1. HOM Power Challenge for CEPC
Jiyuan Zhai
On behalf of the CEPC SRF Design Group of IHEP
TTC Meeting, December 2-5, 2014, KEK

2. Content CEPC-SppC Project CEPC SRF Parameters HOM Damping Challenges
Summary

3. CEPC-SppC Project 50-70 km in circumference
CEPC is a 240 GeV Circular Electron Positron Collider, proposed to carry out high precision study on Higgs boson, which can be upgraded to a 70 TeV or higher pp collider SppC (construction time: ), to study the new physics beyond the Standard Model.
BTC
IP1
IP3 e+ e-
e+ e- Linac
LTB
CEPC Collider Ring
CEPC Booster
SppC ME Booster
SppC LE Booster
IP4
IP2
SppC Collider Ring
Proton Linac
SppC HE Booster
50-70 km in circumference

4. Potential Site: Qinhuangdao, Hebei Province, China

5. CEPC Design Baseline Tunnel circumference: ~ 54 km
Tunnel size: ~ 6.5 m (LEP tunnel: 3.76 m)
8 arcs and 8 straight sections: 4 straight for IPs and RF, another 4 for RF, injection and beam dump, etc.
A 6 GeV linac on the surface (with the option for an XFEL in the future)
A full-energy 120 GeV Booster in the tunnel
A 240 GeV e+e- Collider in the same tunnel underneath the Booster
A single beam pipe for both e+ and e- beams (similar to LEP, CESR)
Synchrotron radiation budget: 50 MW per beam
Two SRF systems:
Booster: 1.3 GHz 9-cell cavity, similar to the ILC, XFEL, LCLS-II
Collider: 650 MHz 5-cell cavity, similar to the China ADS, PIP-II

6. CEPC Main Technical Challenges
IR optics with appropriate dynamic aperture for off-momentum up to 2%
Pretzel orbit:
Tolerance for orbit offset in the RF
Tolerance for orbit offset in the quads and sextupoles
Bunch tilt effect due to transverse wakefield
Machine-detector interface (MDI)
Low energy injection (6 GeV) to the Booster:
Earth field effect: bend field ~30 Gs, measured earth field ~0.5  0.03 Gs
HOM damper for the Collider RF cavity
High average beam current: 2 x 16.6 mA
High HOM loss: 3.5 kW per cavity

7. CEPC SRF System Layout Total SRF 1.4 km, 12 GeV RF voltage
Booster magnet field cycle
extraction to main ring
120 GeV
collider and booster in one tunnel
one vacuum chamber for collider
injection from linac
6 GeV
4 IPs, ~ 1 km each
4 straights, ~ 850 m each
RF section length ~ 175 m
Cavity voltage ramping
Total SRF 1.4 km, 12 GeV RF voltage
104.5 MW beam power, 2 MW HOM power
124 MW RF power, > 200 MW RF AC power
126 kW (4.2 K equiv.) installed cryogenic power
Booster RF and cryogenic duty factor ~ 22 % for continuous injection.
Each collider or booster cavity is driven by an individual RF power source.

8. CEPC Top-level Parameters related to SRF System (1)
Unit
Main Ring
Booster
Injection
Booster Extraction
Beam energy
GeV
120 6
Circumference km
54.752
Luminosity / IP
cm-2s-1
2.04E+34 -
Energy loss / turn
3.11
17.6 keV
2.81
SR power MW
103.42
16.2 W
2.46
Revolution period s
1.83E-04
Revolution frequency
kHz
5.4755
Bunch charge nC
60.56
3.35
3.2
Bunch number
100 50
Beam current (total) mA
33.2
0.92
0.87
Bunch spacing μs
1.825
3.65
Beam spectrum spacing
MHz
0.55
0.274

9. CEPC Top-level Parameters related to SRF System (2)
Unit
Main Ring
Booster
Injection
Booster Extraction
Momentum compaction -
3.36E-05
7.69E-05
Energy spread %
0.1629
0.1 (linac)
0.127
Bunch length mm
2.65
1.5 (linac)
2.66
RF voltage GV
6.87
5.12
RF frequency
GHz
0.65
1.3
Harmonic number
118800
237423
Synchrotron tune
0.180
Energy acceptance (RF)
5.99
17.307
2.091
Transverse damping time
turns
77.05
(125 s)
85.2 (15.56 ms)
Longitudinal damping time
38.52
(62 s)
42.7 (7.789 ms)
Lifetime Bhabha
min
51.77
Lifetime BS (sim.) 47

