1. Initial Study of Synchrotron Radiation Issues for the CEPC Interaction Region M. Sullivan SLAC National Accelerator Laboratory for the CEPC14 Workshop Oct. 9-13, 2014

2. Outline Introduction – Background sources Bend magnets Final focus – BSC Bend Magnets in the Chromaticity Correction sections Final Focus quadrupoles Detector beam pipe Summary Issues to keep in mind Some Conclusions 2

3. Introduction Primary Background sources from SR – Last bend magnet before the IP The last bend magnet is a source of SR background since it sends a SR fan through the IP One way to minimize the contribution from this magnet is to make the last part of the bend magnet as soft a possible However, the soft part of the last bend has to have enough bending to cover the IP area (have to look at the geometry) – Farther from the IP is better 3

4. Synchrotron Radiation Spectrum Half of the total energy is above the critical energy 91.6% of the photons are below the critical energy 4

5. Introduction (cont.) The final focus magnets also make significant SR backgrounds One of the biggest concerns is how many beam sigmas do the beam particles populate B-factories generally assume at least 10 beam sigmas in X and 10 fully coupled (half of the total X+Y emittance) beam sigmas in Y as beam-stay-clear definitions 5

6. Introduction (3) More on BSC definitions – A more careful BSC definition should include energy spread and dispersion in the calculation and this is more important for the CEPC design which has a large energy spread (1.5%  p/p) – Another factor is some value for Closed Orbit Distortion (COD) usually 0.5-1.0 mm 6 COD x and CODy are values (usually ~1 mm) for allowed orbit errors n and m are the number of beam sigmas defined for the BSC (typically 10)

7. FF SR Issues Generally the SR from the last vertically focusing magnet can be shielded from the IP beam pipe and vertex detector (see next slide) The SR from the horizontally focusing magnet is much more difficult to control For the CEPC the critical energies of the photons from the high sigma particles is high enough to penetrate most materials as well as start showers in the beam pipe 7

8. SR from the last vertical focusing magnet can usually be shielded from the IP detector beam pipe. This is more difficult when the photons penetrate the beam pipe SR from the last horizontal focusing magnet is more difficult to block because the beam is over-focused in order to compensate for the defocusing from the last quadrupole SR from the final focusing magnets 8

9. Chromaticity Correction Bends We understand that the CCB lattice sections are a first draft design and have not been optimized – Yesterday learned that a new design has been made Both designs have the Y chromaticity correction section closest to the IP and this block has 4 horizontal bend magnets Then an X chromaticity correction section follows the Y section and it also has 4 horizontal bend magnets These sections are on both sides of the IP for a total of 16 bending magnets 9

10. CC bends (2) Designs Field (kG)Length (m)Bend angle (mrad)/mag Crit. Energy (k c )(MeV) Total SR Power (kW) Old5.23753.3754.4165.0168965 2.5 kG field2.53.3752.1082.3942043 2.0 kG field2.03.3751.6861.9151307 1.0 kG field1.03.3750.8430.958327 Beam parameters Current16.6 mA Energy 120 GeV The old design produced 9 MW of SR power (9% of the entire ring SR power). This is too much for so small a region. Would like to keep the total SR power near the IP to 1 MW or less. The new design is very close to this value. 10 Suggest trying to stay near or below. New design achieves this.

11. Synchrotron Beam Pipe Wall Power Another criteria to watch is how high the power density is on the beam pipe wall The current design has 4.416 mrad bend angles in the CCB horizontal bend magnets If we assume a 2 cm radius beam pipe then the first 1/8 th of the magnet generates a SR fan that strikes the beam pipe over a distance of 0.86 m. The power in this 1/8 th fan is 35.02 kW which gives us an average power density of 40.7 W/mm. This is twice as much as can be absorbed by dispersion strengthened Cu (GlidCoP  ) and about 20 times higher than the arc value – This has been addressed with the newer CCBs that have softer, longer bend magnets 11

12. SR beam pipe wall power (2) Table of wall power values for various bend magnet strengths in the CCB Magnet strength (kG) 1/8 th fan power (kW) SR fan (k c ) (MeV) Bend angle for 1/8 th fan Length of beam pipe struck (m) Power density W/mmComments 5.237535.0195.0160.5220.86040.70Older design 2.57.9792.3940.2641.5675.09 2.05.1061.9150.2101.8962.69 1.05.8630.9580.4842.0762.82New design 15.5 m 12 As seen from the table, there is rapid improvement in the beam wall power levels as the magnetic strength of the bend magnets decreases. This improvement comes from reduced field strength as well as reduced bending angle. The power reduction due a longer softer bend magnet can be seen in the last row from the newer design.

