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FCC-FS EPOL group and FCCIS WP2.5 meeting 21
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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.
Videoconference
FCC-FS EPOL group meeting
- Zoom Meeting ID
- 63437787216
- Host
- Alain Blondel
- Alternative host
- Jacqueline Keintzel
- Useful links
- Join via phone
- Zoom URL
-
List of actions:
Experiments:
- Start preparing proposal for tests at KARA with present set-up.
- Could a Compton polarimeter be installed at KARA? At which cost?
- Does the KARA lattice allow generating bumps and localized depolarizing sources?
- What is the available space for polarimeter at KARA and could a beam line be used?
Polarization:
- Vertical dispersion matching to be followed up.
Effects on ECM:
- A broad study is required to understand the cross-talk between IPs with different collision offsets and OSVDs.
- What is an acceptable value for a RF-frequency shift (dp/p ~ 1e-4?) for OSVD for colliding bunches?
- Define parameters (strength, rise time, etc.) and suitable locations for kickers measuring OSVD for pilot bunches.
Polarimeter:
- Can we benefit from more than one polarimeter, and if yes, at which cost?
- What are the systematics for RDP and FSP measurements?
- What is the best location for one polarimeter?
- How stable is the laser wavelength's central value, laser polarization and spectral width and what is required for RDP and FSP measurements?
- Which laser systems are applicable for pilot bunches and which for colliding bunches?
- How many scattering events do we need for which photon detection efficiency and polarimeter detector requirements?
Depolarizer:
- What are the systematics for RDP and FSP measurements?
- Which minimum beam polarization level is needed (is less than 10% ok) ?
- What is the presently foreseen operation mode (collision on and off or continuously on)?
- Where is the best location for a depolarizer (would next to the final focus work)?
Y. Wu presents on updates on spin orbit bumps and polarization progress. In previously shown simulations the simplified DESY formalism has been used to minimize Fourier coefficients 0 and 1 with 4 orbit bumps. It is shown that by applying this technique with 8 random orbit bumps the 90% maximum polarization is achievable with a vertical closed orbit of 70 µm. J. Wenninger suggests to check generated vertical dispersion since it would lead to emittance growth. G. Wilkinson points out that simulations could also be performed at W-energy. Following a question by B. Härer, D. Barber explains that the main goal of these studies is aligning the n0 vector vertically, to avoid depolarization from the horizontal plane.
Action: Evaluate generated vertical dispersion and possible emittance growth.
Action: What is the situation for W-energy?
Action: What is the impact of BPM errors?
B. Härer presents a detailed layout of KARA with orbit kickers and BPMs. J. Wenninger asks how the spin knobs could be used to measure the polarization. Concerning measurements of the change of Touschek rate to determine the resonant frequency, the biggest errors stem from the count rate. A.-S. Müller had mentioned that a systematic study might even allow to measure the polarization level and a Master thesis project is defined. In principle, a polarimeter could be installed, either in a straight section, after removing a super conducting undulator, or in a dismantled beamline. I. Koop suggests that the counting rate could be normalized to the particle number and possibly also the vertical beam size. Furthermore, he suggests that it could be interesting to approach the integer and then using orbit bumps to increase the level of polarization again. G. Wilkinson comments that establishing required parameters for a possible polarimeter should be investigated. B. Härer agrees and adds that one should also consider financal implications.
Action: Define polarimeter parameters.
I. Koop presents updates on depolarizing strategies for colliding bunches. Local vertical orbit bumps (pi-bumps) are generated in the arcs via RF-kickers. While at W-energy,a large spin rotation is achievable, it is rather low at Z-energy. I. Koop proposes changing the optics for about 800 m in one arc section by increasing the optics functions, leading to a larger w-function at Z-mode. J. Wenninger comments that this drastic change would impact the chromaticity correction, dynamic aperture, impact on errors, emittance, etc. and should be studied carefully.
Action: What would be the impact on the optics, DA, emittance with this optics change?
Z. Zhang presents that the principle of introducing horizontal dispersion is implemented by inserting quadrupoles in-between sliced dipoles before the final focus. Following a question by D. Shatilov, A. Faus-Golfe comments that the crossing angle is included. The impact of beamstrahlung needs to be studied, in presence of large vertical dispersion, which could lead to beam-blowup and might be too large.
Action: What is the impact of beamstrahlung on monochromatization schemes?
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