CEPC optics and booster optics Workshop on the Circular Electron-Positron Collider - EU edition CEPC optics and booster optics Dou Wang on behalf CEPC AP group May 24~26, 2018. Rome, Italy.

Outline Introduction Optics for collider ring Optics for booster Summary

Introduction Injection energy: 10GeV 100km Injection energy: 10GeV 10 GeV linac provides electron and positron beams for booster. Top up injection for collider ring ~ 3% current decay Booster is in the same tunnel as collider ring, above the collider ring. Double ring collider with 2 IPs, crab-waist collision Collider ring adopt twin-aperture quadrupoles and dipoles in the ARC

CEPC CDR Parameters Higgs W Z（3T） Z（2T） Number of IPs 2   Higgs W Z（3T） Z（2T） Number of IPs 2 Beam energy (GeV) 120 80 45.5 Circumference (km) 100 Synchrotron radiation loss/turn (GeV) 1.73 0.34 0.036 Crossing angle at IP (mrad) 16.5×2 Piwinski angle 2.58 7.0 23.8 Number of particles/bunch Ne (1010) 15.0 12.0 8.0 Bunch number (bunch spacing) 242 (0.68s) 1524 (0.21s) 12000 (25ns+10%gap) Beam current (mA) 17.4 87.9 461.0 Synchrotron radiation power /beam (MW) 30 16.5 Bending radius (km) 10.7 Momentum compact (10-5) 1.11  function at IP x* / y* (m) 0.36/0.0015 0.2/0.0015 0.2/0.001 Emittance ex/ey (nm) 1.21/0.0031 0.54/0.0016 0.18/0.004 0.18/0.0016 Beam size at IP sx /sy (m) 20.9/0.068 13.9/0.049 6.0/0.078 6.0/0.04 Beam-beam parameters x/y 0.031/0.109 0.013/0.106 0.0041/0.056 0.0041/0.072 RF voltage VRF (GV) 2.17 0.47 0.10 RF frequency f RF (MHz) (harmonic) 650 (216816) Natural bunch length z (mm) 2.72 2.98 2.42 Bunch length z (mm) 3.26 5.9 8.5 HOM power/cavity (2 cell) (kw) 0.54 0.75 1.94 Natural energy spread (%) 0.1 0.066 0.038 Energy acceptance requirement (%) 1.35 0.4 0.23 Energy acceptance by RF (%) 2.06 1.47 1.7 Photon number due to beamstrahlung 0.29 0.35 0.55 Lifetime _simulation (min) Lifetime (hour) 0.67 1.4 4.0 2.1 F (hour glass) 0.89 0.94 0.99 Luminosity/IP L (1034cm-2s-1) 2.93 10.1 16.6 32.1

Collider ring optics-IR Crab-waist scheme* with local chromaticity correction L*=2.2m, C =33mrad, GQD0 =136T/m, GQF1 =111T/m, LQD0 =2.0m, LQF1 =1.48m Asymmetric IR layout (ref to: K. Oide, ICHEP16). IP upstream: Ec < 120 keV within 400m, last bend Ec = 45 keV IP downstream: Ec < 300 keV within 250m, last bend Ec = 97 keV up to 3rd order chromaticity, 3rd and 4th order RDT, 1st order tune shift corrected Break down of -I due to energy deviation corrected with ARC sextupoles geometry @IR *M. Zobov et al.， Test of “Crab-Waist” Collisions at the DAΦNE Φ Factory, Phys. Rev. Lett. 104, 174801(2010)

Collider ring optics-ARC Distance of two ring is 0.35m to adopt twin-aperture Q & B magnets (save power & cost). FODO cell, 90/90, non-interleaved sextupole scheme

Collider ring optics-RF Common cavities for Higgs mode, bunches filled in half ring for e+ and e-. An electrostatic separator combined with a dipole magnet - bend only one beam (ref: K. Oide, ICHEP16) Dedicated small beta function to reduce the multi-bunch instability caused by RF cavities Independent cavities for W & Z mode, bunches filled in full ring. The outer diameter of RF cavity is 1.5m. Distance of two ring is 1.0m. Z mode: Bunch spacing > 25ns Separator Esep=1.8 MV/m , Lsep=50m

Collider ring optics-injection Independent magnets for two rings 0.3m of longitudinal distance between two quadrupoles of two rings. Phase tuning Injection x=600 m at injection point for off-axis injection Injection section 66.7 m

Injection scheme Off-axis injection with horizontal bump On-axis injection in vertical plane with vertical local bump

DA optimization Multi-objective optimization based on differential evolution algorithm (MODE) SAD is used Track for a transverse damping time 10 IR sexts + 32 arc sexts +8 phase (Max. free various=254) RF ON (SR oscillation) Damping at each element Radiation fluctuation ON (100 samples) Sawtooth on with tapering Crab waist=100% Coupling=0.26% Higgs Coupling=0.3% W Coupling=2.2% DA requirement  Higgs W Z with on-axis injection 8x 12y1.35% - with off-axis injection 13x12y1.35% 15x 9y0.4% 17x 9y0.23% Z

Collider ring error study-1 LOCO based on AT is used to correct COD, beta, dispersion and coupling. Tracking DA by SAD. About 1500 BPMs, 1500 horizontal correctors and 1500 vertical correctors are placed in the collider ring. (4 per betatron wave) Correction steps: 1/5 quadrupole (~600 of 3000 quadrupoles) for optics correction About 200 Skew-quads (1/5 sextupoles) are used in the coupling correction Component x (mm) y (mm) z (mrad) Dipole 0.05 0.1 Quadrupole w/o FF 0.03 Sextupole 1. correct COD w/o sextupoles 2. correct optics w/ sextupoles 3. correct coupling Component Field error Dipole 0.01% Quadrupole (w/o FF) 0.02%