10. CEPC SRF System Parameters
CEPC-Collider
CEPC-Booster
LEP2
Cavity Type
650 MHz 5-cell
Nitrogen-doped Nb
1.3 GHz 9-cell
352 MHz 4-cell Nb/Cu sputtered
Cavity number
384
256
288
Vcav / VRF
17.9 MV / 6.87 GeV
20 MV / 5.12 GeV
12 MV / 3.46 GeV
Eacc (MV/m)
15.5
19.3
6 ~ 7.5 Q0 2K 2K
4.2K
Cryo AC power (MW)
~15
~2.3 (21% DF)
6.1
Qualified test
20 4E10
23 2E10 /
Cryomodule number
96 (4 cav. / module)
32 (8 cav. / module)
72 (4 cav. / module)
CW RF power / cav. (kW)
280 20
125
RF source number
384 (330 kW)
256 (25 kW)
36 (1.2 MW/8 cav)
HOM damper (W)
10k ferrite +1k hook
50 (hook+ceramic)
300 (hook)

11. SRF Program in Possible CEPC Timeline
2015
2020
2027
Pre-CDR
Pre-studies
( )
R&D
Engineering Design
( , 5 yrs.)
Construction
( , 7 yrs.)
SRF
Pre-design
SRF Initial Technology R&D
( , 5 yrs.)
HOM damping, high Q0 cavity, coupler window, heat load
SRF Pre-Production R&D
( , 4 yrs.)
SRF Production and Installation
( , 5 yrs.)
IHEP site infrastructure
Large scale SRF lab (infrastructure) on CEPC site and in industry (2018)
CEPC SRF Team (~6)
CEPC SRF Core Team (main ring 10 + booster 10 in parallel) (start from 2017)
Facility upgrade, additional space
SRF ~ 200 FTEs (start from 2020), engineers and technicians
Components Prototyping & Performance Demonstration:
Cavity (four for each), helium vessel, vertical test
Power coupler (four for each), HOM dampers, tuners
Short (two-cavity) module for each type, horizontal test
Booster SRF in three years ( ):
test two 1300 MHz cavities per week
assemble and test one cryomodule per month
Main ring SRF in four years ( ):
test two 650 MHz cavities per week
assemble and test two cryomodules per month
Demonstrate Robustness of Fabrication and Assembly Processes of Cryomodule and its Components. Establish procedures, quality control steps, test set ups, assembly sequences, etc. for the production run. Build and test two booster cryomodules and three main ring cryomodules.

12. HOM Power
Very small beam spectral line spacing: main ring 0.55 MHz, booster MHz.
Impossible to detune HOM modes away from beam spectral lines with large HOM frequency scattering from cavity to cavity
by fabrication tolerances and RF tuning of the fundamental mode
DESY TESLA cavity measured spread: TM011 9 MHz, TE111 5 MHz, TM110 1~6 MHz, TE121 2 MHz
HOM power damping of 3.5 kW for each 650 MHz 5-cell cavity and 21 kW for each cryomodule of the CEPC main ring.
Possible resonant excitation for low frequency modes below cut-off.
For the main ring cavity TM011 power at Qext = 1E4 is 937 W (310 W of the single pass case).

13. HOM Spectrum
80 % above cut-off frequency , propagate through cavities and finally absorbed by the two HOM absorbers at room temperature outside the cryomodule.
Each absorber has to damp about 10 kW HOM power, can’t be in the cryogenic region.
LEP2: 300 W / cavity
above cut-off : 200 W
KEKB and SuperKEKB ferrite HOM absorbers 20 kW. IHEP 4.5 kW tested. XFEL/ILC type HOM coupler and absorber can easily handle the booster HOM power. Ceramic HOM absorber at 70 K in the cryomodule beam line. LEP/LHC-type HOM coupler for the kW level power capacity. BNL also developing kW class coaxial HOM couplers. Waveguide at the cavity beam pipe is also a possible solution for the main ring cavity HOM power extraction, but with large size waveguides, more complicated structure and interfaces of the cryomodule, and large heat load.

14. HOM Damping Scheme
Inter-cavity beam pipe (and bellow) diameter will determine the fraction of HOM power propagating out of the cryomodule.
Let most HOMs confine in one cavity and extract them out or let them go through cavities? How much will trap, especially the grey zone?
LEP2 type
Conservative scheme: two cavities in one module.
Another possible solution is hybrid module with water cooled HOM absorber inside (Carlo Pagani).
Need further study. Critical issue.

