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FCC-ee Arc Half-Cell Senior advisor Panel Meeting #1
Notes of FCC-ee Arc Half-Cell Senior advisor Panel Meeting
30th September 2022
List of people attending the meeting:
F. Carra (chairman), K. Artoos, L. Baudin, M. Benedikt, A. Bertarelli, F. Bertinelli, C. Eriksson, P. Fessia, H. Mainaud Durand, F. Lackner, P. Raimondi, T. Raubenheimer, C. Tetrault, 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/1204097/)
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)
Tor showed slides presenting the 3 phases of the Project which can be recapped as it follows: Phase1: 3D model, Phase 2: Engineering Design, Phase 3: Construction Of the Half Arc Cell of FCC-ee... The goal of the project is to optimize the design a single cell “sufficiently detailed state”, the integration should also include the booster.
The components (and their characteristics) of each arc cell should be defined (presence of quadrupole, sextuple, length of the dipole…)
Strong 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)
2)
F. Carra showed a presentation and underlined that the first step would be freezing a preliminary design and main design parameters.
In the collider, recent discussion exhibited interference and incompatibility of the current design of magnet and vacuum systems of the dipoles themselves, and the dipole interconnection. The design of the absorber (and the associated water-cooling circuit) is still to be detailed. However, in terms of required space, the associated volume envelop would require enlarging the separation distance between e- and e+ lines. F. Carra explained that the firstly considered beam to beam transversal distance (300 mm) has been increased to 350 mm because of interferences between vacuum and magnet systems, in particular, the margin between the vacuum chamber insolation layer and the Synchrotron Radiation absorbers on one side, and the busbars and magnet yoke on the other side has been increased. From beam optics aspects this modification has been accepted by K. Oide.
The design of booster components is not yet done, and in general the state of the associated functional specifications is very preliminary. F. Carra stressed that many assumptions are being taken on this aspect.
J-Ph. Tock asked about the scale considered from the mockup. F. Carra said that it would be important to study in a 1:1 scale at least 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). Anyhow, the ongoing exercise (Phase I), until end of the year, focuses on the determination arc configuration, and a more detail view on what to build could be refined towards (or after) the end of the exercise, also as a function of the available budget.
P. Fessia asked how the field “lost” in the interconnect will be recovered? Increasing the current? C. Eriksson mentioned that overlapping plates are planned to be installed.
F. Bertinelli asked details about operation of the booster. The booster has only one chamber, so the booster is operated in cycles between 2 phases: acceleration and injection of electrons and then positrons.
General configuration of the cross section
P. Fessia asked about the width of the trays, these supports have to be dimensioned accounting the weight of the pipes they are supported, for instance the DN 600 of the water-cooling circuit. F. Valchkova said it is size of the trays is based on the estimated power consumption, it has not been further detailed. In general, F. Carra proposed to re-evaluate the space allocated to the electrical cables and the water cooling pipes. A survey would be launch on the margin taken by each of the user (action F. Carra/L. Baudin to contact G. Peon). P. Fessia observed that the support of the water pipe could be suppressed if the water pipe is installed in the tranchee. F. Valchkova agreed but remind that some connections between that main pipe and the cooled devices must be implemented.
M. Benedikt asked details about the space occupation of the survey system. H. Mainaud Durand justified the 30 cm space allocation because 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 3 times 30 cm space around the beam line of the collider and the booster. If the requirements are relaxed only 1 straight wire might be enough. It is worth noting that the development of laser-structured based alignment method will not change these constrains of space as both wire and laser systems need straight path between the components of the circular collider.
As a matter of comparison, F. Bertinelli reminded that LHC tunnel diameter is 3.8 m. F. Bertinelli asked about the space requirement for FCC-hh, as the tunnel should also be able to accommodate the hadron collider. T. Raubenheimer, M. Benedikt, F. Zimmermann, and F. Carra confirmed that the space should not be an issue for the installation of the FCC-hh. (as offline follow-up F. Carra has crosschecked this with M. Giovannozzi, who confirmed that point). Indeed, FCC-hh does not have a booster and the diameter of the tunnel (5.5 m) has been chosen based on preliminary study done for FCC-hh. Compared to the FCC-hh configuration presented in the CDR, the size of the QRL could be divided by 2 (or 3) as the need of cryogenic power has significantly decreased.
In horizontal configuration
M. Benedikt asked a justification for the 110 cm between the collider and booster in horizontal configuration. F. Valchkova said there are further technical constrains. (action F. Valchkova, l. Baudin and f. Carra to crosscheck these constrains)
T. Raubenheimer asked about the possibility of move the accelerator a side to have a central corridor. J. Wickstrom proposed the idea of a horizontal configuration with a large corridor in the center of the cross section between the two beam lines. (action F. Valchkova to study this configuration) That raises questions about how to access that corridor knowing that the shafts are in the inside of the ring.
