1. The 13th Symposium on Accelerator Physics
CEPC Parameter Choice
Dou Wang, Chenghui Yu, Yuan Zhang, Yiwei Wang, Huiping Geng, Sha Bai, Na Wang, Jiyuan Zhai, Xiaohao Cui, Jie Gao, Qing Qin
Ji Shou, Hu Nan Province, China August, 2017

2. Physics goals of CEPC Electron-positron collider (45.5, 80, 120 GeV)
Higgs Factory
Precision study of Higgs (mH, JPC, couplings)
Looking for hints of new physics
Luminosity > 2.0×1034cm-2s-1
Z & W factory
Precision test of standard model
Rare decays
Luminosity > 1.0×1034cm-2s-1
Flavor factory: b, c, t and QCD studies

3. Four stages towards CDR
Sawtooth effect
Beam loading
COD correction
Collision tuning
Since May 2015
Since Oct 2012
Since May 2016
Since Nov 2016

4. CEPC layout
方案二
方案一

5. CEPC bunch distribution
W & Z
C0=100km
C0=100km
Higgs

6. Beamstrahlung effect Typical issue for energy-frontier e+e− colliders
During collision, the deflected particles will lose part of its energy due to the synchrotron radiation.
Extra energy spread
Beam loss for large energy deviation  life time reduction
Detector background (photons, hadrons…)
Divergence angle  interfere the detection of small-angle events
Constraint for energy spread
Constraint for life time
Large energy acceptance is essential!
 Harder DA!

7. Crab waist collision large Piwinski’s angle
overlapping area much smaller than σz  small βy
Crab waist sextupoles to supress betatron resonances

8. Machine constraints / given parameters
Energy E0
Circumference C0
NIP
Beam power P0
y*
Emittance coupling factor 
Bending radius 
Piwinski angle 
y enhancement by crab waist Fl ~1.5
Energy acceptance (DA)
Phase advance per cell (FODO)
100km

9. Constraint for CEPC parameter choice
Limit of Beam-beam tune shift
Fl: y enhancement by crab waist
Beam lifetime due to beamstrahlung
BS life time: 30 min
Beamstrahlung energy spread
A=0/BS (A5)
HOM power per cavity (coaxial coupler)

10. Parameter choice – step 1
Beam-beam limit:
Fl: y enhancement by crab waist, ~ 1.5 for Higgs, 1.9 for W and 2.6 for Z.

11. Parameter choice – step 2

12. Parameter choice – step 3
BS life time: 30 min
y:
-- phase advance/cell, -- bending angle/cell.
Estimate :

13. Parameter choice – step 4

14. Parameter choice – step 5
Effective bunch length: overlap area of colliding bunches
Hour glass effect:

15. Parameter choice – step 6
Vrf , s
Energy acceptance from RF:

16. Parameter choice – step 7
Beam lifetime due to radiative Bhabha scattering
Beam lifetime due to Beamstrahlung
HOM power per cavity
HOM loss factor:
*V.I. Telnov, "Issues with current designs for e+e- and gammagamma colliders“, PoS Photon2013 (2013)

17. Cross-section for radiative Bhabha scattering

18. Lifetime due to radiative Bhabha
For CEPC(Pre-CDR) and FCCee,
Life time of FCCee: 72 min
Life time of CEPC(Pre-CDR) : 55 min
For CEPC(CDR)
Life time of CEPC(CDR) : 100 min
Life time due to beamstrahlung and Bhabha at the same level for CEPC

