1. CEPC injector high field S-band accelerating structure design and R&D
Zhang Jingru, Pei Guoxi, Wang Xiangjian
April 8th, 2016, IHEP, Beijing

2. Outline CEPC injector S-band accelerating structure design
High power test bench
Summary & future work

3. CEPC injector Linac layout Main parameters of CEPC injector Parameter
Symbol
Unit
Value
E- beam energy
Ee-
GeV 6
E+ beam energy
Ee+
Repetition rate
frep Hz 50
E- bunch population
Ne-
2×1010
E+ bunch population
Ne+
Energy spread (E+/E-) σE
<1×10-3
Emitance (E-)
0.1 mm mrad
Emitance (E+)

4. CEPC injector Main parameters of the accelerating structure
Disk loaded
travelling wave type
f0=2856MHz
constant gradient
about 3 meters long
BEPCII
Operation frequency
2856
MHz
Operation temperature
40.0 OC
Number of cells
84 regular cells
2 coupler cells
Section length
3050 mm
Phase advance per cell
2/3 – mode
Cell length
Disk thickness (t)
5.84
Iris diameter (2a)
Cavity diameter (2b)
Shunt impedance (r0)
M/m
Q factor
Group velocity (vg/c)
Filling time
823 ns
Attenuation parameter
0.5383
Neper

5. CEPC injector
BEPCII working accelerating gradient：~15MV/m (With SLED, ~1us)
BEPCII upgrade working accelerating gradient ：~22MV/m (With SLED, ~1us)
New design: Up to 30MV/m

6. S-band accelerating structure design
Cavity shape optimization
Racetrack dual feed coupler design

7. S-band accelerating structure design
Cavity shape optimization
Superfish
The first cavity of the structure
Q factor increase more than 5.4%, shunt impedance increase 6.8%
Q=13680,R=49.6Mohm/m
Q factor increase more than 12%, shunt impedance increase 10.9%

8. S-band accelerating structure design
Cavity shape optimization
Ellipticity 1
1.1
1.3
1.6
1.8 2
kilp
0.067
0.065
0.0622
0.059
0.058
0.06
Emax/E0
3.086
2.9752
2.8457
2.7035
2.685
2.7486
Decrease 13%

9. S-band accelerating structure design
Racetrack dual feed coupler design
The time dependent multipole fields in the coupler induce a transverse kick along the bunch
The transverse variation of Ez field creates a transverse magnetic field which deflects the beam
Offsetting the coupler center
About 1°phase asymmetry
Ideal electromagnetic structure
Center bias coupler
center
∆ 𝑝 𝑥 =- 𝑒∆𝑧 𝐸 0 2ω𝑎 [∆𝜃∗𝑠𝑖𝑛𝜑− ∆𝐸 𝐸 0 cos𝜑]
Dual feed coupler
Dual feed racetrack coupler
𝜀 𝑛−𝑓𝑖𝑛𝑎𝑙 = 𝜀 𝑛−𝑖𝑛𝑖𝑡𝑖𝑎𝑙 2 + 𝜎 𝑥 𝜎 ∆ 𝑝 𝑥 𝑚𝑐 2

10. S-band accelerating structure design
Racetrack dual feed coupler design
RF in
CST Microwave Studio
Kyhl’s method
RF out
Reflection coefficient at work frequency
Input coupler simulation model

11. S-band accelerating structure design
Racetrack dual feed coupler design
RF in
RF out
Longitudinal electric field on axis
Phase shift per cavity
Input coupler simulation model

12. S-band accelerating structure design
Racetrack dual feed coupler design
△E/E0= /4794=0.1%
r=5mm
Phase asymmetry: 0.06°

13. High power test bench
Lack of accelerating structure high power characteristic measurement system

14. High power test bench
Breakdown locations & Breakdown rate (BDR) By acoustic sensors
Measurement of dark current Picoammeter / Analyzer magnet / Faraday cup 1 2

15. Summary & future work Optimized first cavity size
Finished dual feed input coupler design Preliminary
Considering improvement high power test bench
Design the whole structure
R&D big beam accelerating structure. After the positron source, big whole structure can increase positron flux gain to 80%

16. Thanks!