. For this purpose, a web application, the "FCC Footprint Explorer", has been developed that permits moving, resizing and rotating the collider trace on a map that also shows the territorial sensibility grids. The search can also be carried out in a semi-automatic fashion. After defining allowed ranges of geometrical parameters and "target areas" for individual surface sites, a software module generates random combinations of parameters and calculates how many surface sites fall inside target areas. Scenarios with a sufficiently high number of hits (10, 11, 12) can be stored and further analysed and optimised by hand.

Next, a manual selection among several thousands of interesting scenarios needs to take place. Those selected scenarios permit identifying classes of scenarios with similar characteristics.

Once scenario classes are identified, a manual process starts from some hundred scenarios to optimise towards some tens of hand-picked scenarios that consider further constraints and requirements that are based on the OECD guideline for the site selection of industrial parcs. During the process the following additional information sources are consulted and considered for the scenario developments:

• Urbanistic planning documents e.g PLU, PLUI, PLUH, SCOTT in France, Plan Directeur Cantonal, Plans Directeurs Communaux in Switzerland
• Societal known constraints and known development plans until 2030 reported by french host state authorities, for instance Cerema, DDT01 and DDT74, SGAR, prefectures of the departments
• Societal known constraints and known development plans until 2030 reported by swiss host state authorities, for instance sector in charge of the urbanism at the DT of the state and republic of Geneva, GESDEC, OCAN, Directorate of the Geneva airport, SIG
• Feedback from selected land owners in France and Switzerland
• Site walk overs and selected analysis of the fauna and flora initial state to determine the ecological quality level
• Potentials for synergies are identified using map exploration and existing project documentations as driver to place surface sites in the vicinity of such development potentials.

Reduce

Once a number of candidate scenarios that can "in principle be feasible" are identified, the merits of the different scenarios are documented with the help of a multi criteria analysis ("MCA") as is best practice for large-scale projects. The MCA consists of a ranking of a set of criteria for each site that is part of the scenario. These sets do also include aspects that interlink individual surface sites, such as for instance the underground geological conditions, roads and connections to resources (e.g. electricity and water).

The ranking for each criterium is supplied according to the MCA guide sheet.

Compensate

-- JohannesGutleber - 2020-07-10