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Preparing LHC commissioning
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6/R-012 - conference room (CERN)
Preparing LHC commissioning
News as reported directly on the Indico entry.
Stephane Fartoukh, 2024 configuration
Change of IR1 polarity to mitigate radiation dose to the IR1 triplet and D1, without rotating the crossing plane (as this required going to flat optics, which was perceived as a small risk for 2024, along for a deterioration of forward physics performance). This affects 3 other cycles: VdM, ion and pp reference run.
IR1 Q4s are switched off and the de-Gauss cycle is in place. Q2 has slightly larger powering with this new optics.
At injection peak beta function in IR1 is increased.
At collision H dispersion is reduced at XRP and TCL6 (but still larger than in IR5). Background to FASER might need mitigation by reducing the TCL6 gap (tbc). Collimation is exploring this. Stefano comments that there is hope to stay with the larger gap. Stephane confirms that the baseline plan is to stay with the larger gap.
At 30cm the IR aperture is deteriorated by ~0.5sigma in the vertical plane. A possible cure is to keep TCT constant gap in mm from beta*=32.5cm.
DA with new optics is the same as for 2023.
Crossing angle for 2024 is set constant to 160 urad.
Optics down to 20cm are available. There is an ongoing discussion on how to exploit this for performance. Jorg has another talk on the commissioning in this week's LMC.
Ion beta* in 2024 will be the same as for 2023.
Vdm optics are ready. Peak beta-function increases compared to 2023 VdM.
Q4 was used in the past to correct the linear perturbation of the wire. A possible cure is to introduce the wire linear optics correction with a new matched point. This is under discussion.
Triplet trims have changed polarity and LSA is aware of this. This will require dedicated polarity check at injection.
Going below beta=25-26cm requires a change of crossing angle, which also affects wire operation. Stefano suggests to avoid a too complex commissioning. Stephane recalls that the plan would be to level at a reduced lumi to avoid degradation of the lifetime. Rogelio recalls the possibility to squeeze only H beta but Stephane sees no time to develop this optics before commissioning.
Tobias Persson, Optics commissioning
Plan of the optics commissioning is presented. Ideal plan is to remove Q4 corrections with hopefully aiming similar energies in both beams, which might require a bit extra time for IR5. Global corrections go as usual. Sextupole and higher order corrections in IR1 need to be redone.
Additional measurements and corrections include: spool pieces set-up, arc sextupole correction, MO response, IR4 K-mod.
Q6 in IR1 has no longer low values so it could be used for global corrections.
Some software issues are reported and are being followed up.
Good chances to start commissioning already 9th of March. Likely VdM and ions to be measured over Easter.
Ideally we would squeeze without local corrections in IR1. Stephane comments that one could be more pragmatic including some guessed corrections at least for the squeeze.
Ewen on behalf of Sasha Horney, Sextupole correction at LHC injection.
Using MAD-NG, thanks to Laurent, the optimization of sextupolar strenghts at injection is explored. The RDT f1020 is routinely visible in all linear optics measurements in the LHC. A change of f1020 during the ramp is also seen. This RDT is mainly driven by arc MS sextupoles.
RDT matching is at single location and monitoring impact around the ring. In first correction f1020 and 3Qx are well corrected but other RDTs increase, in particular integer resonance Qx. In a second correction the strength change is minimized but features similar performance in terms of RDTs. A third correction succeeds to improve all RDTs but with large changes in MS strength. Further attemipts are tried.
Stephane suggests to keep family 1 and family 2 of the arc sextupoles to avoid introducing chromatic aberrations.
Federica Murgia, SPS survey
There are significant discrepancies between different survey databases (GEODE, LDB). As a starting point, within the E2A project, Federica takes the SPS case. Difference between GEODE and LDB goes up to 4mm. In GEODE and at SPS installion time only the cord length is used so apparently some mistakes were made and the current fix in MADX would be to increase the lengths of the drifts between dipoles. Optimizing these parameters a very good modeling is achieved.
In 2003 energy calibrations study revealed a 60mm too long SPS. The current fix reduces this by 14mm. So fix is in the right direction!
The open question is whether to change the machine modeling or realigning the SPS.
A beam-based quadrupole center measurement could be used to see if it brings some understanding, as the 2003 study was using the sextupoles.
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