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FCC-ee Arc Half-Cell Senior Advisor Panel Meeting #2
Notes of FCC-ee Arc Half-Cell Senior advisor Panel Meeting #2
22nd November 2022
List of people attending the meeting:
F. Carra (chairman), J. Bauche, L. Baudin, M. Benedikt, A. Bertarelli, F. Bertinelli, C. Eriksson, K. Hanke, F. Lackner, R. Losito, W. Niewiem, T. Raubenheimer, S. Rorison, M. Timmins,
J-Ph Tock, F. Valchkova, L. Von Freeden, J. Wickstrom, F. Zimmermann
Slides of presentation are available in the Indico page related to the meeting (https://indico.cern.ch/event/1221522/)
Agenda:
- Presentation of the meeting (T. Raubenheimer)
- Presentation of status of FCC-ee half arc cell project (F. Carra)
Disclaimer: The meeting minutes herein are a summarization of the content of the discussions and conversation during the presentation to the senior advisor panel, it does not aim at being exhaustive. For the contents of the presented slides, the reader should be referred to the indico event.
1)
T. Raubenheimer opened the meeting. The presentation in front of the Advisor Panel is considered as a platform for discussions. Feedback will be collected as individual thoughts rather than a common report to be able to raise a broad spectrum of concerns or opinions. F. Carra added that based on the individual feedback from the advisors, priority actions have been identified. A road map has been set for the coming months.
2)
F. Carra showed a presentation and underlined that the first step would be freezing a preliminary design and main design parameters.
F. Carra thanks the fruitful interaction required with Technical Infrastructures Pillar (especially, Integration Working Group) and other Working Group of Accelerator pillar (especially FCC-ee booster design, and Technology R&D).
F. Carra gave a recapitulative from the first Senior Advisor Meeting, he cited the 6 priority tasks identified, he emphasized on the main conclusions about technical aspect, in particular, the consensus about the interest about using a girder to support the components of the collider’s short straight section (SSS). About alignment and stability, F. Carra reminded that the specificati8ons are targets used to validate the design of the component’s supports, current specifications seem feasible but challenging.
Short Straight Section (SSS)
F. Carra presented a proposal for the configuration of the arc half-cell, where the dipole length would be constant independently of the type of cell (i.e., with 0, 1 ,2 sextupoles). For the cell where the number of sextupoles is not 2, the short straight section would include a dipole installed on the girder. That would allow to standardize the length of all the main dipoles and the girders. A. Bertarelli commented that it is a relevant strategy of standardisation allowing modularity for the maintenance of the components. F. Bertinelli asked the estimated masses of the components. J. Bauche gave the following numbers: Quadrupoles = 5.5 tons, Sextupoles = 700 kg, Dipole: 70 kg/m., that leads to a total mass of the girder of 11 tons. F. Carra added that this strategy allows the use of a common chamber for all the magnets of the SSS (interest for the standardisation the vacuum chambers, more compact integration without flanges and below, lower impedance). J. Bauche remarked that the common chamber requires the design of splitable magnets, including the dipoles used in the SSS. F. Carra added that indeed, the girder use goes in that direction, as assembling splitable magnets in the tunnel, with tight tolerances, would not be impossible. M. Timmins and A. Bertarelli raised the concerns of the alignment: relative alignment of the components installed on the same girder with the same vacuum chamber and alignment between of dipole installed on the SSS and the main dipole.
F. Zimmermann commented that the configuration has attractive features, even if it is probably an overkilling for the alignment of the dipole. F. Carra agreed that, indeed, at least the “Type A” cell (that with only one quad and not sexts), the girder would not be strictly necessary; however, type A is the version with less repetitions, so cost-wise not using a girder there can help, but not substantially.
F. Zimmermann asked if movers or/and correctors will be used. T. Raubenheimer answered that the strategy for alignment still needs to be investigated.
Girder and Jacks
F. Carra showed a preliminary design of the girder to be used to support the magnet of the arc short straight section. It is made of welded standard I-beam profiles. In the considered configuration it is 6 m long and weighs 3 tonnes. F. Bertinelli asked what the expected vertical deflection of the configuration is. M. Timmins and L. Baudin answered that under static load the calculated deflection is less than 1 mm.
F. Carra listed the advantages of using the girders (preassembly in the laboratory, faster assembly procedure (highlighted by J. Wickstrom) modularity, possibility to use one single chamber for the magnets of the short straight section for a more compact configuration with less bellows, less flanges, lower impedance).
