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6th FiDeL run III meeting
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Europe/Zurich
Date: 2020-10-16
Place: everywhere
Presents:
E. Todesco, W. Kozanecki, M. Hostettler, L. Fiscarelli (all present with remote connection)
This new series of meetings is triggered by W. Kozanecki, chairman of the LHC luminosity calibration and monitoring working group. During the Van der Meer scans, whose aim is to have a measurement of luminosity, the precision of the scan is limited by a residual effect that could be due to hysteresis or non reproducibility of magnets. This affects the measurement to the level of 0.1%. Several publications of HEP results are on hold waiting for a refinement of this measurement.
The magnets driving the Van der Meer scan are the dipole correctors in Q4 and Q5, i.e. MCBC and MCBY. It is agreed to have a zero order model of the hysteresis in these magnets to verify if this could be the origin of this issue.
A zero order model for non linear component of transfer function of MCBC and MCBY [E. Todesco]
Ezio presents a model based on the fit of magnetic measurements during the production (00's) of the transfer function around zero current for cycles of current going up to the maximum amplitude. This provides an upper bound to any effect that can be due to these magnets, and a very pessimistic estimate for the VdM scans, where the magnets are used with cycles of much lower amplitude than nominal current.
The hysteresis is due to two different sources: iron magnetization, giving a non zero field independent of the current around zero A, and the superconductor hysteresis, giving a larger contribution around zero field and decreasing with larger fields. The model is presented and will be used by Michi to have a zero order estimate of these effects on the VdM scans.
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