CEPC Booster Design Huiping Geng Presented for Chuang

CEPC Booster Design Huiping Geng Presented for Chuang Zhang FCC Week 2015, 23 -27 March, 2015 Marriot Georgetown Hotel

The CEPC Booster General description Lattice Low injection energy issues Beam transfer Summary

1. General Description Booster is in the same tunnel of the CEPC collider, and will be installed on its up-side with about the same circumference as the collider, while bypasses are arranged to keep away from detectors. Providing beams for the collider with top-up frequency up to 0. 1 Hz. Using 1. 3 GHz RF system; The injection energy of the booster is 6 Ge. V; Magnetic Field is as low as 31 Gs at injection.

The CEPC Layout ½ RF RF IP 1 RF P. S. BTCe+ 8 arcs 5852. 8 m each ½ RF (4 IPs, 1132. 8 m each) BTCe- One collider RF station: Ø 650 MHz five-cell SRF cavities; Ø 4 cavities/module Ø 12 modules, 8 m each Ø RF length 120 m D = 17. 428 km IP 4 RF IP 2 RF C = 54. 752 km 4 arc straights 849. 6 m each RF • LTB IP 3 P. S. • RF P. S. • Linac ½ RF • One booster RF station: 1. 3 GHz 9 -cell SRF cavities; 8 cavities/module 4 modules, 12 m each RF length 48 m

Main parameters of CEPC booster Single bunch injection from linac (E=6 Ge. V, Ip=3. 2 n. C, frep=50 Hz, ex, y=0. 1 -0. 3 mm mrad) to booster; Assuming 5% of current decay in the collider between two top-ups ; Booster operates with repetition frequency of 0. 1 Hz. Overall efficiency from linac to the collider is assumed as 90 %. SR power density of 45 W/m is much lower than in BEPCII of 415 W/m.

2. Lattice Similar arc arrangement to the collider Simple structure: FODO cells + Disp. Suppr. + Straight / bypass Cell length： To optimize cell number, emittance and aperture Straight sections For RF cavities, injection, extraction, etc. 6

2. 1. Choice of cell length

2. 2 Lattice functions: booster vs. collider FODO cell ARC

SUP and Ring Lattice: booster vs. collider SUP Ring

2. 3 Bypasses Two bypasses are arranged to skirt the detectors at IP 1 and IP 3 of the collider. 10

Bypasses Length of half bypass: L=(4+4 f 1+1. 5 f 2) Lc Width of the bypass: W=(9. 5+9 f 1) c. Lc L = 10. 5 Lc= 752. 482 m (f 1=1. 0, f 2=1. 0) W 18. 5 c. Lc = 13. 0 m (f 1=1. 0) (MAD: 12. 662 m) By adjusting f 1 and f 2, both length and width of bypass can be adjusted to fit the FFS length and detector width. No additional bending cell is required! 11

The bypass lattice ARC DIS BPI 1 RFC -DIS FODO DIS BPI 2

Orbit length change 3 Lc 4(Lc+Dl) 7 Lc 3 Lc 4(Lc+Dl) 15 Lc 3 Lc 4 Lc 7 Lc 4 Lc 3 L c s LBP = LAS+SS+8 Dl =21 LC + DL DL=8 Dl =LC d =Lc(1/cos 3 C-1) DL=0. 25 m MAD calculation: DL = 2 0. 1934 = 0. 3868 m To increase the straight length from 7 Lc to 7 Lc+2 DL LBooster=Ccollider+2 DL = 54752. 7936 m

2. 4 Dynamic aperture With two family sextupoles, xc=0. 5 n y (s y ) r = ey/ex = 0. 01 Dp/p = +2% Dp/p = +1% Dp/p = 0 Dp/p = -1% Dp/p = -2% n x (s )x

Lattice Parameters

3. Low injection energy and low field issue The bending field of CEPC booster is 614 Gs at 120 Ge. V; To reduce the cost of linac injector, the injection beam energy for booster is chosen as low as 6 Ge. V with the magnetic field of 30. 7 Gs. It needs to be tested if the magnetic field could be stable enough at such a low field against the earth field of 0. 5 -0. 6 Gs and its variation? Try to do magnetic measurement using existing magnet at low field strength.