Nevertheless, F. Carra underlined drawbacks of those girder, in particular, they could amplify the ground motion on some frequency due to vibration modes and the additional cost estimated at 50 kCHF/girder, which, considering that ~3000 girders would be needed, gives a significant total. A. Bertarelli asked details about this cost estimation of 50 kCHF/girder, this price seems overestimated to him. M. Timmins explained that this price was accounting for the raw material, welding and the machining needed to need the tight geometrical specifications. A. Bertarelli proposed to investigate the use of other material (aluminium?) or other technics (using commercial extruded profile and precisely assemble the components with a shim system rather than aiming at a high mechanical alignment). M. Benedikt said that cost estimation is expected with a break down into raw material and manufacturing operations and based on the cost of similar components in existing machines. J. Wickstrom suggested to organise a meeting to share the experience gathered at the Swiss Light Source at PSI (Action à M. Timmins and L. Baudin to trigger the action and organisation)
M. Timmins stated the in addition of the girder itself the cost estimation and detailed design study of the jacks should also be performed, so far, HL jacks were used. F. Carra added that knowing that the loads applied on the jacks are lower the design of the jack can be optimised in term of space occupation and cost. The cost of this design optimization is completely negligible, if it then allows savings of several hundred CHF on a component that must be produced in ~10k of units. (Action à M. Timmins and L. Baudin to trigger the action) F. Bertinelli asked about the stroke needed for the FCC-ee jacks. L. Baudin answered that the current stroke considered by BE/GM colleagues is +-20 mm
General configuration of the cross section
To introduce the topic, F. Carra showed pictures from facilities where accelerators are installed in the same vertical plan (Hera) r in the same horizontal plan (SuperKEKB). F. Bertinelli asked if presented facilities are set up in a tunnel or inside an experimental hall. He remarked that in case of installing the facility in a hall the space restriction, and consequent design decision might be strictly different. F. Zimmermann added that constrains for the beam aspects may have been preponderant to explain the decisions made about the design for those facilities.
Vertical configuration
F. Carra presented the update of the component’s and support’s design in the vertical configuration. He explained the interest of an azimuthal shift of the booster compared to the collider in order to systematically install the booster Quad/Sext on the top of a collider’s dipole which is more compact than a quadrupole and, therefore, allows to decrease the height of the booster. F. Carra mentioned the discussions with colleagues working at the integration, beam optics, booster’s design to know what the degrees of freedom are in term of booster installation, to identify any showstopper. From the mechanical aspect, the preferred configuration would be to install the booster on the top of its vertical support (that results in a transversal shift of the booster ring compared to the configuration shown at the FCC week 2022). R. Losito identified accessibility issues for the components placed on the external side of the collider, he recommended to early integrate assembly and maintenance considerations in decisions about the design of the supporting system.
F. Bertinelli observed that the access on both sides of the accelerators should be guarantee for installation and maintenance. He asked how it is foreseen to move from the inner side of the tunnel to the outside. J. Bauche and F. Valchkova showed that the free space between the ground and the collider dipole is approximately 90 cm. F. Carra mentioned that HSE has agreed on evacuation scenario at the ground level.
F. Carra showed the progress of design of the booster support. Compared to the integration model, a more robust design is now being produced, leading to an increase of the resonance frequencies calculated for the support. Work is still on progress.
Horizontal configuration
F. Carra presented the updated version of the cross-section, in horizontal configuration, in particular the DN 550 of the water-cooling circuit. F. Valchkova reminded that that is an internal diameter, without flanges and insolation of the tunnel. In general, F. Carra proposed to regularly re-evaluate the space allocated to the electrical cables and the water cooling pipes. An exercise of updating the requirements from each user has already been triggered with G. Peon. F. Carra hopes that with this, we will be able to decrease the diameter down to ~400 mm. In addition of the heat to evacuate, the users must also state what is the maximal temperature difference accepted between the inlet and the outlet of the cooling system. J. Bauche reminded that 20 degrees are acceptable, but he reminded that hotter tube would lead to more energy dissipated in the tunnel which must be handled by the ventilation system, so not ideal. J. Bauche concluded that the exercise is ongoing on MSC side, also following the updates to the design.
F. Carra presented a horizontal configuration where the support of the water pipe has been suppressed and the water pipe has installed in the trenches. F. Valchkova reminded that connections between that main pipe and the cooled devices must be implemented at every quadrupole (26 m).
J-Ph. Tock observed that in horizontal configuration the access to the collider is easier.
K. Hanke underlined drawbacks of the horizontal configuration. He mentioned that the space in the tunnel is inefficiently used (fully packed in the horizontal plan of the collider, less space for transport, and unused space around. the height of 2 m). Furthermore because of the Synchrotron Radiation no components should be installed in the plan of the collider. It has been commented by Lorraine Bobb from Diamonds Light Source at the FCC-ee BI workshop.