19. CEPC CDR parameters Higgs W Z-low lum. Z-high lum. Number of IPs 2
Higgs W
Z-low lum.
Z-high lum.
Number of IPs 2
Energy (GeV)
120 80
45.5
Circumference (km)
100
SR loss/turn (GeV)
1.61
0.32
0.033
Half crossing angle (mrad)
16.5
Piwinski angle
2.28
3.6
6.33
Ne/bunch (1010)
9.68
2.3
Bunch number
420
5700
3510
27000
Beam current (mA)
19.5
98.6
38.8
298.5
SR power /beam (MW)
31.4
31.3
1.3
9.9
Bending radius (km)
11.4
Momentum compaction (10-5)
1.15
IP x/y (m)
0.36/0.002
Emittance x/y (nm)
1.18/0.0036
0.52/0.0017
0.17/0.0038
Transverse IP (um)
20.6/0.085
13.7/0.059
7.81/0.087
x/y/IP
0.025/0.085
0.014/0.068
0.017/0.053
RF Phase (degree)
128
134.7
151
VRF (GV)
2.03
0.45
0.069
f RF (MHz) (harmonic)
650
650 (217800)
Nature z (mm)
2.75
2.98
2.92
Total z (mm)
2.85
3.0
HOM power/cavity (kw)
0.42 (2cell)
0.38 (2cell)
0.096 (2cell)
0.74 (2cell)
Energy spread (%)
0.096
0.064
0.036
Energy acceptance (%)
1.1
Energy acceptance by RF (%)
1.98
1.46
1.2 n
0.19
0.11
0.12
Life time due to beamstrahlung_cal (minute) 63
F (hour glass)
0.93
0.963
0.987
Lmax/IP (1034cm-2s-1)
2.0
5.6
1.0
7.7

20. Higgs Luminosity vs. crossing angle
Keep beamstrahlung life time constant (52min)
100km
luminosity
emittance

21. Higgs Luminosity vs. y*
Keep beamstrahlung life time constant (52min)
100km
luminosity
emittance

22. Higgs Luminosity vs. SR power
100km
goal

23. MDI related parameters
old
new
L* (m)
1.5
2.2
Crossing angle (mrad) 30 33
Strength of QD0 (T/m)
200
150
Strength of detector solenoid (T)
3.5
3.0
Strength of anti-solenoid (T) 13
7.0

24. Vertical emittance induced by solenoid
Vertical emittance growth
Higgs: pm
W: pm
Z: pm
Vertical emittance at Z pole is most dangerous with solenoid!
Coupling: 0.3% for Higgs and W
Real model
Larger coupling factor (2.2%) at Z pole
Detector

25. CEPC upgrade （wangdou20161219-100km_1mmy）tt
H-high lumi.
H-low pow. W Z
Number of IPs 2
Energy (GeV)
175
120 80
45.5
Circumference (km)
100
SR loss/turn (GeV)
7.55
1.67
0.33
0.034
Half crossing angle (mrad) 15
Piwinski angle
1.6
2.5
3.57
5.69
Ne/bunch (1011)
1.41
1.12
1.05
0.46
Bunch number 98
555
211
1000
16666
65716
Beam current (mA)
6.64
29.97
11.4
50.6
367.7
1449.7
SR power /beam (MW) 50 19
16.7
12.7
Bending radius (km) 11
Momentum compaction (10-5)
1.3
0.96
3.1
3.3
IP x/y (m)
0.2/0.002
0.3/0.001
0.3 /0.001
0.1 /0.001
0.12/0.001
Emittance x/y (nm)
3.19/0.0097
1.01/0.0031
2.68/0.008
0.93/0.0049
Transverse IP (um)
25.3/0.14
17.4/0.055
16.4/0.09
10.5/0.07
x/y/IP
0.016/0.055
0.029/0.083
0.0082/0.055
0.0075/0.054
RF Phase (degree)
122.2
123.3
149
160.8
VRF (GV)
8.92
2.0
0.63
0.11
f RF (MHz) (harmonic)
650
650 (217800)
Nature z (mm)
2.62
2.72
3.8
3.93
Total z (mm)
2.7
2.9
3.9
4.0
HOM power/cavity (kw)
0.53(5cell)
0.75(2cell)
0.28(2cell)
1.0 (2cell)
1.6(1cell)
6.25(1cell)
Energy spread (%)
0.14
0.098
0.065
0.037
Energy acceptance (%)
1.5
Energy acceptance by RF (%)
2.6
1.8
1.1 n
0.23
0.26
0.18
Life time due to beamstrahlung_cal (minute) 52
F (hour glass)
0.89
0.83
0.84
0.91
Lmax/IP (1034cm-2s-1)
0.62
5.42
2.06
4.08
18.0
70.97

26. 100km CEPC luminosity potential (1mmy+50MW/beam)H z

27. CEPC Luminosity vs circumference
* Fabiola Gianotti, Future Circular ColliderDesign Study, ICFA meeting, J-PARC,

28. Nonlinearity sources Amplitude dependent non-linear tune shift:
The undisturbed linear part,
The kinematic part,
The influence of the fringe field,
The octupole component.
Amplitude dependent non-linear tune shift:
So,

