1. Fusan Chen, Wen Kang, Mei Yang*
CEPC Magnet System
Fusan Chen, Wen Kang, Mei Yang*
IHEP, CAS
Workshop on the Circular Electron-Positron Collider - EU edition
Thursday 24 May Saturday 26 May 2018

2. Outline CEPC collider magnet(F. Chen, M. Yang, X. Sun)
Overview
Design of the dual aperture dipole
Design of the dual aperture quadrupole
Design of the sextupole
CEPC booster magnet(W. Kang)
Design considerations of Low field dipole magnet
Low field dipole magnet R&D
Summary

3. Collider magnet overview
The magnets cover almost 80% of the 100km ring.
* Beam center separation:350 mm
The most concern issues for collider magnet
Manufacturing cost
Power consumption
Radiation shielding
Field quality
Dipole
Quad.
Sext.
Corrector
Total
Dual aperture
2384
2392 -
13742
Single aperture
80*2+2
480*2+172
932*2
2904*2
Total length [km]
71.5
5.9
1.0
2.5
80.8
Power [MW]
7.0
20.2
4.6
2.2 34

4. Basic design consideration
To reduce the manufacturing cost
Make coil with aluminum instead of copper.
Decrease the filling factor of core to reduce the steel.
E.g. steel-concrete combined core referring to LEP.
To reduce the power consumption
Choose the gap/aperture as small as possible.
Use dual aperture dipole and dual aperture quadrupole.
Dual aperture magnet save about 50% power.
Use dipole/sextupole combined function magnet.
Reduce the sextupole strength.
Increase the cross section of the magnet coils.
High-voltage low-current design to reduce the cable loss.
Possibility of using permanent magnet.

5. Design of dual aperture dipole
The magnetic length of the dipole is 28.7m.
The core is divided into 5 segments for easy fabrication.
Different polarity sextupoles combined in the first and the last segments of one dipole
To reduce the difficulty and power consumption of individual sextupoles
No additional power supply is needed. SF SD

6. Design of dual aperture dipole
Two types dual aperture dipole segments
The first and the last segments: sextupole combined
The three middle segments: dipole only
Main coils: 1 turn  2 turns to reduce the power consumption of cables.
Trim coils:
For strength tapering
±1.5% adjustability
Radiation shielding:
30mm thickness

7. Design of dual aperture dipole.
Design parameters
Magnetic length [m]
28.686
Magntetic strength [T]
0.037
Aperture [mm] 70
Main coil
Turns 2
Material
Aluminum
Conductor specs. [mm]
30×54
Current [A]
1058
Current density [A/mm2]
0.67
Voltage [V]
2.58
Power consumption [kW]
2.73
Trim coil
1×4
60×3
16.7
0.093
0.36
0.006
Cooling water
loop number 1
pressure [kg/cm2] 6
Velocity [m/s]
1.75
Flux [l/s]
0.138
Temperature rise [°C]
4.7
Overall size (H×V×L) [mm]
530×200×30500

8. Design of dual aperture quad.
Polarity: F/D
Main coils: large cross section to reduce power consumption
Trim coils:
±1.5% adjustability
Gap between cores:
50mm
Magnetic shielding:
11.52mm
Radiation shielding:
30mm

9. Design of dual aperture quad.
Purpose of the magnetic shielding
Compensate the odd order harmonics.
The variation is proportional to the shielding thickness.
All odd order harmonics can be compensated by tuning the shielding thickness.
Bn/B2
Before shielding
After shielding 1
-1.3 3
-169.9
-0.1 4
0.4
0.5 5
-2.3
0.0 6 7
0.2 10 14

10. Design of dual aperture quad.
Design parameters
Magnetic length [m] 2
Gradient [T/m]
8.42
Aperture [mm] 76
Main coil
Turns
64×2
Material
Aluminum
Conductor specs. [mm]
11×11, φ7, R1
Current [A]
154
Current density [A/mm2]
1.89
Voltage [V]
34.1
Power consumption [kW]
5.3
Trim coil
20×4
copper
4×2, R0.2
7.2
0.9
5.4
0.04
Cooling water
loop number 4
pressure [kg/cm2] 6
Velocity [m/s]
1.3
Flux [l/s]
0.201
Temperature rise [°C]
6.3
Overall size (H×V×L) [mm]
700×600×2000

11. Design of sextupole
The sextupoles of the two rings are single aperture magnets installed side by side.
The core size is limited by the 350mm beam separation.
The space between two sextupoles is only 10mm.
Copper coils to reduce the power consumption.
Wedge shaped pole
Powered with independent ps.
No trim coil
Radiation shielding
20mm

12. Design of sextupole Design parameters
With the help of the combined function dipoles, the gradient can be halved to 253T/m2 and the power consumption is reduced to a quarter.
Type SF SD
Magnetic length [m]
0.7
1.4
Gradient [T/m2]
506*
Aperture [mm] 80
Coil
Turns
26×6
Material
Copper
Conductor specs. [mm]
7×7, φ3, R1
7×7, φ4, R1
Current [A]
168.4
Current density [A/mm2]
4.1
4.7
Voltage [V]
19.5
41.4
Power consumption [kW]
3.3
7.0
Cooling water
loop number 12
pressure [kg/cm2] 6
Velocity [m/s]
2.1
1.8
Flux [l/s]
0.175
0.269
Temperature rise [°C]
4.5
6.2
Overall size (H×V×L) [mm]
350×350×700
350×350×1400