15. HOM Heat Load Budget
Main Ring
Booster
HOM power / cavity
3.5 kW
5.3 W
HOM power / module
21 kW
56 W
HOM 2K heat load / module
13 W
7.2 W
HOM 5K heat load / module
39 W
3.2 W
HOM 80K heat load / module
390 W
52.8 W
Percent of total cryogenic load
22 %
15 %
Estimated
upper limit,
design goal,
enough margin
Main ring cryomodule: RF shielded bellows (copper plated) and gate valves, flange connection with gap-free gaskets
Booster heat load scaled from ILC TDR. Duty factor of booster HOM power is 50 % for continuous injection (much lowered, e.g. 1-5%, for longer beam lifetime => long injection cycle and/or low charge).

16. Heat Load Estimation
Assume 10 kW HOM power propagating through the beam tubes and bellows (thin copper film RRR=30, in abnormal skin effect regime), power dissipation < 2 2K.
Even RRR=1 for copper plating (in normal skin effect regime), power dissipation < 10 W/m. Note 300 kW power through input coupler connected to 2 K and 5 K.
HOM power loss in cavity at 2 K less than 0.3 W (all modes Qext < 106)
Heat load at 5 K and 80 K dominated by HOM cable heating. Assume 0.1 dB/m power dissipation of the LEP/LHC-type rigid coaxial line (copper plated stainless steel) and 1 m length, total heat load is 10 W for the resonant excitation of TM011 mode.
Further study: HOM propagating, heat load calculation, statistical approach to study heat load at 2 K (LCLS-II), trap mode … beam test?

17. Main Ring 650MHz Cavity HOM Q Limit
Monopole Mode
f (GHz)
R/Q (Ω)
Qlimit
σf = 0 MHz
σf = 0.5 MHz
σf = 5 MHz
TM011
1.173
84.8
5.1E+5
2.9E7
5.8E7
TM020
1.350
5.5
6.8E+6
3.7E7
7.5E7
Dipole Mode
R/Q (Ω/m)
TE111
0.824
832.2
2.3E+4
1.2E6
2.4E6
TM110
0.930
681.2
2.8E+4
1.5E6
3.0E6
TE112
1.225
36.2
5.2E+5
1.9E6
3.7E6
TM111
1.440
101.5
1.9E+5
1.0E7
2.0E7
Large HOM frequency spread from cavity to cavity relaxes the Q requirement. However, some modes far above cut-off frequency may become trapped among cavities in the cryomodule due to the large frequency spread
Further work: enlarge the iris diameter to decrease loss factors while keep relatively high R/Q and low surface field of the fundamental mode, identify trapped modes within the cavity and cryomodule, reduce the cavity cell number or design asymmetry end cells to avoid trapped modes (e.g. TE121)

18. Booster 1.3GHz Cavity Impedance

19. Transverse Mode Coupling Instability (TMCI) at Booster Injection Energy (6 GeV)
The single bunch threshold current is 27 A (I0th=1.36mA).
This is higher than the design bunch current, but doesn’t leave enough safe margin.
 Objects
number
ky [V/pC]
Resistive wall (Al) 1
4.4104
RF cavities
256
5.7103
The transverse kick factor is dominated by the resistive wall impedance.
More impedance objects will be included when the vacuum components are designed.

20. Coupled Bunch Instability at Booster Injection Energy (6 GeV)
Monopole Mode
f (GHz)
R/Q (Ω) Q*
f ** (MHz)
TM011
2.450
156
58600 9
TM012
3.845 44
240000 1
Dipole Mode
R/Q (Ω/m) Q
TE111
1.739
4283
3400 5
TM110
1.874
2293
50200
TM111
2.577
4336
50000
TE121
3.087
196
43700
* Data of TESLA 9-cell cavity
** J. Sekutowicz. EuCARD Editorial Series on Accelerator Science and Technology, Vol.01 (2007)
Most dangerous longitudinal mode growth rate: 472 ms.
Most dangerous transverse mode growth rate: 22 ms.
Feedback system will be needed and able to mitigate the multi-bunch instabilities. 20 20