29. Kinematic effects Then,
Hamiltonian includes the high-order terms of Px and Py.
nonlinear kinematic effect originated from the large angles of particles in the interaction region is responsible for the large tune-shift which in turn limits the dynamic aperture.
Then,
A.Bogomyagkov, S.Glykhov, E.Levichev, P.Piminov

30. Quadrupole fringe fields
With a simple model of two matched parabolas for fringe field
A.Bogomyagkov, S.Glykhov, E.Levichev, P.Piminov

31. Seminar at CERN, March 24th 2014
Chromatic sextupoles
Vertical chromatic sextupole pair separated by –I transformer gives the following coordinate transformation in the first order*)
Pair of sextupoles
Octupole
By analogy to the octupole and using the expression for the FF chromaticity we found for the vertical detuning (2 pairs)
*) A.Bogomyagkov, S.Glykhov, E.Levichev, P.Piminov
Seminar at CERN, March 24th 2014

32. CEPC amplitude-tune dependence
x=0.22 y=0.002
Cxx (m-1)
Cxy (m-1)
Cyy (m-1)
Kinematic effects
3.7
271
44762
Fringe field (QD0+QF1)
x=0.144 y=0.002
Cxx (m-1)
Cxy (m-1)
Cyy (m-1)
Kinematic effects
8.6
414
44762
Fringe field (QD0+QF1)
Larger x* give help to DA while keeping y*!
* Nonlinear effect of sextupole pairs can be corrected by the attached weak sextupole pairs.

33. Luminosity vs. betax*

34. Energy acceptance vs. betax*
Larger x* release the difficulty of DA study.

35. Summary
A consistent design method for CEPC parameter choice with carb waist scheme has been created.
The 100km Double Ring configuration with shared SCRF has been defined as baseline in order to avoid the sawtooth and beam loading effects.
CEPC was optimized at Higgs energy. W and Z just make do with what they have.
2mm y* and 31MW SR power for CDR. Further upgrade was also considered.
Requirement for energy acceptance reduced to 1.1% by enlarging the ring and x*.
Recently, x* was increased to 0.36m for better DA.

36. Thanks for your attention！

37. Back up

38. parameters for CEPC double ring （wangdou20170306-100km_2mmy）
Pre-CDR
Higgs W Z
Number of IPs 2
Energy (GeV)
120 80
45.5
Circumference (km) 54
100
SR loss/turn (GeV)
3.1
1.67
0.33
0.034
Half crossing angle (mrad)
16.5
Piwinski angle
3.19
5.69
4.29
Ne/bunch (1011)
3.79
0.968
0.365
0.455
Bunch number 50
412
5534
21300
Beam current (mA)
16.6
19.2
97.1
465.8
SR power /beam (MW)
51.7 32
16.1
Bending radius (km)
6.1 11
Momentum compaction (10-5)
3.4
1.14
4.49
IP x/y (m)
0.8/0.0012
0.171/0.002
0.171 /0.002
0.16/0.002
Emittance x/y (nm)
6.12/0.018
1.31/0.004
0.57/0.0017
1.48/0.0078
Transverse IP (um)
69.97/0.15
15.0/0.089
9.9/0.059
15.4/0.125
x/y/IP
0.118/0.083
0.013/0.083
0.0055/0.062
0.008/0.054
RF Phase (degree)
153.0
128
126.9
165.3
VRF (GV)
6.87
2.1
0.41
0.14
f RF (MHz) (harmonic)
650
650 (217800)
Nature z (mm)
2.14
2.72
3.37
3.97
Total z (mm)
2.65
2.9
4.0
HOM power/cavity (kw)
3.6 (5cell)
0.41(2cell)
0.36(2cell)
1.99(2cell)
Energy spread (%)
0.13
0.098
0.065
0.037
Energy acceptance (%)
1.5
Energy acceptance by RF (%) 6
1.1 n
0.23
0.26
0.15
0.12
Life time due to beamstrahlung_cal (minute) 47 52
F (hour glass)
0.68
0.96
0.98
Lmax/IP (1034cm-2s-1)
2.04
2.0
5.15
11.9

39. Z luminosity vs. coupling