13. Final Focus magnets The CEPC design has two final focus quads These two magnets have to be quite strong in order to focus the very high energy beams – 516 T/m for Q1 – 364 T/m for Q2 The newer design still has fairly strong magnets – 300 T/m These very strong fields will generate an intense synchrotron radiation beam along the colliding beam axis The detector beam pipe must clear this intense SR beam 13

14. Table of SR photons at the IP from the FF magnets for one beam using the older design 14

15. FF SR photons The energy spectrum of the photons slightly softens as we get closer to the beam axis The average critical energy is about 20 MeV The perpendicular power density gets very high at the beam axis The total SR power for one beam going through one pair of FF magnets is 24 kW so the total power for both FF magnets and both beams is 96.4 kW (193 kW for both IRs) 15

16. FF SR photons (2) It may be possible to have an IP beam pipe with an inner radius of 2 cm although there are still some photons that do strike this radius beam pipe 16

17. FF SR photons (3) A 2.5 cm radius beam pipe is better There is also the issue of SR from the last upstream bend magnet 17

18. Bend radiation on the IP beam pipe The last bend magnet from the chromaticity correction block will send a fan of SR down into the interaction region – The end of the magnet is located 16.83 m from the IP – We will assume a 3 cm radius beam pipe until we get to the IP where we will assume a 2.5 cm radius pipe +/- 0.5 m from the IP. – The middle part of the magnet is bending the beam 2.208 mrad and each 0.552 mrad of bending produces 35.02 kW of SR power – This issue has been improved with the most recent design that has softer bend magnets but this is still important especially if the design assumes cryogenic beam pipes under the FF quads 18

19. SR fan from the Last Bend Magnet 19 SR beam from FF magnets 18 kW 28 W/mm 14 W/mm These values are out of date but there will still be SR power in this region These values are about 10 times less with the newer design

20. Suggested masking and shielding scheme 20 Unfortunately, the power density goes up on this kind of design but with the softer bends this should work The masking for the IP does not work because the incident power density is too high – 18kW/500mm = 36 W/mm The new design with softer bends helps here

21. Suggested FF quad change Would like to suggest some changes to the FF quads Suggest increasing the length of the magnets and moving them farther apart – Make both magnets 1 m long – May need to make longer to get to possible field strengths (see E. Paoloni’s talk) – 2 m drift between Q2 and Q1 (was 1.44 m) If we can move the Q1 down to a 2 m L* then the maximum beta comes down from nearly 6 km to about 4 km – This makes the chromaticity correction a little easier – New design has L* of 1.5 m which is even better 21

22. FF change (cont.) These proposed changes reduce the SR power from the FF quads by a factor of 2 which is a big help Making  y* larger is a big help May need to do more to improve dynamic aperture? This change increases the amount of SR hits on the IP beam pipe mainly due to backing up the X focusing magnet (Q2) – Smaller angle tracks can now strike the detector beam pipe 22

23. Beta functions 23 The current design is roughly the same (see Y. Wang’s talk) but with softer bend magnets

24. Summary There is quite a bit of SR power in the earlier design of the CEPC local chromaticity correction – 9 MW (currently 1.7 MW) – twice this for 2 IRs – The new chromaticity correction schemes with the softer bend magnets help a lot The final focus magnets may need further optimization and perhaps the magnet strengths can be further lowered 24

25. Summary (cont.) I do not think the beam pipe under the final focus magnets can be cryogenic – there is probably too much SR power very close by – One has to protect the beam pipe from not only primary photons but also secondary and perhaps even tertiary photons (Single bounce and double bounce photons and shower debris from higher energy gammas) 25

26. Single bounce SR Example 26 These photons will deposit energy on the cryogenic beam pipe

27. Partial List of Issues to keep in mind There is a lot of SR Critical energies are high – Highly penetrating photons – Difficult to shield Backscatter and Forward scattering photons Try keep Beam-Stay-Clear as large as possible – Also important for beam loss particles Keep an eye on the last bend magnet 27

28. Issues list (2) Strength of the FF quads generate intense SR beams – Backscatter from these photon beams Quadrupole strengths in the CCB lattice – Some are fairly strong and generate SR beams like the FF magnets Have to make sure cryogenic beam pipe can work for FF quads – KEKB (and superKEKB) use cryo beam pipes – Last bending magnet is very far away 28

29. Issues list (3) Detector concerns will become more detailed as the detector design develops – The detector already influences the IR design Very important that there is good communication between the detector group and the accelerator group Small changes in design from one group may have large consequences for the other group 29

30. Issues list (4) Secondary backgrounds from the machine can be important for the detector – Absorber shower debris – Neutrons – Muons – Forward scatter and backscatter SR – … Detailed simulations are necessary (super KEKB talk from Nakayama) 30

31. Some Conclusions The IR design is still under going optimization which is excellent since this is one of most difficult parts of the ring and there are a large number of parameters that force over- constrained solutions – One probably never completely finishes the optimization (better can be the enemy of good enough) Compromise must come into play and the best machine will be a set of compromises Still a lot to do but significant progress has already been made 31

32. Thank You And very special thanks to the support team here who recovered my talk from the hard drive of my computer and let me use a local laptop to finish the talk 32