Choose timezone
Your profile timezone:
Triplet misalignments in Run 2
→
Europe/Zurich
6/2-008 (CERN)
Frigyes presented the local dispersion measurement at the TOTEM published in this note:
http://cds.cern.ch/record/2256433/files/CERN-TOTEM-NOTE-2017-002.pdf
The 2016 measurement already showed an asymmetry between the left and right measurements, -9.7cm and -6.7cm, respectively.
The MADX left and right values are symmetric and are about the average of the measured ones.
These same features are confirmed in later years and for various crossing angles.
Initially one could attribute this to a wrong crossing angle. One would need:
>>>> Beam 1 225(+-10) murad
>>>> Beam 2 110(+-20) murad
But the required error would not be easily compatible with other observations.
Alternatively the same effect could be explained by an orbit error originating in Q1 which results in 3 mm orbit deviation in Q3 that can change the dispersion by 1 cm as presented by Riccardo here:
https://indico.cern.ch/event/516795/contributions/1196631/attachments/1253585/1850572/CT-PPS_bump_proposal_20160406.pdf
(slide 5 case "aggressive bump" proposed at the time, but discarded being too pushed )
Frigyes performed a model fitting study were varying crossing angle below 10 % and misaligning MQX quads by 100 – 400 μm
can reproduce measured local dispersion. The values were considered realistic by everyone.
Yet the 'perfect' asymmetry left-to-right on the measured local dispersion and its systematic origin remains suspicious and deserves
deeper understanding. For this two measurements have been proposed:
-Experiment (@ low beta*, top energy) with zero crossing angle to remove the possible bias from crossing angle. Number of bunches could be above 60.
TOTEM should think about how long it would be needed for data collection. Further discussions will be needed.
-Experiment with triplets switched off (ballistic optics, top energy). Number of bunches to be discussed. Interesting settings could be
without TCLs. The optics used in 2017 had triplets plus Q4 switched off. The madx for IR5 can be found here:
/afs/cern.ch/eng/lhc/optics/runII/2017/IR5/ir5_align.madx
If further instructions are needed for TOTEM to look at this, please let us know. We should have further discussions.
Ewen presented measurements of beam position wrt the center of sextupoles in the IR5. Large offsets 2-3mm are observed incompatible with Riccardo's
crossing angle deviation. For now we should attribute to the misalignment of the IR sextupole but further studies will be required as this is very important
for the machine performance.
There are minutes attached to this event.
Show them.