Login

FCC-FS EPOL group and FCCIS WP2.5 meeting 20

→ 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
Useful links
Join via phone
Zoom URL
Hide

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?

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?

S. Nikitin presents that 2 RD kickers (2 pilot bunches scanned independently) are required for RDP and a third one (iron kicker) to eliminate polarization for physics bunches. The kicker line-width should be known and smaller then the width of the spin-line for fine-scanning of the spin resonance. With 2 kickers it is possible to determine direction and drift rate of the spin resonance. I. Koop comments that the main difference between the depolarizer at Z- and W-energy is that for the latter the depolarizer is weaker. The expected natural longitudinal polarization is 0.001 for colliding bunches, controlling it to 10e-5 the kicker harmonic amplitude should be 5x10e-6, creating a free-betatron-oscillation. J. Wenninger comments this should be translated to an orbit oscillation to understand its impact on optics and luminosity. J. Wenninger comments that the depolarizer should be located close to the IR. A. Blondel comments to have closed-orbit bumps for kickers, which needs 2-3 kickers per type. I. Koop suggests placing depolarizers close to the final focus. 

Action: Which minimum beam polarization level is needed (is less than 10% ok) ?

Action: What is the presently foreseen operation mode (collision on and off or continuously on)? 

Action: Where is the best location for a depolarizer (would next to the final focus work)?

I. Koop presents the dependence of the spin response function on the beam energy. At Z-energy the gap between the spin tune and the vertical betatron tune could be chosen as small as +/-0.02. At W-mode it must be as large as +/-0.25. Furthermore, he suggests to place the depolarizer at Z at a location where the spin response function is in the order of 1000 (close to final focus). Remaining at one fixed frequency for the colliding bunches would require a too large kick and thus it is suggested to also sweep the frequencies for colliding bunches. S. Nikitin comments that in here presented simulations the line-width of the depolarizer is assumed too small, leading to large required kicks. A. Blondel comments that iteration between various teams (optics etc.) will be needed and closed bumps could help.

AOB:

B. Härer asks A. Blondel for clarification on spin knobs. These create a spin rotation without disturbing the rest, such as pi or 2pi bumps. A. Blondel asks if kickers would be available to generate such bumps and also a static depolarization source, which should be investigated. 

B. Härer comments that at LEP and FCC polarimeter are about 100m, which could not be installed at KARA. A. Blondel comments that the length would scale with the energy. J. Wenninger comments that scattering angles would be larger and one could aim putting together some numbers for KARA. G. Wilkinson replies that this will be followed up. 

Action: Does the KARA lattice allow generating bumps and localized depolarizing sources?

Action: What is the available space for polarimeter at KARA and could a beam line be used?

There are minutes attached to this event. Show them.
Powered by Indico v3.3.6-pre