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FCC-FS EPOL group and FCCIS WP2.5 meeting 18 - Joint with FCC-ee tuning meeting
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Europe/Zurich
Description
The FCC technical and financial feasibility study comprises a work package (EPOL) on precision determination of the centre of mass energy at FCCee. using resonant depolarisation of the beams, in conjunction with precise measurement of the energy spread and other parameters using physics events in the detectors, and other beam diagnostics in particular to control the collision parameters. Specific equipment involves polarimeters for both beams, polarisation wigglers, and depolarising RF kickers. The possible mono-chromatization of the beams in view of a measurement of the e+ e- —> H (125) process will also be studied and special requirements investigated.
Short group meetings are foreseen at 16:30 on Thursday typically every two weeks.
FCC-FS EPOL group meeting
Zoom Meeting ID
63437787216
Host
Alain Blondel
Alternative host
Jacqueline Keintzel
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Zoom URL
A. Blondel presents thoughts on EPOL experiments at KARA. with the present set-up one could investigate implementation of strong static depolarizing bumps and the study of modifications in the relationship between resonant depolarization frequency and energy in presence spin resonances in the lattice.
Since the measurements are based on measuring the change of Touschek lifetime no spin orientation is measured and thus spin precession can not be measured.
Action: Start preparing proposal for tests with present set-up.
Action: Could a Compton polarimeter be installed at KARA? At which cost?
J. Keintzel presents first studies for BBA determining the transverse offset of an arc quadrupole. K. Oide suggest to use all available BPMs. L. van Riesen-Haupt comments that one could include the systematic error between the center of the quadrupole and the nearest BPM.
A. Blondel presents Opposite Sign Vertical Dispersion (OSVD) and beam-beam (BB)-offset measurements. Using the BPMs closest to the IP with a 1 µm precision one can measure the beam-beam offset for colliding bunches with a precision of roughly 0.4 nm. OSVD is proposed being measured applying a change of the RF-frequency and detecting the orbit change. During discussions between A. Blondel, K. Oide, T. Pieloni, T. Raubenheimer, D. Shatilov, R. Tomas and F. Zimmermann it is concluded that this shift must be very small to not introduce flip-flop effect, and cross-talk between the four IPs could spoil these measurements, which requires realistic and extensive simulations.
Action: A broad study is required to understand the cross-talk between IPs with different collision offsets and OSVDs.
Action: What is an acceptable value for a RF-frequency shift (dp/p ~ 1e-4?) ?
K. Oide present dispersion measurements at the IP. It is proposed to use horizontal kickers to change the path length of pilot bunches only to measure the dispersion without changing the RF-frequency. The kickers should introduce closed orbit bumps, changing the path length and consequently the energy of the pilot bunches, which would allow to measure the IP Dy of the pilot bunches via BPM readings. These kickers could be installed in one of the long straight sections and raise their excitation amplitude adiabatically without affecting the colliding bunches.
Action: Define parameters (strength, rise time, etc.) and suitable locations for such kickers.
Y. Wu presents updates on harmonic spin matching (HSM). Using 4 bumps (each requiring 3 vertical orbit kickers) it is shown that a maximum polarization level above 90% could be achieved, despite an rms vertical orbit of 70 µm. First studies of deterministic HSM show that polarization can roughly doubled to 20%. D. Barber comments that scanning the 4 polarization bumps would not seem practical and
a deterministic approach would be preferred. A. Blondel suggests exploring 2pi bumps using 5 orbit kickers. D. Barber and A. Blondel raise the question whether horizontal or vertical dispersion has stronger depolarizing effects.
Action: Vertical dispersion matching to be followed up.
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