13. Outline CEPC booster magnet(W. Kang)
CEPC collider magnet(F. Chen, M. Yang, X. Sun)
Overview
Design of the dual aperture dipole
Design of the dual aperture quadrupole
Design of the sextupole
CEPC booster magnet(W. Kang)
Design considerations
Low field dipole magnet R&D

14. Main parameters of the CEPCB magnets
Basic parameters of the three kinds of magnets

15. Low field dipole magnet
Main parameters of low dipole magnet
Challenges and design considerations
Remnant field and coercive force
Earth field effect
Large quantities cost saving
Field measurement ：NMR and wire system
Quantity
13312
Max. strength(T)
0.0614
Min. strength(T)
0.0031
Repetitive frequency(Hz)
0.1
Gap height(mm) 40
Good field region(mm) 52
Field uniformity
5e-4

16. Design considerations
Earth field effect
0.3~0.5Gs.
Unavoidable: covers 1%-2% of the min. working field.
A H type of yoke is chosen.
Closed structure has a better shielding against the earth field.

17. Design considerations
Remnant field and coercive force.
4~6 Gs at the center of the magnet gap.
Repeatability at different excitations.
Solutions:
Find low remnant field and coercive force material.
Increase the field in the iron--Dilution.
Remove the core-hollow coil scheme.

18. Design considerations
Weight and Cost saving
Less iron and cheaper material
Dilution at both directions
Aluminum coil instead of Copper
Simple structure-one turn coil
Iron laminations not machining.
Field simulation and Prototype magnet study.
The existing BH curve has no points at very low field. New BH data should be tested.
The calculated field uniformity at 29 Gs and 300 Gs are the same but have large differences when measured.

19. CEPCB first prototype magnet
Two refereces: LEP & LHeC
LEP
Packing factor=0.27
LHeC
Packing factor=0.333

20. CEPCB first prototype magnet
Core dilution in two directions
The return yoke of the core was made as thin as possible.
In the pole areas of the laminations, some holes were stamped.
Iron to aluminum ratio is 1:2.
Coil: Aluminium bus bar; One turn per pole.

21. CEPCB first prototype magnet
Measurement results by Hall probe system.
The measured remnant field in the magnet gap is about 4-6 Gs, which is 13%-20% of the low field of 30Gs.
Due to remnant field, the field errors at low field becomes 10 times larger than that at high field
Because of the remnant field effect and bad measurement precision of the Hall probe system at 30Gs, the field reproducibility at low field becomes 40 times worse than that at high field.

22. New design of CEPCB low field dipole
Dipole with the core(new design)
In order to improve the field quality at low field level, the dipole magnet with core is re-designed according to the new requirements of the CDR.
Core dilution is still used.
Silicon steel and aluminium laminations with a ratio of 1:1.
The return yoke of the cores will be made as thin as possible.
In the pole areas of the laminations, some holes will be stamped.

23. New design of CEPCB low field dipole
To compensate errors of the simulation modelization, the simulated field uniformity at the high field level is optimized to be 10 times smaller than the requirements. It means that the measured field uniformity at low field level will probably meet the requirements even though it actually becomes 10 times worse than the simulated results.
To reduce the influence of remnant field, the oriented low carbon silicon steel laminations with lower coercive force instead of non-oriented laminations will be used to stack the cores of the magnet.

24. New Design of CEPCB low field dipole
Dipole without core
Because the remnant field of the iron cores is the key element that destroys the field quality at low field level, a dipole magnet design without core is proposed.
An optimized design of dipole magnet with cosθ type coils can meet the field quality at both high and low field levels.
The iron cylinder outside of the coils is used to shield the surrounding field as well as to increase the central field.

25. New design of CEPCB low field dipole
The main disadvantage of the magnet without core is that the excitation efficiency is only 50% of that in the magnet with core.
To improve the excitation efficiency of the magnet, the top and bottom of the cylinder can be flatted, the simulation result shows that it can increase the excitation efficiency by 5%.
In addition, the flat top and bottom has a function to adjust the field distribution and improve field quality.

26. New design of CEPCB low field dipole
The upper and lower coils of the magnet are formed by two aluminum bars with the same cross section areas, each coil has two layers and two turns.
Because of the low field level and small magnetic force, for 1 m long magnet, three supporters at different positions in longitudinal direction are strong enough to fix the coil bars .
To verify the design of the magnet without core, a subscale prototype dipole magnet will be developed.

27. Summary
A twin aperture quadrupole prototype will be built to verify the design and mechanical structure. Further optimizations will be done according to the measurement results.
A subscale prototype dipole magnet for CEPCB was developed, however the measured field errors and reproducibility at low field level became 10 times worse than that at high field level unexpectedly .
New dipole magnet prototype with and without core will be constructed and studied.