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

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

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+)

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 34.989783 Disk thickness (t) 5.84 Iris diameter (2a) 26.220-19.093 Cavity diameter (2b) 83.458-81.762 Shunt impedance (r0) 53.708-63.294 M/m Q factor 13783-13711 Group velocity (vg/c) 0.02004-0.0063 Filling time 823 ns Attenuation parameter 0.5383 Neper

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

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

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%

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%

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 2 𝜎 ∆ 𝑝 𝑥 𝑚𝑐 2

S-band accelerating structure design Racetrack dual feed coupler design RF in CST Microwave Studio Kyhl’s method S12=0.00059@2856MHz RF out Reflection coefficient at work frequency Input coupler simulation model

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

S-band accelerating structure design Racetrack dual feed coupler design △E/E0=5.17867/4794=0.1% r=5mm Phase asymmetry: 0.06°

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

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

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 hole(2a=31mm @KEKB) accelerating structure. After the positron source, big whole structure can increase positron flux gain to 80%

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