21. ~100 kW per cavity if 800 MHz cavity! 50MHz 4MHz
Parameter
LEP2
FCC-ee Z
Z (c.w.) W H t
Ebeam [GeV]
104 45 80
120
175
circumference [km]
26.7
100
current [mA]
3.0
1450
1431
152 30
6.6
PSR,tot [MW] 22
no. bunches 4
16700
29791
4490
1360 98
Nb [1011]
4.2
1.8
1.0
0.7
0.46
1.4
ex [nm] 29
0.14
3.3
0.94 2
ey [pm]
250 60 1
b*x [m]
1.2
0.5
b*y [mm] 50
s*y [nm]
3500 32 84 44
sz,SR [mm]
11.5
1.64
2.7
1.01
0.81
1.16
sz,tot [mm] (w beamstr.)
2.56
5.9
1.49
1.17
hourglass factor Fhg
0.99
0.64
0.79
0.80
0.73
L/IP [1034 cm-2s-1]
0.01 28
212 12 6
1.7
tbeam [min]
434
298 39 73 21
F. Zimmermann, HF2014
10% HOM power
of CEPC
20 times HOM power
of CEPC.
~100 kW per cavity if 800 MHz cavity!
50MHz
4MHz
28.8 nC
7 nC

22. Thin Film - Nb3Sn for CEPC?
S. Posen & M. Liepe, PRSTAB (2014)

23. Summary and Outlook http://cepc.ihep.ac.cn
CEPC SRF system: unique challenge and opportunity for China & world SRF accelerator community. Need strong international collaboration (TTC) to succeed on schedule.
Three main challenges, especially the HOM power issue
Huge HOM power extraction with low heat load (efficient, low cost)
Very high power CW coupler (robust, clean assembly and low heat load)
Cavity with very high Q0 at MV/m (use state-of-the-art technology)
SRF R&D planned for extensive development of key technology, personnel, infrastructure and industrialization.
Cooperation or synergy with other projects (R&D and industrialization): CADS & PIP-II, Euro-XFEL, LCLS-II, ERLs & ILC, FCC-ee

24. 秦皇島 Qinghuangdao

25. Back ups

26. Yifang Wang, IHEP

27. Yifang Wang, IHEP

28. Yifang Wang, IHEP

29. CEPC SRF Injector Linac Parameters
Unit
Value
RF voltage GV 6
RF frequency
GHz
1.3
Bunch charge for CEPC nC
3.2
Bunch repetition rate for CEPC Hz
100
Bunch charge for XFEL pC
Bunch repetition rate for XFEL
MHz 1
Aver. beam current for XFEL mA
0.1
Cavity CW gradient
MV/m 20
Cavity effective length (9-cell) m
1.038
Number of cavities
289
Cavities in one cryomodule 8
Cryomodule length
12.2
Number of cryomodules 36
SRF Linac length
500
Parameter
Unit
Value
R/Q Ω
1036
Geometry factor
270
Iris diameter mm 70
Operating temperature K 2
Cavity quality factor
2×1010
4K equiv cryogenic installed kW 40
Total cryogenic AC power MW
8.5
External Q of input coupler
2E+07
RF power per cavity
6.4
Solid State Amplifier power 8
Number of SSA
289
Total RF source power
2.3
Total RF AC power
4.6
RF&Cryo AC power
13.1
Alternative to normal conducting linac. Simultaneous XFEL user facility?

30. CEPC Cryomodule Layout
Euro-XFEL/ILC/LCLS-II type X
Enlarge two- phase pipe and chimney. Omit 5K shield while keep 5K intercept for power coupler.
As simple as possible.
scaled from 1.3 GHz cryomodule X

31. Cavity Design Parameters
Unit
Main Ring
Booster
Cavity frequency
MHz
650
1300
Number of cells - 5 9
Cavity effective length m
1.154
1.038
Cavity iris diameter mm
156 70
Beam tube diameter
170 78
Cell-to-cell coupling
3 %
1.87 %
R/Q Ω
514
1036
Geometry factor
268
270
Epeak/Eacc
2.4 2
Bpeak/Eacc
mT/(MV/m)
4.23
4.26
Cavity longitudinal loss factor* k∥ HOM
V/pC
1.8
3.34
Cavity transverse loss factor* k⊥
V/pC/m
35.3
Qualified gradient
MV/m 20
23.5
Qualified Q0 at qualified gradient
4E10
2E10
* main ring bunch length 2.65 mm, booster bunch length 2.66 mm

32. FNAL 650 MHz Single Cell Cavity
Melnychuk, IPAC2014
CEPC new
PIP-II (Project X)
CEPC old
Bp/Eacc=3.75
CEPC cavity with large iris Bp/Eacc = 4.2

33. Booster Cavity Bandwidth
Qopt = 4E7 (BW 33 Hz, LCLS-II), df = 10 Hz, 10 kW input power, SSA kW, hard for transient LLRF control. LLRF control difference between booster and SRF linac with low beam loading?
Baseline: over coupled (Qext =1E7, df = 50 Hz ), 20 kW input power, SSA 25 kW