1. Conceptual Design of CEPC Interaction Region Superconducting Magnets
Yingshun Zhu
Accelerator Center, Magnet Group
Institute of High Energy Physics, Chinese Academy of Sciences
CEPC-SPPC Symposium

2. Contents Conceptual design of superconducting magnets in single ring
Design progress of QD0 in partial double ring
Summary

3. Conceptual design of superconducting magnets in single ring
Two types high gradient quadrupole magnets are needed in CEPC Interaction Region：
There are two types of quadrupole magnets in the CEPC Interaction Region. The magnetic field at the pole region exceeds 7T, and these two magnets are inside the detector solenoid field of about 3.5T.
These quadrupole magnets are iron-free magnets, and Nb3Sn technology must be used. The quadrupole coils have the same cross section, but with different lengths.
The coils are made of Rutherford Type Nb3Sn Cables, and are clamped by stainless steel collar.
The magnetic field calculation is performed by OPERA. After optimization, detailed coil cross section is obtained.

4. 2D flux lines
Magnetic flux density distribution
Magnetic flux density distribution
3D model
Field distribution

5. Main design parameters of CEPC interaction region quadrupole magnets

6. Anti-solenoid coil
To minimize the effect of the longitudinal detector solenoid field on the accelerator beam, anti-solenoid coils are introduced just outside the quadrupoles.
The total integral longitudinal field generated by the detector solenoid and anti-solenoid coils is nearly zero.
Coils of anti-solenoid are made of NbTi-Cu Conductor.
Magnetic field calculation is performed using axi-symmetric model, and main design parameters are obtained.
2D flux lines of Anti-QD
Longitudinal field distribution of Anti-QD

7. Cross section of quadrupole coil and anti-solenoid coil

8. Design Progress of QD0 in partial double ring
Compact high gradient QD0 quadrupole magnets are needed in interaction region of the CEPC partial double ring.

9. The field gradient of QD0 is as high as 200T/m.
The minimum distance between two QD0 magnets centerline is only 45mm.The coil inner radius is 12.5mm, tight radial space is available.
Serpentine winding coil using direct winding technology is selected for its high efficiency and high compactness. (experience at BEPCII)
Serpentine coil is first developed at BNL, and has been used in BEPCII, J-PARC, ATF2, ILC baseline design, etc.
Coil winding at BNL

10. Coil turns per pole is 110, excitation current is 340A.
The QD0 magnet is iron-free, and its coils are made of 0.5mm round NbTi-Cu conductor using direct winding technology.
Eight Serpentine coil layers are used for the QD0 coils. Collar is not needed.
Coil turns per pole is 110, excitation current is 340A.
2D magnetic field calculation is performed. After optimization, the field quality for standalone QD0 is very good.
Magnetic flux density distribution
2D flux lines

11. 2D field harmonics（unit, 1×10-4）
Coil layout

12. The field of one QD0 magnet is affected by another QD0 magnet
The field of one QD0 magnet is affected by another QD0 magnet. Field cross talk of the two QD0 magnets is studied.

13. Since the small distance between the two QD0 magnets, the field cross talk is serious.
Each multipole field inside one QD0 bore is affected by the cross talk, and the largest one is a dipole field.
One layer of shield coil is introduced just outside the quadrupole coil to improve the field quality, and the effect is obvious.

14. Summary
It is challenging to develop high gradient superconducting magnets in CEPC Interaction Region.
Conceptual design of superconducting magnets in single ring is performed.
Serpentine coil using direct winding technology is suitable for QD0 in partial double ring. Cross talk effect should be further compensated in 3D field calculation.

15. Thanks for your attention!
CEPC-SPPC Symposium