1. M. Sullivan Apr 27, 2017 MDI meeting
Update of the SR
M. Sullivan
Apr 27, 2017
MDI meeting

2. Outline Current beam pipe layout Improved SR calculations
How Lumi-monitor fits in
Improved SR calculations
Binning for critical energy histograms was found to be too coarse. This has been resolved.
New tt SR background estimate
HH running SR background estimate
WW running SR background estimate
Summary

3. Here is where the beam pipe needs to be enlarged
LumiCal
HOM absorbers

6. Features The natural chamber wall geometry is OK
All LumiCal tracks now have a 20 mrad or higher angle of incidence to the beam pipe walls
There looks to be enough room on the other side of the beam pipe for a NEG vacuum pump
Now we come to the next question…..

7. Shielding
The question now arises about shielding the central detector from SR from the last bend magnet
The LumiCal needs a window in the beam pipe where we can no longer put shielding
For the Z running this is probably not an issue
The photon energies are very low (Ave scattered=1.8 keV)
But for the Top running this is more important
Ave scattered is 200 keV
Effectively the central part of the beam pipe increases from +/-12.5 cm to +/-50 cm due to the LumiCal window
This will increase the number of photons going into the central chamber (factor of 10? More?)
GEANT simulation comparison will tell us the answer

8. tt scattered photon energy spectrum in Jan.
The photons that cause trouble are coming from the high energy tail of the bend radiation

9. Scattered energy spectrum for tt in Mar.
out of 100 million incident scatter through the tip
I found an inaccuracy in my code where the binning for the critical energy histograms was calculated too coarsely.
Came about because there are very strong FF quads and a very soft last bend magnet

10. Energy scattered/incident ratio

11. Scattered energy spectrum from HH
out of 1billion incident scatter through the tip
Number incident > 10 keV is 187 million/xing so 81,000 scatter from the tip

12. Scattered energy spectrum for WW
out of 10 billion incident scatter through the tip
Number incident > 10 keV is 28 million/xing so 109 scatter from the tip

13. WW scat. ener. with expanded scale

15. For the Top running
2.1109 photons are incident on the mask tip from every beam bunch (9.9108 > 10 keV)
0.52% scatter through the mask tip (5.15 million)
Of these, 1.4% go through 1 cm of Ta (72,000)
Or 0.16% go through 2 cm of Ta (8200)
This is looking like it might be OK
We will probably need 2 cm of shielding (Ta, W or Pb) near the mask tip
Once the photons are going through the shield at a 45 deg or smaller angle of incidence 1 cm of shielding should be enough
A detailed GEANT simulation should verify this

16. What about the other running points?
SR photon critical energies are listed here
The critical energy of the last soft bend magnet
For the Top (175 GeV) – 100 keV
For the Higgs (125 GeV) – 40 keV
For the WW (80 GeV) – keV
For the Z (46 GeV) – keV (3.6 keV?)
More simulation runs with the GEANT4 model of the beam pipe
Do we want/need the LumiCal for the Higgs?
Stay tuned……

17. Summary The LumiCal looks OK at the Z running
Even with smaller beta* optics (needs to be checked)
We need a Be beam pipe for the LumiCal window in order to minimize the RL to the LumiCal and to minimize the HOM power in this region
The LumiCal window will cause central detector SR backgrounds to increase at the Top running because of the higher energy of the scattered photons and some reduced shielding but in general SR backgrounds look like they can be controlled

18. FF quads
Magnet designs are converging and the next big step is to get initial dimensions for the cryostats
Can we have warm bores as asked?
How much actual space is needed for the magnets and correction coils and compensation coils

19. Conclusions
A great deal of progress has been made in deciding on baseline parameters and general layout of the IR
SR at the Z does not appear to be an issue due to the very low photon energies
SR backgrounds at the Higgs and Top will be more of an issue. The photon energies are significantly higher, but at first blush it looks like they can be controlled