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

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

3. 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.

4. 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, m each)
BTCe+
BTCe-
8 arcs
m each
D = km RF
IP4
IP2 RF
C = 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

5. 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.

6. 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.

7. 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.

8. Lattice parameters scale to cell length

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

10. SUP and Ring Lattice: booster vs. collider

11. Bypass design Length of half bypass: L=(5+4f1+1.5f2)Lc
Width of the bypass: W=(9.5+9f1) cLc
L = 10.5Lc= 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!

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

13. 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

14. Lattice Parameters

15. 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 Gs and its variation?
Try to find a way to increase the bending field at injection.

16. 3.1 Low field stability test
A BEPC bending magnet:
IB=1060A;
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。

17. 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

18. 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  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.

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

20. 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

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

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

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

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

25. SR related parameters: WB vs. NB

26. 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.

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

28. 4.1 Transfer From Linac to Booster
Matching section
Vertical slope line
Match and Switch
Arcs
Match to the booster

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

30. 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

31. 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.

32. 4.2 . Transfer from booster to collider
C. S., H
I. S., H
C. S., V
I. S., V

33. Vertical transfer

34. 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.

35. 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.