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MQSX replacement and review of misalignments in IR8
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Videoconference
MQSX replacement and review of misalignments in IR8
- Zoom Meeting ID
- 67502452128
- Host
- Rogelio Tomas Garcia
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02/02/2022 LNO meeting on IR8 misalignment and consequences in case of MQSX failure
Welcome 2022 + News, Rogelio :
The 2022 LHC Performance "Chamonix" workshop took place remotely at CERN last week.
Conclusions will be discussed in tomorrow's close out meeting.
RUN3 commissioning is starting for the injectors. LINAC4 is starting today.
The new MAD-X version 5.08.00 has been released. A first bug fix is planned soon, addressing minor issues related to coupling, exact drift behavior relevant at very low energies, and translation of K0 in MAKETHIN.
Impact of IR8 misalignments, Riccardo, slides
The alignment of IR8 was checked in LS2. The report by the alignment group lists the magnets with more than 0.2 mm offsets from the smoothed reference curve. We were asked to comment if realignment was needed. Riccardo looked at possible consequences for RUN3. He finds that the apertures remain sufficient both at injection and for physics. For this, he applied the aperture criteria (emittance and tolerances) as currently used for HL-LHC studies. Chiara Bracco from SY-ABT joined this meeting. She looked into possible consequences on injection failures and finds that the protection in case of injection failures by the TDI remains sufficient.
It was concluded that the alignment team should be encouraged to realign the magnets, where this can easily be done to avoid accumulation of drifts during the run. If the MBX with banana shape cannot be realigned we should request to monitor it and to think of future techniques to realign it.
LHCb proposes a second major luminosity upgrade for RUN5 requiring major changes in IR8. MBX will have to be shielded resulting in a reduction of aperture by 5mm (radius). There will also be a request to move Q5 by 1.5 m in LS4. Details remain to be studied and will be followed by HL-LHC WP2.
Replacement of MQSX, Tobias, slides
With the extension of RUN3, the risk increases that magnets in the high luminosity regions IR1,5 suffer radiation damage and fail. The expected radiation doses were evaluated by the FLUKA team and presented in the Chamonix meeting by F. Cerutti, predicting that several of the MQSX magnets could reach what has been assumed as damage limit. The MQSX magnets are used for coupling correction. The MQSX strength required for the coupling correction were re-measured in the 2021 beam test and will be measured again in the re-commissioning this year.
Tobias finds that the coupling introduced by the loss of a single MQSX can be corrected globally but would leave a local distortion at the IR were the failure occurred, resulting in a loss of luminosity at that IR. Other possible consequences on beam-beam and witness bunch stability remain to be evaluated by Xavier and colleagues from the CEI section. The reduction in luminosity becomes significant at the lowest beta* values planned for operation in RUN3. A small squeeze only in one IR could overcome the luminosity loss or imbalance. However loosing 2 MQSX in the same IR has no other (hardware-free) solution than increasing beta* (1m?).
Several mitigation methods have been proposed : orbit offsets in sextupoles (skew-quad by feed-down), construction + installation of warm skew quadrupoles, or a small (of order 1 mrad) tilt of the Q2 and or Q3 magnets. Tobias shows first ideas on related MD studies. These studies should be started in commissioning making luminosity scans versus MQSX strength imbalance.
He finds that the maximum strength of the MCSSX, MCSX sextupoles is not sufficient to compensate for the loss of a MQSX magnet.
Further follow up is planned. One of the next steps should be to provide Mikko Karppinen with specs of the warm quadrupole next to D1 so they could evaluate cost of the magnet and chances to install it without breaking vacuum. Riccardo also commented that 2-in-1 skew quads could be placed next to Q4, however it does look a bit more complex.
Rogelio mentioned that Stephane initially proposed a telescopic squeeze to enhance the effect of the arc MQS, however first considerations made him think that with the current powering scheme of the MQS they cannot replace a failing MQSX in both beams.
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