CEPC Magnet System Fusan Chen Wen Kang Mei

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

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) • Overview • Design considerations of Low field dipole magnet • Low field dipole magnet R&D • Summary 2

Collider magnet overview • The magnets cover almost 80% of the 100 km ring. Dipole Quad. Sext. Corrector Total Dual aperture 2384 2392 - - 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 13742 * Beam center separation: 350 mm • The most concern issues for collider magnet • • Manufacturing cost Power consumption Radiation shielding Field quality 3

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. 4

Design of dual aperture dipole • The magnetic length of the dipole is 28. 7 m. • 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 SF SD SD SF • No additional power supply is needed. 5

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: • 30 mm thickness 6

Design of dual aperture dipole. • Design parameters Magnetic length [m] Magntetic strength [T] Aperture [mm] Turns Material Conductor specs. [mm] Current [A] Main coil Current density [A/mm 2] Voltage [V] Power consumption [k. W] Turns Material Conductor specs. [mm] Current [A] Trim coil Current density [A/mm 2] Voltage [V] Power consumption [k. W] loop number pressure [kg/cm 2] Cooling water Velocity [m/s] Flux [l/s] Temperature rise [°C] Overall size (H×V×L) [mm] 28. 686 0. 037 70 2 Aluminum 30× 54 1058 0. 67 2. 58 2. 73 1× 4 Aluminum 60× 3 16. 7 0. 093 0. 36 0. 006 1 6 1. 75 0. 138 4. 7 530× 200× 30500 7

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: • 50 mm • Magnetic shielding: • 11. 52 mm • Radiation shielding: • 30 mm 8

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/B 2 1 3 4 5 6 7 10 14 Before shielding -2132. 6 -169. 9 0. 4 -2. 3 -0. 1 0. 2 0. 0 After shielding -1. 3 -0. 1 0. 5 0. 0 0. 0 9

Design of dual aperture quad. • Design parameters Magnetic length [m] Gradient [T/m] Aperture [mm] Turns Material Conductor specs. [mm] Main coil Current [A] Current density [A/mm 2] Voltage [V] Power consumption [k. W] Turns Material Conductor specs. [mm] Current [A] Trim coil Current density [A/mm 2] Voltage [V] Power consumption [k. W] loop number pressure [kg/cm 2] Cooling water Velocity [m/s] Flux [l/s] Temperature rise [°C] Overall size (H×V×L) [mm] 2 8. 42 76 64× 2 Aluminum 11× 11, φ7, R 1 154 1. 89 34. 1 5. 3 20× 4 copper 4× 2, R 0. 2 7. 2 0. 9 5. 4 0. 04 4 6 1. 3 0. 201 6. 3 700× 600× 2000 10

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

Design of sextupole • Design parameters Type Magnetic length [m] Gradient [T/m 2] Aperture [mm] Turns Material Conductor specs. [mm] Current [A] Coil Current density [A/mm 2] Voltage [V] Power consumption [k. W] loop number pressure [kg/cm 2] Cooling Velocity [m/s] water Flux [l/s] Temperature rise [°C] Overall size (H×V×L) [mm] SF 0. 7 7× 7, φ3, R 1 4. 1 19. 5 3. 3 2. 1 0. 175 4. 5 350× 700 506* 80 26× 6 Copper 168. 4 12 6 SD 1. 4 7× 7, φ4, R 1 4. 7 41. 4 7. 0 1. 8 0. 269 6. 2 350× 1400 * With the help of the combined function dipoles, the gradient can be halved to 253 T/m 2 and the power consumption is reduced to a quarter. 12

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 • Low field dipole magnet R&D 13

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

Low field dipole magnet • Main parameters of low dipole magnet 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 5 e-4 • Challenges and design considerations • • Remnant field and coercive force Earth field effect Large quantities----cost saving Field measurement ：NMR and wire system 15

Design considerations • Earth field effect • 0. 3~0. 5 Gs. • 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. 16

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. 17

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. 18

CEPCB first prototype magnet • Two refereces: LEP & LHe. C LEP Packing factor=0. 27 LHe. C Packing factor=0. 333 19

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. 20

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 30 Gs. • 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 30 Gs, the field reproducibility at low field becomes 40 times worse than that at high field. 21

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. 1) Silicon steel and aluminium laminations with a ratio of 1: 1. 2) The return yoke of the cores will be made as thin as possible. 3) In the pole areas of the laminations, some holes will be stamped. 22

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. 23

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. 24

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. 25

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. 26

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. 27