Yingshun Zhu Design progress of QD0 in CEPC Interaction Region Accelerator Center, Magnet Group Institute of High Energy Physics, Chinese Academy of Sciences 2016.5.4

Contents Overview of final focus quadrupole magnets in Interaction Region Design progress of QD0 in CEPC partial double ring Summary

Overview of final focus quadrupole magnets in Interaction Region Final focus quadrupole magnets in Interaction Region of high energy colliders： BEPCII. SuperKEKB CLIC ILC FCC-ee

Final focus quadrupole magnets in BEPCII Interaction Region：

Superconducting magnets in BEPCII Interaction Region are designed and developed by IHEP and BNL. Serpentine coil is used for quadrupole coil, and is wound layer by layer using direct winding technology at BNL.

The development of Backup Superconducting magnet for BEPCII Interaction Region is in progress. A Superconducting model quadrupole magnet is fabricated ; the development of automatic winding machine using direct winding technology is to be finished.

Overview of final focus magnets in Interaction Region Final focus quadrupole magnets in SuperKEKB Interaction Region ：

Overview of final focus magnets in Interaction Region Final focus quadrupole magnet QD0 in CLIC Interaction Region ：

Overview of final focus magnets in Interaction Region Final focus quadrupole magnet QD0 in ILC Interaction Region (baseline) ：

Overview of final focus magnets in Interaction Region Final focus quadrupole magnet QD0 in FCC-ee Interaction Region ： CERN model of CCT IR quadrupole BINP prototype IR quadr. 2 cm aperture, 100 T/m

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.

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

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

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

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.

The ideal way for reducing the field cross talk is to add a layer of quadrupole shield coil with minus polarity. The quadrupole shield coil reduces the central field gradient. So it is not suitable since the limited radial space.

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.

Anti-solenoid coil To minimize the effect of the longitudinal detector solenoid field on the accelerator beam, anti-solenoid coils are introduced. The integral longitudinal field generated by the detector solenoid and anti-solenoid coils should be zero. Screening solenoid (at the same location of QD0): The longitudinal field inside the quadrupole should be 0. Center field of screening solenoid :3.3T, magnetic length: the same as QD0. Compensating solenoid (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 )

Summary Several final focus quadrupole magnets in Interaction Region of high energy colliders in the world are reviewed.. It is challenging to develop high gradient superconducting magnets in CEPC Interaction Region. Serpentine coil using direct winding technology is suitable for QD0 in partial double ring. It is quite difficult to reduce the field cross talk effect since the limited radial space; The space available for anti-solenoid coils is also very limited.

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