1. Some CEPC SRF considerations
Zhenchao LIU

2. Contents Introduction Input coupler power issue
RF deflecting and synchrotron radiation in cavity

3. Primary parameter for CEPC double ring （wangdou20160219）
Pre-CDR
H-high lumi.
H-low power Z
Number of IPs 2
Energy (GeV)
120
45.5
Circumference (km) 54
SR loss/turn (GeV)
3.1
2.96
0.062
Half crossing angle (mrad)
14.5 15
11.5
Piwinski angle
2.5
2.6
8.5
Ne/bunch (1011)
3.79
2.85
2.81
2.67
0.46
Bunch number 50 40 44
1100
Beam current (mA)
16.6
16.9
10.1
10.5
45.4
SR power /beam (MW)
51.7 30
31.2
2.8
Bending radius (km)
6.1
6.2
Momentum compaction (10-5)
3.4
3.0
2.2
3.5
IP x/y (m)
0.8/0.0012
0.306/0.0012
0.25/
0.22/0.001
0.268 /
0.08/0.001
Emittance x/y (nm)
6.12/0.018
3.34/0.01
2.45/0.0074
2.67/0.008
2.06 /0.0062
0.62/0.002
Transverse IP (um)
69.97/0.15
32/0.11
24.8/0.1
24.3/0.09
23.5/0.088
7/0.046
x/IP
0.118
0.04
0.03
0.032
0.005
y/IP
0.083
0.11
0.084
VRF (GV)
6.87
3.7
3.62
3.6
3.53
0.12
f RF (MHz)
650
Nature z (mm)
2.14
3.3
3.2
3.9
Total z (mm)
2.65
4.4
4.1
4.2
4.0
HOM power/cavity (kw)
1.5
1.3
0.99
Energy spread (%)
0.13
0.05
Energy acceptance (%)
Energy acceptance by RF (%) 6
2.1
1.1 n
0.23
0.49
0.47
0.27
Life time due to beamstrahlung_cal (minute) 47 53 36 41 32
F (hour glass)
0.68
0.73
0.82
0.69
0.81
0.95
Lmax/IP (1034cm-2s-1)
2.04
2.97
2.03
2.01
3.61

4. Advantage: Avoid pretzel orbit Accommodate more bunches at Z/W energy
Reduce AC power with crab waist collision
bypass (pp)
bypass (pp)

5. Primary parameter for CEPC double ring （wangdou20160219）
Pre-CDR
H-high lumi.
H-low power Z
Number of IPs 2
Energy (GeV)
120
45.5
Circumference (km) 54
SR loss/turn (GeV)
3.1
2.96
0.062
Half crossing angle (mrad)
14.5 15
11.5
Piwinski angle
2.5
2.6
8.5
Ne/bunch (1011)
3.79
2.85
2.81
2.67
0.46
Bunch number 50 40 44
1100
Beam current (mA)
16.6
16.9
10.1
10.5
45.4
SR power /beam (MW)
51.7 30
31.2
2.8
Bending radius (km)
6.1
6.2
Momentum compaction (10-5)
3.4
3.0
2.2
3.5
IP x/y (m)
0.8/0.0012
0.306/0.0012
0.25/
0.22/0.001
0.268 /
0.08/0.001
Emittance x/y (nm)
6.12/0.018
3.34/0.01
2.45/0.0074
2.67/0.008
2.06 /0.0062
0.62/0.002
Transverse IP (um)
69.97/0.15
32/0.11
24.8/0.1
24.3/0.09
23.5/0.088
7/0.046
x/IP
0.118
0.04
0.03
0.032
0.005
y/IP
0.083
0.11
0.084
VRF (GV)
6.87
3.7
3.62
3.6
3.53
0.12
f RF (MHz)
650
Nature z (mm)
2.14
3.3
3.2
3.9
Total z (mm)
2.65
4.4
4.1
4.2
4.0
HOM power/cavity (kw)
1.5
1.3
0.99
Energy spread (%)
0.13
0.05
Energy acceptance (%)
Energy acceptance by RF (%) 6
2.1
1.1 n
0.23
0.49
0.47
0.27
Life time due to beamstrahlung_cal (minute) 47 53 36 41 32
F (hour glass)
0.68
0.73
0.82
0.69
0.81
0.95
Lmax/IP (1034cm-2s-1)
2.04
2.97
2.03
2.01
3.61

