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Wrapping up LHC optics commissioning and MDs
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Wrapping up LHC optics commissioning and MDs
Wednesday 29/6/2022. Wrapping up LHC optics commissioning and MDs
3Qy compensation at LHC injection, Ewen, slides
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Ewen reminds that the LHC does not have correctors specifically designed for 3Qy correction. He reports from MDs performed last Saturday. In the MDs, MCSSX correctors designed for local IR corrections at top energy, were used to corrected 3Qy at the LHC injection energy of 450 GeV. The strength of the 3Qy resonance driving term was monitored by observing the height of the -2Qy peak in tune spectra recorded with AC-dipole excitation. The f0030 RDT (resonance driving term) is 2-3 times higher in LHCB1 compared to B2 and 4-5 times stronger than predicted by basic LHC models. The MD showed that it is possible to reduce this driving term with MCSSX correctors and to improve the beam lifetime close to the resonance at injection (and reduce emittance growth) without major unwanted side effects. The corrections were first tested for a flat machine and then repeated with x-ing. The MCSSX act on both beams. With the knobs tested in the MD (using the MCSSX correctors right of IP1,5,8) the B2 compensation reduced B1 by 20%. Stephane commented the beta-functions of B1,B2 are different left and right of the IP and that it should be possible to improve both beams by using left and right MCSSX correctors. Ewen agrees the knobs can certainly be optimized but also reminds that 2 of the 8 MCSSX correctors are broken and cannot be used.
In the following discussion there was a consensus that the observations reported here are very promising to improve injection and early ramp and should be followed up by further studies. A proposal should be made to MPP (by Kostas) to use these techniques in regular operation like scrubbing at injection energy. It would also be important to understand the source of 3Qy (alignment, Landau octupoles, local coupling).
Both Tobias and Ewen are scheduled for 10 minute presentations in the LMC next week and discussed in the next two presentations their plans.
Linear Optics, Tobias
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There has been much work on linear optics corrections, reducing significantly beta-beat and coupling for the operational optics and correcting the waist at the interaction points to provide equal luminosities to IP1,5, that Tobias will summarize in the planned LMC presentation.
Non-linear Optics, Ewen
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Ewen discussed what he would like to cover in his planned LMC presentation, see also the last slides of his first presentation in the meeting. He plans to summarize the nonlinear optics measurement and correction activities in the commissioning and in particular what has been new in RUN3 like first decapole RDT measurement and higher order chromaticity Q'4, Q'5 measurements. He also plans to say that these efforts serve as a dry run for HL-LHC and mention where further efforts are needed.
Ewen also shows that raw BBQ data recorded in time window just before + during AC-dipole excitaton can be used to obtain cleaner tune spectra than available from regular BPMs.
MD on Amplitude detuning and b6, Joschua
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Joschua reported from a recent MD measuring tune shifts with amplitude. The measurements were performed for the flat machine as well as with crossing angles at beta* = 30 cm and new corrections calculated.
The observed tune slopes are mostly similar to what has been observed earlier in commissioning, and changes in settings applied in the meantime like waist corrections appear to not affect much the results.
More measurements were performed with crossing bumps only in IP5 for both signs and then again with the full crossing scheme now the new corrections applied. Analysis of these kicks will be done offline.
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