1. Design Progress of CEPC Interaction Region Superconducting Quadrupole and Sextupole magnets
Yingshun Zhu
Accelerator Center, Magnet Group
Institute of High Energy Physics, Chinese Academy of Sciences
CEPC-SppC Study Group Meeting

2. Contents Introduction
Design progress of Superconducting Magnets in CEPC Interaction
Region
QD0, Anti-solenoid, Sextupole magnet
Summary

3. Introduction
Most magnets needed for CEPC Accelerator are conventional magnets.
Several superconducting magnets are required in CEPC Interaction Region.
Compact high gradient QD0 quadrupole magnets are needed in interaction region of the CEPC partial double ring/ double ring.
To minimize the effect of the longitudinal detector solenoid field on the accelerator beam, compensating solenoid and screening solenoid are needed.
In addition, high strength superconducting sextupole magnet is needed.

4. Design Progress of QD0
Compact high gradient QD0 quadrupole magnets are needed in interaction region of the CEPC partial double ring/double ring.
(L*=1.5m, crossing angle 30 mrad)
QD0 is a double aperture superconducting magnet. Its field gradient is 200T/m, which is the highest in present Electron machines.
The minimum distance between two aperture centerline is only 45mm, and the coil inner radius is 12.5mm. Very 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, Super-KEKB, ILC QD0 baseline design, etc.

5. Four pole coils in one layer is wound once.
Serpentine coil is a novel coil pattern developed at BNL for direct winding of superconducting magnets. It allows winding a coil layer of an arbitrary multipolarity in one continuous winding process and greatly simplifies the magnet design and production.
Four pole coils in one layer is wound once.
Planar view of Serpentine winding
Coil winding at BNL

6. The QD0 magnet is iron-free small-aperture long magnet.
Its coils will be 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 using OPERA. After optimization, the field quality in each aperture is very good.
Magnetic flux density distribution
2D flux lines

7. 2D field harmonics（unit, 1×10-4）
Coil layout (single aperture)

8. The field in one aperture is affected by the coil in another aperture
The field in one aperture is affected by the coil in another aperture. Field cross talk of the two apertures is studied.

9. Multipole field in each aperture with different aperture central distance（unit, 1×10-4）

10. Since the small distance between the two QD0 magnets, the field cross talk is serious.
Each multipole field inside one bore is affected by the cross talk, and the most serious one is the sextupole field.
One layer of shield coil is introduced just outside the quadrupole coil to improve the field quality, and the effect is obvious. Each integrated multipole field can be reduced to below 5×10-4.

11. Main design parameters of QD0.

12. Design Progress of sextupole magnet
Requirement of superconducting sextupole magnet in CEPC interaction region.
Since the field in the sextupole magnet is not very high, Serpentine sextupole coil using direct winding technology is used.
The conductor of sextupole coil is the same as in QD0.
The sextupole magnet is also iron-free.
Eight Serpentine coil layers are used. Coil turns per pole is 104.

13. 2D magnetic field calculation is performed
2D magnetic field calculation is performed. After optimization, the layout of the sextupole coil is obtained.
2D flux lines
Magnetic flux density distribution

14. Main parameters of sextupole magnet
Field harmonics（unit, 1×10-4）

15. Anti-solenoid coil
To minimize the effect of the longitudinal detector solenoid field on the accelerator beam, solenoid coils are introduced.
The total integral longitudinal field generated by the detector solenoid and solenoid coils should be zero.
Screening solenoid (at the same location of QD0): The longitudinal field inside the quadrupole bore should be 0.
Center field of screening solenoid :3.3T, magnetic length: the same as QD0.
Compensating solenoid options (before QD0):
1) length 1m, Center field: 5.2T (NbTi technology )
2) length 0.7m, Center field: 7.4T (NbTi technology )
3) length 0.4m, Center field: 13T (Nb3Sn technology )

16. Anti-solenoid coil
Coil of screening solenoid will be made of NbTi-Cu Conductor.
Because of the limited available space in the CEPC Interaction Region, Nb3Sn technology is chosen for Compensating solenoid with a central field of 13T.

17. Field calculation of solenoid coil
Magnetic field calculation is performed using axi-symmetric model in OPERA, and main design parameters are obtained.
Flux lines of compensating solenoid
Field distribution of compensating solenoid
Flux lines of screening solenoid

18. Preliminary design parameters of solenoids:

19. Summary
It is challenging to develop high strength superconducting magnets in CEPC Interaction Region.
Serpentine coil using direct winding technology is suitable for QD0 and sextupole magnet. Cross talk effect between two apertures in QD0 can be compensated to be acceptable.
Due to the limited longitudinal space, high field compensating solenoid with a central field of 13T is used.
Conceptual design of superconducting magnets in CEPC Interaction Region is performed, and some prototypes will be proposed.

20. Thanks for your attention!
CEPC-SppC Study Group Meeting