Design Study of the CEPC Booster C. Zhang, IHEP HF2014 October 11, 2014

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 installed in its up-side with same circumference as the collider, while bypasses are arranged to keep away from detectors. Provide beams for the collider with top-up frequency of 0.1 Hz. Use 1.3GHz RF system; Booster injection energy is 6 (10) GeV; Field is only 30 (50) Gs at injection.

One collider RF station: One booster RF station: The CEPC Layout ½ RF ½ RF One collider RF station: 650 MHz five-cell SRF cavities; 4 cavities/module 12 modules, 8 m each RF length 120 m P.S. RF IP1 RF (4 IPs, 1132.8 m each) BTCe+ BTCe- 8 arcs 5852.8 m each D = 17.428 km RF IP4 IP2 RF C = 54.752 km One booster RF station: 1.3 GHz 9-cell SRF cavities; 8 cavities/module 4 modules, 12 m each RF length 48 m 4 arc straights 849.6 m each RF LTB IP3 P.S. RF P.S. Linac ½ RF ½ RF

Main parameters of CEPC booster Single bunch injection from linac 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 45W/m is much lower than in BEPCII of 415W/m.

CEPC Linac for booster injection X.P. Li Conventional S-band linac with frequency of 2856MHz; 4GeV 10nC electron beam on the positron converter; 0.2 GeV positron recycling line.

2. Lattice Similar arc arrangement to the collider Simple structure: 60FODO cells + dispersion suppressor + bypass Cell length： To optimize cell (magnet) number, emittance and aperture Straight sections For RF cavities, injection, extraction, etc.

Lattice parameters scale to cell length

Lattice functions: booster vs. collider FODO cell ARC

SUP and Ring Lattice: booster vs. collider

Bypass design Length of half bypass: L=(5+4f1+1.5f2)Lc Width of the bypass: W=(9.5+9f1) cLc L = 10.5Lc= 752.482 m (f1=1.0, f2=1.0) W = 18.5cLc = 13.0m (f1=1.0) By adjusting f1 and f2, both length and width of bypass can be adjusted to fit the FFS length and detector width. No additional bending cell is required!

The bypass lattice ARC DIS BPI1 -DIS FODO DIS BPI2 RFC

With two family sixtopoles, xc=0.5 Dynamic aperture With two family sixtopoles, xc=0.5 ny (sy) Dp/p = +2% Dp/p = +1% Dp/p = 0 Dp/p = -1% Dp/p = -2% r = ey/ex = 0.01 nx (s)x

Lattice Parameters

3. Injection energy and low field issue The bending field of CEPC booster is 614Gs at 120 GeV; To reduce the cost of linac injector, the injection beam energy for booster is chosen as low as 6 GeV 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 find a way to increase the bending field at injection.

3.1 Low field stability test A BEPC bending magnet: B0=9028Gs @ IB=1060A; B0=30Gs @ IB=3A 。 Power supplies for ADS: 3A/5V, 15A/8V ; DI/I < 110-4。 Hall probe system will be used for field measurement; DB= 0.1 Gs  DB/B0~310-3。

Magnetic field stability measured in 24 hours IB=3A, B30Gs, sB=710-4 IB=6A, B  60Gs, sB=1.310-3 sB=1.810-3 IB=9A, B90Gs, sB=1.810-3

Low field stability test The earth field outside the magnet: Bx=0.550.026Gs, By=0.450.027Gs, Bz=0.250.03 Gs  B= 0.80.04Gs Inside the magnet, By=7.00.05Gs is dominated by residual field, Bx=0.40.04Gs 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 24h field stability (sB) 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 GeV could be feasible in view of magnetic field stability.

Mitigation Wiggling band scheme Increase linac energy  Pre-booster 10 GeV, 12 GeV Pre-booster

3.2 Wiggling-Bends Four bending magnets with same length in a half cell, two outside bends are excited by a bipolar power supply; The magnetic field of the bends is properly set during the energy ramping to keep the total bending angle constant. Both “Wiggling-Bends”, “Normal-Bends” and other combinations can be tested in operation. QF QF B2 B2 B2 B2 B1 B1 B1 B1 B2, L/2, 2 /2 B2, L/2, 2 /2 B1, L/2, 1 /2 B1, L/2, 1 /2 B1, L/2, 1 /2 B1, L/2, 1 /2 QD

WB field vs. time during energy ramping B2(t)=Bej B1(t) = B1(0)+ (Bej-Binj)*t/Tramp E(t) B1(0)= -0.9Bej

Based on the Lattice Aug.2014 Lattice with WB’s W.B. N.B. Based on the Lattice Aug.2014

A SUP Lattice functions: W.B. vs. N.B.

The bypass : W.B. vs. N.B.

SR related parameters: WB vs. NB

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.

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

Match from linac to booster Matching Switch e+ (or e-) arc Vertical slope line VSL Match and switch to e Arc Match to booster MTB From end of VSL to Booster

Booster injection e beams are injected from outside of the booster ring; Horizontal septum is used to bend beams into the booster; 2 kickers downstream of injected beams kick the beams into the booster orbit.

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

Vertical transfer

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

Summary Conception design study on CEPC-Booster is carried out; There is no showstopper found in the design, in view point 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. There are some technical challenges, such as the low HOM 1.3 GHz SC cavities, supports & alignment, as well as low cost components. The design study needs to be detailed and deepened.