6. Accelerator gradient decrease in one RF cavity (H-high lumi.)
The peak beam power is
Assume 4.9MW input power & 15MV/m （5cell cavity） t IP IP t
Final Eacc/Initial Eacc
Z.C. Liu modified on J.Y. Zhai’s figure
RF cycles

7. Accelerator gradient decrease in one RF cavity (H-high lumi.)
The peak RF power is very high at the bunch train passing period.
The cavity field gradient will decrease a lot if the input power much lower than the beam power.
The peak beam power is
Assume 300kW input power & 15MV/m （5cell cavity）
Final Eacc/Initial Eacc
Final Eacc/Initial Eacc
Initial Eacc(MV/m)
RF cycles
Field decrease in one cavity at the bunch train passing period
Field decrease vs. various initial field gradient of the cavity

8. Accelerator gradient decrease in one RF cavity (H-high lumi.)
The head bunch and tail bunch get different energy.
The ratio of Etail/Ehead is about this will make the bunch energy spread larger than 0.13% .
Higher field gradient will give a higher Etail/Ehead when the input power is not enough.
Final Eacc/Initial Eacc
Initial Eacc(MV/m)
Field decrease vs. various initial field gradient of the cavity

9. Situation for the cavity close to IP
The field decrease is much worse for the cavity close to the IP as another bunch train is coming when one bunch train passed.
Assume 300kW input power （5cell cavity）
The ratio of Final Eacc/Initial Eacc is down to 0.2 at low initial field.
Final Eacc/Initial Eacc
Initial Eacc(MV/m)
These cavities

10. Primary parameter for CEPC double ring （wangdou20160219）
Pre-CDR
H-high lumi.
H-low power Z
Number of IPs 2
Energy (GeV)
120
45.5
Circumference (km) 54
SR loss/turn (GeV)
3.1
2.96
0.062
Half crossing angle (mrad)
14.5 15
11.5
Piwinski angle
2.5
2.6
8.5
Ne/bunch (1011)
3.79
2.85
2.81
2.67
0.46
Bunch number 50 40 44
1100
Beam current (mA)
16.6
16.9
10.1
10.5
45.4
SR power /beam (MW)
51.7 30
31.2
2.8
Bending radius (km)
6.1
6.2
Momentum compaction (10-5)
3.4
3.0
2.2
3.5
IP x/y (m)
0.8/0.0012
0.306/0.0012
0.25/
0.22/0.001
0.268 /
0.08/0.001
Emittance x/y (nm)
6.12/0.018
3.34/0.01
2.45/0.0074
2.67/0.008
2.06 /0.0062
0.62/0.002
Transverse IP (um)
69.97/0.15
32/0.11
24.8/0.1
24.3/0.09
23.5/0.088
7/0.046
x/IP
0.118
0.04
0.03
0.032
0.005
y/IP
0.083
0.11
0.084
VRF (GV)
6.87
3.7
3.62
3.6
3.53
0.12
f RF (MHz)
650
Nature z (mm)
2.14
3.3
3.2
3.9
Total z (mm)
2.65
4.4
4.1
4.2
4.0
HOM power/cavity (kw)
1.5
1.3
0.99
Energy spread (%)
0.13
0.05
Energy acceptance (%)
Energy acceptance by RF (%) 6
2.1
1.1 n
0.23
0.49
0.47
0.27
Life time due to beamstrahlung_cal (minute) 47 53 36 41 32
F (hour glass)
0.68
0.73
0.82
0.69
0.81
0.95
Lmax/IP (1034cm-2s-1)
2.04
2.97
2.03
2.01
3.61

11. Accelerator gradient decrease in one RF cavity(H-low power)
The peak RF power is very high at the bunch train passing period.
The cavity field gradient will decrease a lot if the input power much lower than the beam power.
The peak beam power is
Assume 300kW input power & 15MV/m （5cell cavity）
Final Eacc/Initial Eacc
Final Eacc/Initial Eacc
Initial Eacc(MV/m)
RF cycles
Field decrease in one cavity at the bunch train passing period
Field decrease vs. various initial field gradient of the cavity

