Sullivan presented talk on IR design for FCC-ee tt machine.
SR was traced from last soft bend magnet and through the final focus quads.
The beam envelope was traced out to 15 sigma x and 50 sigma y with
a beam tail distribution as given nin the presentation.
A central beam pipe with a 10 mm radius intercepts significant SR photons
from the soft bend as well as the final focus magnets. The radiation from the
FF magnets comes from beam particles that are between 40-50 vertical beam sigmas.
Collimation of these beam particles would eliminate this background from SR
but one is still left with the soft bend radiation.
The upstream masks for the soft bend SR have 10mm radius and are located 2m
upstream of the IP.
A 15mm radius beam pipe no longer intercepts SR from either the soft bend or the FF quads.
However, there is no margin at 15 mm radius.
Present recommendation is to have an IP beam pipe with a 20 mm radius.
Photon backscatter rate from the downstream SR mask at +2m was simulated.
Accounting for solid angle acceptance and a backscatter rate of 1.3%,
about 700 photons/beam crossing penetrate the central Be beam pipe.
The average energy of these photons is 200 keV.
The total for both forward and backward scattering plus 2 beams means
about 2800 photons/crossing go through the Be beampipe.
In order to minimize the photon rate into the central chamber from these backscattered
photons some shielding is needed on the beam pipe between the FF quads and the central Be chamber.
Simulation indicates that as little as 0.5 mm of Ta is sufficient for SR photons.
It is unlikely this thin shielding is enough to also shield from other beam
related backgrounds (BGB, Coulomb, etc.)
The present cartoon design will have a cavity in the beam pipe at the IP.
HOM power will collect here and should be estimated especially for the Z running.
HOM absorbers might be installed in the crotch area of the central beam pipe.
Resistive wall losses may be an issue for the IP area.
The skin depth is quite deep (1-2mm) and this may cause RF interference
in the detectors close to the beam pipe.