3. 1 Low field stability test A BEPC bending magnet: B 0=9028 Gs @ IB=1060 A; Power supplies for ADS: 3 A/5 V, 15 A/8 V ; DI/I < 1 10 -4。 BHall @ system IB=3 A 。 0=30 Gs probe will be used for field measurement; DB= 0. 1 Gs DB/B 0~3 10 -3。

Magnetic field stability measured in 24 hours IB=3 A, B 30 Gs, s. B=7 10 -4 IB=6 A, B 60 Gs, s. B=1. 3 10 -3 s. B=1. 8 10 -3 IB=9 A, B 90 Gs, s. B=1. 8 10 -3

Low field stability test The earth field outside the magnet: Bx=0. 55 0. 026 Gs, By=0. 45 0. 027 Gs, Bz=0. 25 0. 03 Gs B= 0. 8 0. 04 Gs Inside the magnet, By=7. 0 0. 05 Gs is dominated by residual field, Bx=0. 4 0. 04 Gs reduced due to the shielding while Bz=0. 25 0. 03 Gs. The reason of the measured field variation (field itself or measurement error) is being investigated; The 24 h field stability (s. B) for 30 Gs-150 Gs is about (1 -2) 10 -3; The magnet ramps smoothly around the low fields with accuracy better than 1 10 -3; The field error DBy/By 10 -4 for x (-60, 60) mm and By (30 -150) Gs The injected beam energy for booster of 6 Ge. V could be feasible in view of magnetic field stability.

Mitigation Wiggling band scheme Increase linac energy 10 Ge. V, 12 Ge. V Accumulating pre-booster

3. 3 Instability issues Beam stability at injection is concerned Ebooster, inj=0. 05 Ecollider vs. Ibooster, =0. 05 Icollider; Almost no synchrotron radiation damping; HOM of 1. 3 GHz SC cavities, CB instability; Resistive wall instability; Transverse mode coupling instability; ECI and ion effects? Bunch-by-bunch feedback to stabilize beams.

Instability issues (N. Wang) The transverse mode coupling The resistive wall instability Considering the impedance generated from the resistive wall and the RF cavity, the single bunch threshold current of 27 A, higher than the design bunch current of 18 A, but doesn’t leave much margin. The growth time for the most dangerous mode is 34 ms in the vertical plane. The growth rate is much shorter than the radiation damping time, transverse feedback system is needed to stabilize the beams.

The growth rate of the first few HOM’s * k∥ mode= 2πf·(R/Q)/4 [V/p. C] ** k⊥ mode = 2πf·(R/Q)/4 [V/(p. C·m)] Longitudinal (td < 0. 5 s) and transverse (td< 20 ms) feedback systems should be equipped to stabilize beams.

4. Beam transfer e+ inj. e- inj.

4. 1 Transfer From Linac to Booster Matching section Vertical slope line Match and Switch Arcs Match to the booster

Vertical slope line 4 FODO match from linac Slope = 1: 10, L~500 m, Dx, max~2 m Vertical slope line Match to e arcs

Match from linac to booster Matching Switch e+ (or e-) arc From linac to the end of VSL Match and switch to e Arc Match to booster MTB From Linac to Booster

4. 2. Beam injection to booster e beams are injected from outside of the booster ring; Horizontal septum is used to bend beams into the booster; A single kicker downstream of injected beams kick the beams into the booster orbit.

4. 3. Transfer from booster to collider C. S. , H I. S. , H C. S. , V I. S. , V

Booster ejection Single kicker + 4 orbit bumps are used for beam extraction vertically from the booster; Septum magnets are applied to bend beams vertically into BTC; Maximum extraction rate is 100 Hz.

Vertical transfer

Summary Conceptual design study on CEPC-Booster has been carried out; There is no showstopper found in the design, from the point of view of lattice, bypasses, dynamic aperture, beam transfer and requirement to technical systems. The issues related to the low energy injection remain a central concern in the design. The schemes of extending the linac injection energy and/or adding a pre-booster are being considered. There are some technical challenges, such as the low HOM in 1. 3 GHz SC cavities, supports & alignment etc. The design study will keep moving on.

Our Pre-CDR is available at: • http: //cepc. ihep. ac. cn/pre. CDR/volume. html

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