12. Primary parameter for CEPC double ring （wangdou20160219）
Pre-CDR
H-high lumi.
H-low power Z
Number of IPs 2
Energy (GeV)
120
45.5
Circumference (km) 54
SR loss/turn (GeV)
3.1
2.96
0.062
Half crossing angle (mrad)
14.5 15
11.5
Piwinski angle
2.5
2.6
8.5
Ne/bunch (1011)
3.79
2.85
2.81
2.67
0.46
Bunch number 50 40 44
1100
Beam current (mA)
16.6
16.9
10.1
10.5
45.4
SR power /beam (MW)
51.7 30
31.2
2.8
Bending radius (km)
6.1
6.2
Momentum compaction (10-5)
3.4
3.0
2.2
3.5
IP x/y (m)
0.8/0.0012
0.306/0.0012
0.25/
0.22/0.001
0.268 /
0.08/0.001
Emittance x/y (nm)
6.12/0.018
3.34/0.01
2.45/0.0074
2.67/0.008
2.06 /0.0062
0.62/0.002
Transverse IP (um)
69.97/0.15
32/0.11
24.8/0.1
24.3/0.09
23.5/0.088
7/0.046
x/IP
0.118
0.04
0.03
0.032
0.005
y/IP
0.083
0.11
0.084
VRF (GV)
6.87
3.7
3.62
3.6
3.53
0.12
f RF (MHz)
650
Nature z (mm)
2.14
3.3
3.2
3.9
Total z (mm)
2.65
4.4
4.1
4.2
4.0
HOM power/cavity (kw)
1.5
1.3
0.99
Energy spread (%)
0.13
0.05
Energy acceptance (%)
Energy acceptance by RF (%) 6
2.1
1.1 n
0.23
0.49
0.47
0.27
Life time due to beamstrahlung_cal (minute) 47 53 36 41 32
F (hour glass)
0.68
0.73
0.82
0.69
0.81
0.95
Lmax/IP (1034cm-2s-1)
2.04
2.97
2.03
2.01
3.61

13. Accelerator gradient decrease in one RF cavity (Z)
The peak RF power is very high at the bunch train passing period.
The cavity field gradient will decrease a lot if the input power much lower than the beam power.
The peak beam power is
Assume 300kW input power & 15MV/m （5cell cavity）
Final Eacc/Initial Eacc
Final Eacc/Initial Eacc
Initial Eacc(MV/m)
RF cycles
Field decrease in one cavity at the bunch train passing period
Field decrease vs. various initial field gradient of the cavity

14. Input power requirements
The input power in pulse should be equal to the beam power of bunch train (if do not consider reflection ……).
Or the bunch distance from each other is enough to power up the cavity field again.
Higher Eacc is much better when input power is not enough as the stored energy in cavity is proportional to Eacc^2.
Reliable ceramic window for the high peak input power.
The design is very challenging for the RF!!!!!

15. New parameters
By J.Y. Zhai’s report

16. Limitations on the power coupler
The average power of input coupler is ~300kW
The peak power of input coupler is ~1MW .
The maximum power is limited by the ceramic window

18. S. Belomestnykh，OVERVIEW OF INPUT POWER COUPLER DEVELOPMENTS PULSED AND CW，WE305, SRF2007

19. What things to do? Reliable high power input coupler design!
New ceramic window design and new ceramic material which can pass much higher power with low heat.
Better cooling structure for the ceramic window.
Multi-window?
It is also very essential for other project which need high input power!
Developing high power coupler!

20. Beam off-axis in cavity
Bunches may be not on the cavity axis, especially in the single ring condition.
Bunches off-axis will be deflected by the cavity magnetic field
P（W/m）
H（A/m）
B（mT）
Off-axis (mm)

21. Assume Bmax=80mT (Bpk/Eacc=4 & Eacc=20MV/m) Phase 90deg (Emax at 0deg)
Off-axis (mm)
B (mT)
Pmax（W/m)
Ec (keV) 2
1.4
1.1
13.4 5
3.5
7.1
33.5 10
7.0
28.6
67.1 15
10.5
64.4
100.6 25
17.4
176.8
166.7
Consider phase, The P = Pmax sinφ
It is more serious operating on large phase angle！

22. the length of the straight RF part is 849.6m,
the radius of synchrotron radiation is x sin(4.258urad) = 3.6mm
If the beam direction is not parallel to cavity axis, synchrotron radiation of the beam may irradiate on the cavity.
It is a heat source to the cavity LHe system
4.258urad

23. Thanks!