1. Preliminary design of SPPC RF system Jianping DAI 2015/09/11 jpdai@ihep.ac.cn The CEPC-SppC Study Group Meeting, Sept. 11~12, IHEP

2. Contents 2  Introduction  SPPC RF parameters required by beam  Preliminary design of SPPC RF system  Summary

3. Functions of RF systems Transfer the mains power to beam power, and Accelerate the charged particle to higher energy Keep a constant energy of the charged particle in storage rings RF dependent parameters  Bunch length  Energy acceptance  Bunch life time  RF related beam instabilities 3

4. RF parameters required by beam  RF frequency  RF voltage  Cavity impedance 4

5. The main beam and RF parameters of LHC 5

6. 6 Beam parametersCollision Ring circumference (km)54.7 Proton energy (TeV)35.6 Dipole field (T)20 Dipole curvature radius (m)5928 Revolution frequency (kHz)5.481 Energy loss per turn (MeV)2.11 Momentum compaction8E-5 Synchrotron frequency (Hz)5.6 RMS bunch length (cm7.50 Energy spread1.1E-4 Longitudinal emittance [4  ] (eVs) 12.3 Energy acceptance3.4E-4 Current per beam (A)1 The main beam and RF parameters of SPPC RF parametersCollision RF frequency (MHz)400.0 Harmonic number72980 RF voltage (MV)40 RF power supplied during acceleration per beam [30min ramp] (MW) 3.4 Impedance?

7. 7 Bunch length and energy acceptance of SPPC Bunch length vs. RF voltageEnergy acceptance vs. RF voltage frf=200MHz frf=400MHz frf=200MHz frf=400MHz Vrf (MV) Bunch length (cm)Energy acceptance

8. Contents 8  Introduction  SPPC RF parameters required by beam  Preliminary design of SPPC RF system  Summary

9. 9 SPPC RF technology choice Similar to LHC, The final emittance at 35.6TeV of 13.2eVs and a maximum bunch length of 7.5cm, leads to a required maximum voltage of 40MV. Transient beam-loading, coming from the high beam current (1A) combined with the long beam gap (~us) due to the abort gap dominates the design of SPPC RF system and leads to the choice of SC cavities with wide beam aperture (~300mm) With high RF voltage per cavity and the low R/Q due to the wide aperture, the stored energy in the cavity is high and the cavity field phase swing due to reactive beam loading in the beam gap is minimized. The required voltage is achieved with fewer cavities than with a copper system Due to the wide beam aperture, the R/Q of HOMs in the cavity are lower. With strong HOM damping in the cavity, the total machine impedance can be kept low.

10. Challenges of SPPC RF system 10 During acceleration the real power supplied to the beams is relatively small (LHC 275kW/beam), but the installed power required to control these beams is much larger (LHC 2.4MW/beam). The challenge in the design of RF system is to minimize the beam handling power in order to arrive at acceptable levels for the power couplers. LHC: 8MV, 0.5A, 275kW => 2.4MW SPPC: 40MV, 1A*2, 3.4MW*2 => 30*2MW? (By scaling) 1.5MW/cavity => 40 cavities are needed!

11. 11 Cryomodules of LHC RF system Each beam: 8 single cell SC cavities, 16MV. Grouped in 2 cryomodules. 1.The cavities, with their stainless steel helium vessel 2.The mechanical tuner of 180 kHz stroke, that ensures high speed with a resolution of ~20 Hz 3.The insulation vacuum vessel, built in one piece with no welds 4.The HOM couplers, four per cavity, two for longitudinal modes and two for transverse modes 5.The main coupler, a complex mechanical construction capable of changing the coupling to the cavity by a factor of at least 10, to cope with the different modes of operation in the LHC (injection, ramping, storage) Cryomodules of SPPC RF system 40 single cell SC cavities, 40MV. Grouped in 10 cryomodules

12. 12 LHC cryomodule, 8m long, 4 cavities Partial view of the LHC cryomodule

13. 13  SC cavities, made with the same technology successfully used in the CERN LEP collider (copper cavities with a film of niobium sputtered on the RF surface)  400.8MHz, R/Q=45,  Nominal field 5.5MV/m => Vc=2MV  Beam pipe 300  HOM deep damped

14. 14 LHC variable coupler (Qe=10,000-200,000, 500kW, CW) LHC tuner mechanism

15. 15 RF power system of LHC A maximum of 4800kW of RF power will be generated by sixteen 300kW 400MHz klystrons. Each klystron will feed, via a Y-junction circulator and a WR2300 waveguide, a single-cell SC cavity.

16. 16 Low level RF system of LHC The low level RF system comprises four sub-systems:  The cavity controller,  The beam controller,  RF synchronization,  The longitudinal damper

17. 17 Cryogenic losses of LHC SC cavities Static loss per module: 150W, 4K Dynamic loss (Pc) per module: 100W at nominal field (5.5MV/m); 800W at twice the nominal field. Total cryogenic loss per module:  1000W Total cryogenic losses of SPPC SC cavities @ 4K (150w+100w)*40=10,000W

18. 18 Summary The main RF parameters of SPPC are preliminary determined as: frf=400MHz, Vrf =40MV The RF technology similar to LHC is considered: Single-cell large pipe SC cavity. The total cryogenic loss of SPPC RF system is estimated: <10kW @4K The challenge in the design of RF system is to minimize the beam handling power. (High power coupler, >1MW )

19. 19 Thanks!