Collider ring error study-2 Residual COD  30 um (x) after orbit correction Residual COD  50 um (y) after orbit correction (βxy_err/βxy)max =0.01 after optics correction except for the IR region Coupling=0.15% after coupling correction

Higgs DA with errors Misalignment Nominal field errors (B/Q) W/O errors W errors & corrections Misalignment Nominal field errors (B/Q) After corrections Coupling=0.26% Higgs DA Requirement  Realize on-axis injection 8x 12y 1.35% 16x15y 1.5% off-axis injection 13x12y

Booster parameters @ injection (10GeV)   H W Z Beam energy GeV 10 Bunch number 242 1524 6000 Threshold of single bunch current A 25.7 Threshold of beam current (limited by coupled bunch instability) mA 127.5 Bunch charge  nC 0.78 0.63 0.45 Single bunch current 2.3 1.8 1.3 Beam current 0.57 2.86 7.51 Energy spread % 0.0078 Synchrotron radiation loss/turn keV 73.5 Momentum compaction factor 10-5 2.44 Emittance nm 0.025 Natural chromaticity H/V -336/-333 RF voltage MV 62.7 Betatron tune x/y/s 263.2/261.2/0.1 RF energy acceptance 1.9 Damping time s 90.7 Bunch length of linac beam mm 1.0 Energy spread of linac beam 0.16 Emittance of linac beam 40~120

Booster parameters @ extraction   H W Z Off axis injection On axis injection Beam energy GeV 120 80 45.5 Bunch number 242 235+7 1524 6000 Maximum bunch charge nC 0.72 24.0 0.58 0.41 Maximum single bunch current A 2.1 70 1.7 1.2 Threshold of single bunch current 300 Threshold of beam current (limited by RF power) mA 1.0 4.0 10.0 Beam current 0.52 2.63 6.91 Injection duration for top-up (Both beams) s 25.8 35.4 45.8 275.2 Injection interval for top-up 73.1 153.0 438.0 Current decay during injection interval 3% Energy spread % 0.094 0.062 0.036 Synchrotron radiation loss/turn 1.52 0.3 0.032 Momentum compaction factor 10-5 2.44 Emittance nm 3.57 1.59 0.51 Natural chromaticity H/V -336/-333 Betatron tune x/y 263.2/261.2 RF voltage GV 1.97 0.585 0.287 Longitudinal tune 0.13 0.10 RF energy acceptance 1.8 Damping time ms 52 177 963 Natural bunch length mm 2.8 2.4 1.3 Injection duration from empty ring h 0.17 0.25 2.2

Booster optics - ARC 90/ 90 FODO cell Dispersion suppressor 2 cells @ booster = 3 cells @ collider FODO length: 101m Noninterleave sextupole scheme Dispersion suppressor - two standard FODO cell - adjust bend strength- match the geometry of collider ring Dispersion suppressor FODO cell

Booster optics - RF Booster RF straight section at the same location as collider ring Low average beta to reduce the multi-bunch instability by RF cavities - 90/ 90 FODO cell - Average beta: 30 m - Space between quadrupoles :12m RF section

Booster optics – IR bypass In CEPC detector region, booster bypasses the collider ring from the outer side.

Booster geometry Booster has almost same geometry as collider ring except for the two IRs. ARC: booster is in between the two beams of collider ring, error=0.17m -- precision of element length: ~10-5 m -- precision of dipole angle: ~10-7 rad IR: separation between detector center and booster: ~25 m

Booster DA 10GeV 120GeV BSC w damping DA requirement DA results H V Parameters Dipole Quadrupole Sextupole BPM (10Hz) Transverse shift x/y (μm) 50 70 Accuracy (m) 110-7 Longitudinal shift z (μm) 100 150 Tilt (mrad) 10 Tilt about x/y (mrad) 0.2 Gain 5% Tilt about z (mrad) 0.1 Offset after BBA(mm) 3010-3 Nominal field 310-4 210-4   10GeV 120GeV BSC w damping   DA requirement DA results H V 10GeV (x= y =120nm) 4x +5mm 4y +5mm 7.7x +5mm 14.3y +5mm 120GeV (x=3.57nm, y= x*0.003) 6x +3mm 16y +3mm 21.8x +3mm 1006y +3mm

Eddy current effect During ramping, parasitic sextupole field is induced in dipoles due to eddy current. Ramping rate is limited by eddy current effect. Dedicated ramping curve to control the maximum K2. Maximum K2=0.000345 (m-3) Chromaticity distortion needs to be corrected during ramping. Dynamic chromaticity correction and dynamic DA is under studying.

Low field dipole magnets Challenges: Solutions in CDR Field error <29Gs*0.1%=0.029Gs  how to design Field reproducibility <29Gs*0.05%=0.015Gs  how to measure The Earth field ~0.2-0.5 Gs, the remnant field of silicon steel lamination ~ 4-6 Gs. With magnetic core (better material) Without magnetic core (Twice excitation current) Thinking beyond CDR Wiggler dipole scheme Combine the magnets with core and without core …

Summary Linear optics of the CEPC collider ring fulfill requirements of the parameters, geometry, photon background and key hardware. DA of collider ring can fulfill the requirements with limited errors. Further study with errors & corrections is going on. The design of booster optics and geometry fulfill the requirements. Error effects in the booster are tolerable. Eddy current effect in booster is a new issue which is under studying. Low field problem still the bottleneck for booster design. Both technical solutions and physical solutions will be explored.