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FCC-FS EPOL group and FCCIS WP2.5 meeting 19

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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
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Zoom Meeting ID
63437787216
Host
Alain Blondel
Alternative host
Jacqueline Keintzel
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List of actions:

  • Start preparing proposal for tests at KARA with present set-up.
  • Could a Compton polarimeter be installed at KARA? At which cost?
  • 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. 
  • Vertical dispersion matching to be followed up.

G. Wilkinson reminds that the mid-term report documents are required by June. In parallel, we must provide a shorter summary for the Mid-Term Review document itself. A first draft should be available within the next month. 

G. Wilkinson summarized that this meeting is dedicated to the polarimeter. It should measure the 3D polarization vector for pilot bunches and colliding ones. It is aimed to perform RDP and FSP measurements. J. Wenninger comments that at LEP the bunches used for polarization had low lifetime (4-10 hours) due to the Compton scattering rate with the laser. A continuous/lengthy measurement of the polarization of the probe bunches could lead to significant intensity loss. D. Gaskell asks if we will have longitudinal colliding bunches to understand how well one can determine it, since the systematics are different for transverse and longitudinal polarization measurements. T. Lefevre comments that if the polarimeter is located in the EIR it needs to be outside the beamstrahlung dump, probably after the second dipole.

Action: Can we benefit from more than one polarimeter, and yes, at which cost?

Action: What are the systematics for RDP and FSP measurements with polarimeters?

Action: What is the best location for one polarimeter?

A. Martens reports a factor 3 increase in the scattering rate if the polarimeter is placed in an EIR, compared to the RF straight section. Furthermore, possible laser systems for pilot and colliding bunches are presented, for which the laser could be combined with two different amplification systems. Synchrotron radiation should be mitigated by crossing horizontally and vertically with the beam. A trade-off between the relative interaction probability and scattering rate is needed. Following a questions by T. Lefevre, A. Martens comments that the main question is the control of the laser polarization which needs to be demonstrated.

Action: How stable is the laser wavelength's central value, laser polarization and spectral width and what is required for RDP and FSP measurements? 

N. Muchnoi reports that the beam lifetime due to scattering is about 10h, considering a laser with 532nm wavelength, 42nm emittance and a few mrad scattering angle. The transverse laser size should be roughly 3 times the transverse electron/positron beam size. About 50 scattering events occur with this set-up. Increasing this rather low number could be done by elliptical laser beams, which could be produced with a vacuum mirror, leading to a beam lifetime of 2h. A. Martens comments that once the bunches are depolarized, they are no longer required and beam lifetime should not be a constraint. Furthermore, a Q-switched laser can be used for single bunches but present technology might not be sufficient for interaction with individual colliding bunches. A. Martens comments that we have a lot of flexibility for pilot measurements, which are presently investigated. T. Lefevre suggests understanding the required stability of the laser, the shot-to-shot stability, the laser power, etc., Also the present Q-switch laser technology might not be necessarily the best option for colliding bunches. Furthermore, T. Lefevre suggests that moving forward to designing the photon detection systems and iterate between these two systems. 

Action: Which systems are applicable for pilot bunches and which for colliding bunches?

Action: How many scattering events do we need for which photon detection efficiency and detector requirements?

I. Koop presents simulations for RDP for KARA at 2.5 GeV, which suggests the spin can be flipped with the used measurement set-up. However, since the jump in Touschek counting rate is proportional to Py2 and not Py , it is suggested to depolarize bunches by optimizing harmonic w-function. He also presents simulations for RDP for EBS at 6 GeV. In that case the spin tune overlaps with numerous side bands, as shown by analyses of FSP simulations. However, larger synchrotron tunes (e.g 0.00543 instead of 0.00345) could help creating distinguishable lines in the frequency spectrum with FSP. Using RDP, however, an even larger synchrotron tune would be required, which is probably not feasible in the storage ring. It is concluded that RDP is almost impossible in a storage ring with a large synchrotron modulation index B = Qy*sigmaE/Qs, and it should be kept below B<1.5. For the FCC at W-mode with the present optics, this means one should aim finding chose a synchrotron tune of about 0.08 to 0.09. 
D. Barber comments that relevant theory has been derived in the 1980s and can e.g. be found here: 
https://www2.kek.jp/library/riyou/sagashikata-e.html
https://www.i-repository.net/il/meta_pub/G0000128Lib
https://lib-extopc.kek.jp/preprints/PDF/1982/8212/8212147.pdf
https://bib-pubdb1.desy.de/record/224384/files/DESY-M-82-26.pdf

 

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