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Discussion on LCF beam parameters
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Europe/Zurich
6/2-008 (CERN)
Discussion on LCF beam parameters
Present: E. Adli, A. Faus-Golfe, Y. Papaphillippou, A. Robson, J. List, B. List, K. Yokoya, T. Okugi, L. Kennedy (partially)
Topic: Spin transport and polarisation preservation in LCF
Baseline polarisation:
ILC:
e-: 80% at IP
e+: 30-60% at IP
CLIC:
e-: 80% at IP
(option to study/adopt ILC-scheme for e+ polarisation)
Location of spin rotators:
Longitudinal polarisation at sources
Polarisation should be fully longitudinal at IP (else: loss in Pz => less analysing power), for specific applications also transverse polarisation
Fast helicity flip important to harvest all the benefits of polarisation, in particular for calculation of systematic uncertainties
Spin Rotator to transverse polarisation in DR and RTML, to prevent depolarisation
At the end of RTML spin rotator back to longitudinal for ML, and IP
Alignement spin rotators, main linac, x-angle
A net bending angle will always provide a net spin precession. Last spin rotation to longitudinal direction should be where beam line is parallel, and in same direction, as main linac.
(If last spin rotation needs to be at a location where the beam still has a net angle to direction at the IP, the spin rotators needs to be tuned such that spin precession on the way to IP results in fully longitudinal polarisation.)
If beams at the IP(s) are parallel to main linacs, there will be no net bending angle and no precession. This is - by design - the case for ILC, 1IP.
For CLIC 1IP there may remain some work to be done, as the 1IP case is currently not parallel to the main linac (about 1 mrad off, double check with RT).
Challenge 2IP:
The 2 IPs are foreseen with different x-angles (current baseline ~15 mrad, and ~25 mrad), and necessarily at least on of them needs to have a net angle to the ML.
Therefore, if long. polarisation is to be preserved for both IPs, need to adjust spin rotation into the ML, depending on which IP is used.
Goal: being able to serve the two IPs alternating pulse by pulse.
Another possibility for 2IP: one zero angle with respect to ML, with polarisation preserved. The other, larger angle, will not have polarisation fully preserved.
Kaoru:
Spin will not be parallel both in the linac and the IP. However, in the linac, transverse components may be OK
Related : the discussion on longitudinal separation of IPs and related timing issues. Benno:
Would an idea be to build two separate turn arounds, preparing the correct spin rotation for each of the 2 IPs? Could address the spin, and the timing as well. Would allow for fast switching. How fast is needed? => Aim for pulse-by-pulse (10 Hz), we’re assuming pulse-by-pulse kicking for the positron pre-DR spin rotators, why should it be issue here?
Diagnostics:
ILC has upstream (i.e.in the BDS) and downstream (i.e. in the post collision line) beam characterisation, including Compton polarimeters and energy spectrometers, in both locations. The downstream instrumentation requires beam optics to provide a secondary focus of the sept beam. Studies for CLIC post-collision line instrumentation were performed in the past, but not very successful due to the large beam disruption at CLIC.
Decisions/work to be done:
- quantify the spin rotator requirements (the spin rotation angles and axes) in order to ensure fully longitudinal polarisation at a 15mrad and a 25mrad IP for different main linac x-ing angles, e.g. 0mrad, 15mrad, 20 mrad (as for CLIC)
- verify that ILC spin rotator design can match these, or provide conceptual design for a modified spin rotation section
- analogously for timing: what would be the timing difference for the longitudinal IP separation as studied for CLIC?
- come up with a conceptual design for timing adjustment lines, possibly combined with spin rotation section
- compare cost for time adjustment lines to cost for larger IP hall (to allow detectors sitting side-by-side without longitudinal separation)
- evaluate if any showstoppers for pulse-by-pulse (i.e. 10 Hz) kicking between two beam lines (needed both for the positron pre-DR spin rotators and the pre-ML spin rotator / timing adjustment beam lines)
- which requirements arise from the downstream instrumentation (limit disruption, secondary focus…) => can this be accommodated with CLIC-like BDS?
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