A. Bertarelli observed that in the horizontal configuration the radiation dose received by the booster is higher. T. Raubenheimer detailed that dose is due to both reflection and scattering although we don’t know the specification there is a factor of two between the dose on the horizontal plan, outside and on the top of the collider. The dose is maximum on the horizontal plan, inside the collider (1000 kGy).
In vertical configuration
P. Fessia observed that if due to the stability requirement the support of the booster is massive, the space that was gained with installing the booster on the top of the collider will be lost. K. Artoos added that the vibration modes of the booster could be amplified by the support level arm and transmitted to the collider ring via the concrete slab. F. Carra confirmed that these topics need to be addressed in the preliminary study.
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. F. Valchkova showed that the free space between the ground and the collider dipole is approximately 90 cm. HSE agreed on evacuation scenario at the ground level.
F. Carra showed the initial design of the booster support, which keeps into account integration constraints but was not yet optimized from the mechanical point of view. The next step is to design a more optimized system. F. Lackner and J-Ph Tock observed that there is a room for improvement and agreed that some more realistic design should be considered for the future calculations.
Alignment
H. Mainaud Durand asked about the temperature variability between installation and operation. T. Raubenheimer answered that the aim is a fraction of degree. F. Carra observed that this concern is being managed by colleagues from EN-CV. They try to optimise the heat extraction while avoiding the turbulent air flow. K. Artoos observed that the temperature variation has only low frequency effects that might be corrected by beam feedback. It has been stated that the bake-up phase (where vacuum chambers are heated up to 200 degrees for NEG activation) mentioned by M. Timmins, should not have consequence on the alignment as it is a temporary situation between the installation and the operation.
F. Carra presented the Beam Based Alignment strategies proposed by T. Raubenheimer, these new concepts have not been discussed in the project team. Alignment requirements are tight with a dynamic aperture of 10-20 µm.
One of the proposed strategies relies on the preliminary alignment of the quadrupoles (from magnetic center to magnetic center) in the limit of 200 µm over 200 m. H. Mainaud Durand observed that it is already quite a challenging objective, it is done in synchrotrons over approximately 100 girders scaling it to the entire FCC this preliminary operation would take several years! According to T. Raubenheimer it could be achieved using structured laser beam.
Another proposition by T. Raubenheimer, is to rely on trims inserted inside the magnets without using movers. It could accommodate mechanical misalignments in the range of 300 µm. An alignment team would intervene in the tunnel to correct any further misalignment due to ground motion.
K. Artoos observed that without girder, the positioning of the sextuples would require BPM installed specifically on the sextuples. T. Raubenheimer admitted that it would be challenging to do without the BPM, that depends on the resolution.
A. Bertarelli asked how many degrees of freedom would be considered per girder. K. Artoos replied that 5 DOF are needed (2 transversal translations, 2 vertical translations, and the roll). H. Mainaud Durand observed that for each of those DOFS, alignment sensors are needed, even if the beam could be used as mentioned by T. Raubenheimer and F. Zimmermann, for safety reasons (two redundant ways to read the position are needed)
Stability
F. Carra underlined the need of a clearly defined specification in term of vibration requirement, if possible, a value of integrated RMS or a PSD of the magnet center displacement, as well as a clear statement on the lower border of frequency to considered (K. Oide showed that the orbit feedback is needed until 10 Hz, is that realistic?)
T. Raubenheimer answered that the beam feedback should be possible until 10 Hz. 1/10 of the beam emittance (14 nm) should be the order of magnitude of the allowable as maximum displacement of the magnet center of random motion, in the arcs. F. Zimmermann completed that final quadrupoles around the IR will be treated separately in terms of static and dynamic requirements.
Girder
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 3 m long and weighs 3 tonnes.
Preliminary studies of the transfer function between the ground motion and the support of the magnets on the girder have been done by L. Baudin. It shows amplification of the ground motion for some frequencies (especially between 30 and 40 Hz).
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 ~2000 girders would be needed, gives a significant total.
K. Artoos cited the example of the Swiss Light Source where cast material is used for the girder. Considering the large number of girders to be produced that could be considered as a way to reduce the production cost. J. Wickstrom tempered that the cost of such cast girder is mostly driven by the material itself.
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.
M. Benedikt informed that a proposal for the FCC-ee half arc cell mock-up must be put together and presented at the FCCIS in early December.
Actions follow up
- Define an updated table of key requirement and parameters related to the design on the collider and the booster. This table should be based on CDR and updated following presentations given at FCC week.
à F. Carra and L. Baudin to trigger the action, and to crosscheck the tables with the stakeholders. - Re-evaluate the space allocated to the electrical cables and the water cooling pipes. A survey should be launch on the margin taken by each of the user
à F. Carra and L. Baudin to trigger the action, and to crosscheck the tables with the stakeholders. - Check technical constrains for the 110 cm distance between the collider and booster in horizontal configuration.
à F. Valchkova, L. Baudin and F. Carra to crosscheck these constrains) - Collect comments/feedbacks from advisors, identifying related tasks and actions
à F. Carra and L. Baudin
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