F. Lackner proposed to evaluate the effect of water pipe placed in the tranche as induced-vibration in the ground and on the accelerator’s supporting system. Quantitative evaluation should be done and could be done based on previous studies realised for CLIC.
F. Lackner asked details about the space occupation of the survey system, in each of the configuration. F. Carra reminded that 30 cm space allocation is needed by colleagues from BE/GM of the use of straight alignment systems over a distance of 200 m. Depending on the alignment requirement this system must be installed on both sides of the accelerators in the horizontal plan. Therefore, the requirements given in the CDR require 4 times 30 cm space around the beam line of the collider and the booster. F. Valchkova commented that with such space requirement for alignment system, the current horizontal configuration is not compatible with a size of tunnel of 5.5 m in diameter., or it would be no space for person passing at the same time as the transport.
F. Bertinelli observed that intuitive apparent contradiction in between not having space the install the FCC-ee collider and booster horizontally when the FCC-hh more than 1 m in diameter cryo-magnets and QRL are foreseen to be placed horizontally. F. Valchkova commented that the integration must account for all the services (cables, cooling water) which are more constringent on FCC-ee. F. Carra mentioned private discussions with M. Giovannozzi confirming that the integration of FCC-hh is particularly “aggressive”, and still at a less detailed level of study.
F. Valchkova explained that the ground configuration (i.e., the emplacement of the trenches needed for the QRL) has be modified between FCC-ee and FCC-hh. J-Ph. Tock suggested to investigate if the same configuration could not be reused. M. Benedikt underlined that in the currently considered configuration (each arc alimented by 2 cryo-plants) the size of the QRL could be considerably reduced compared to the QRL presented in the CDR. F. Valchkova will check the integration updates with FFC-hh team.
T. Raubenheimer asked about a configuration including a central corridor as it is done in SLAC. F. Valchkova highlighted several difficulties: access from the alcoves and service caverns to the central corridor and the potential concern of installing one of the accelerators on the top of the fresh air conduct as the layer of cast concrete is lower at that place.
F. Carra concluded that after presented the pros and cons of each configuration, the decision will have to be made by the management considering the scale of the entire acceleration complex (RF, MDI, Civil engineering,…)
Mock-up configuration
F. Carra presented the objectives of the mock-up and proposed a configuration for the scale considered from the mockup. A 1:1 scale is proposed for the cross-section of the arc, whereas, in terms of length, characteristic, shorter lengths, could be adopted (e.g., 1-meter magnet prototypes instead of tens of meters). The mock-up should integrate prototypes of other working group (i.e., magnets and vacuum chambers prototypes). In addition, maquettes of equivalent volume (and possibly masses) should complete that mockup. As a follow-up comment, M. Benedikt suggested the possibility to include in the mock-up the “singularities”, such as doors needed for fire safety for compartmentation of the tunnel every 424 m and alcoves for electronics cables and equipment.
Others
R. Losito asked to set a hard deadline to make the decision about the horizontal or vertical configuration. T. Raubenheimer answered and suggested that a review should be organised based on the report foreseen for January 2023. A. Bertarelli suggested to set-up an overview table quantifying the pros and cons, to evaluate and attribute a relative weight to each of the aspect, to help the decision-making process. F. Carra emphasized that any showstopper be identified at the time.
F. Bertinelli suggested to be ready for requests of minimising the tunnel size.
Conclusion
Being in the feasibility study phase, tools must be developed to bring arguments, pros and cons, help the decision on configuration (horizontal vs vertical, girder or not girder), based on defined timeline and requirements. The appropriate level of details must be found.
A following document will be put together, grouping all recommendations collected after the presentation. Therefore, all advisors and participants are invited to send comments, feedbacks and recommendations to F. Carra and L. Baudin. Further actions will be identified based on notes and comments.
The target is to report the activities related t the project to the FCC management in January 2023.
Actions follow up
- Keep updated table of key requirements and parameters related to the design on the collider and the booster. This table was based on CDR and following presentations given at FCC weeks. Updates must be tracked.
à F. Carra and L. Baudin to regularly crosscheck the tables with the stakeholders. - Set-up an overview table quantifying the pros and cons, to evaluate and attribute a relative weight to each of the aspect, to help the decision-making process.
à F. Carra and L. Baudin - Organise a meeting with J. Wickstrom to discuss the technical solution used in the Swiss Light Source about girder and positioning systems including jacks, and experience of cost estimation
à M. Timmins and L. Baudin to trigger the action and organisation - Design optimisation of the jacks in term of space occupation and cost, based on experience from other machines
à M. Timmins and L. Baudin to trigger the action - Evaluate the vibration induced by water cooling pipe
à L. Baudin to trigger the action and contact G. Peon - Collect comments/feedbacks from advisors, identifying related tasks and actions
à F. Carra and L. Baudin
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