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    IT String tests results - W-bellows intra magnet interference issues

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    Present: P. Bestmann, R. De Maria, H. Mainaud Durand, M. Noir, D. Ramos, J. Rosa, V. Rude, M. Sosin (redactor)

    Minutes

    Following the recent motion tests performed on the IT String, an important issue was identified: rotation of a magnet about its longitudinal axis induces significant roll motion of neighbouring magnets due to torsional forces transmitted through the interconnecting W-bellows. This behaviour may limit the FRAS design assumption that up to 1 mrad of roll adjustment can be applied without significantly affecting adjacent magnets. Furthermore, the induced motions introduce a safety concern, as they may lead to loss of contact between the magnets and their support jacks.

    This meeting was therefore convened to discuss the implications of these findings, with particular focus on:

    • the potential impact of these motion limitations on OP operational scenarios;
    • whether the installation procedures should be updated to anticipate and mitigate these effects; and
    • the follow-up actions required from the WP19 and WP3 teams (e.g. additional analyses) before these findings are officially presented at the WGA and TCC meetings.
    • The meeting started with a presentation by M. Noir (see INDICO), introducing the measurement setup and the results of the recent motion tests.
      • During lateral alignment of the magnets, the mechanical coupling between neighbouring magnets is low. The W-bellows are sufficiently compliant in the lateral directions.
      • During roll adjustment, however, the neighbouring magnets follow the rotation of the adjusted magnet. The induced roll of the neighbouring magnets reaches approximately 25–30% of the applied roll, mainly due to the torsional stiffness of the W-bellows.
      • A cross-check of the torque required to achieve the nominal FRAS roll adjustment of 1 mrad showed that approximately 6 t·m of torque would be required (see Y. Leclercq presentation in Alignment WG meeting nr 11; 2018.05.02). Such a high torque could result in loss of contact between neighbouring magnets and their support jacks well before reaching the nominal FRAS roll adjustment limit.
        • For example, loss of contact may already occur at an independent roll offset of approximately 0.31 mrad for a Q2A magnet (19 t).
        • Similarly, for a CP magnet (16 t), the limit is approximately 0.26 mrad.
        • Consequently, the theoretical FRAS adjustment range of ±1 mrad cannot be achieved independently, as the torsional forces transmitted through the W-bellows may unload one of the neighbouring support jacks (S or T).
        • Loss of contact on one support jack (e.g. S) also means that radial adjustment capability may be lost or significantly degraded.
        • The actual limit depends both on the stress state of the W-bellows and on the weight of the connected magnet.
      • These findings reinforce the need to equip every support jack with a load sensor, both to detect loss of contact during operation and to improve understanding of the global mechanical behaviour of the magnet string.
      • The measurements also reveal torsion of the magnet cryostat tube itself, in the order of 0.05–0.1 mrad, depending on the applied roll adjustment.
      • A similar issue has also been observed for the collimator bellows and will be addressed separately.
    • Discussion
      • Delio: The torsional stiffness of the W-bellows is an inherent characteristic of the current interconnection design and is difficult to reduce. Their primary role is to maintain the insulation vacuum and withstand atmospheric pressure while accommodating the specified motion range. Structurally, the bellows can withstand a 1 mrad rotation, but only at relatively high torque, which is transmitted to the neighbouring magnets and explains the observed behaviour.
      • Mateusz: The ±1 mrad FRAS roll adjustment requirement originates from the historical LHC specification for magnet interconnections. However, such large independent roll adjustments have never been experimentally validated on installed LHC triplets.
      • Riccardo: Since redesigning the interconnections is not considered a realistic mechanical solution (as confirmed by Delio), the observed limitation of independent roll adjustment to approximately 0.26–0.31 mrad is not expected to be critical from the operational point of view. The corrector magnets should be capable of compensating for the remaining roll misalignment. The expected correction capability is approximately 0.5 mrad, although this should be confirmed through dedicated simulations.
        • Mateusz: What is the uncertainty in determining the magnetic field roll? Would a correction capability of 0.5 mrad still provide sufficient operational margin? Should the installation procedures therefore be updated?
        • Vivien: Based on the existing calibration data (slide 24), the uncertainty in determining the magnetic field roll is approximately 0.2 mrad. Combined with the expected pre-alignment uncertainty of approximately ±0.1 mrad, the total uncertainty is expected to be around 0.3 mrad. The final roll alignment will rely on the Phase 2 fiducialisation data.
        • Action (Riccardo): Launch more detailed simulations to evaluate the capability of the corrector magnets to compensate for realistic roll offsets.

      • Mateusz, Michel, Vivien: Even if the reduced FRAS roll adjustment range is considered operationally acceptable, performing large roll corrections may still lead to loss of contact on the support jacks of both the adjusted and neighbouring magnets. This further supports the need to install load sensors on all support jacks, which had previously not been considered part of the baseline design.

        Furthermore, it would be beneficial to perform a global mechanical analysis of the complete six-magnet installation, including the W-bellows, cryostat torsional stiffness and support jack kinematics, in order to better understand the overall mechanical behaviour and predict the achievable independent roll adjustments.

        • Delio: Such simulations are feasible but will require time. Before starting, the operational scenarios to be analysed should first be clearly defined, and the expected benefit of the simulations should be assessed.
        • Mateusz: At present, our understanding of the W-bellows-induced behaviour is still limited. Such simulations would therefore be extremely valuable for improving our understanding of the system, preparing future IT String tests and providing quantitative justification for introducing load sensors.
        • Action (Delio, Mateusz): Organise a dedicated discussion to define the simulation scenarios and launch the required mechanical analyses.
    • Next steps
      • Following additional input from Riccardo and Delio, prepare a WGA presentation describing the identified issue together with the proposed mitigation measures.
      • Subsequently, request a TCC presentation to discuss:
        • inclusion of load sensors in the FRAS baseline;
        • the identified mechanical limitations; and
        • the proposed operational and installation mitigations.
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