1. Design of Interaction Region
Gang Xu
Accelerator Physics Group
IHEP, Beijing, Oct. 14, 2001

2. Content Introduction to BEPCII
Requirements to IR design from accelerator physics
Detector boundary condition
IR beam line layout
IR Magnets
Background and consideration of new design
Summary

3. Introduction to BEPCII
2-ring collider— factory type machine
Synchrotron radiation facility
Basic parameters
(next page)

4. Introduction to BEPCII(continued)
Tab. 1 Main parameters
Energy E(GeV)
1.55
Energy spread(10-4)
4.23
Circumference C(m)
237.53
Emittance εx/εy(μm)
0.21/0.003
Energy loss/turn U0(keV)
55.8
Momentum compact α
0.036
RF frequency frf(MHz)
499.8
β*x /β*y(m)
1/0.015
RF Voltage Vrf(MV)
1.5
Chromaticity ξx/ξy
13.3/28.5
Total current/beam I(A)
1.116
Tunes νx/νy/νz
6.64/7.58/0.047
Particle number Nt
5.51×1012
Crossing angle θx(mrad)
±11
SR Power P(kW)
62.3
Bunch spacing Sb(m)
2.4
Bunch number Nb 93
Damping time τx/τy/τz(ms)
44/44/22
Bunch current Ib(mA) 12
Beam-beam parameter
0.04/0.04
Bunch length σz(cm)
Luminosity(1033cm-2s-1)
1.0
Natural bunch length σz0(cm)
1.1
Effective impedance()
0.7

5. Requirements to IR design from accelerator physics
Rapidly separate the beams into two rings
Squeeze the vertical beta function to 1.5cm
Compensate the coupling from detector solenoid
Connect the two outer rings to provide the synchrotron radiation
Keep the background in the acceptable level
All accelerator components in IR must stay in the space limited by the detector
HOM heating due to the mask and the discontinuity of vacuum chamber should be considered

6. Boundary condition
Detector geometric condition

7. Boundary condition(continued)
Summary of the main geometric conditions
The first accelerator component must be after 0.5m away from IP
Before 1.15m, the outer aperture of components must be less than Φ384mm
Before 1.9m, the outer aperture of components must be less than Φ756mm

8. Boundary condition(continued)
Detector solenoid(Bs=1T, new design value 1.2T)

9. Boundary condition(continued)
Aperture of IR vacuum chamber(Half )
For collision beam
1. 14σx+xoffset+10mm c.o.d.(εx=0.25μm)
σy+5mm c.o.d. (full coupling εy=0.125μm)
For injection beam
3. 16σx+xoffset+3mm c.o.d.
For SR photons(collision beam)
4. Out of the trajectories of SR photons
from 10 σ particles

10. IR beam line layout
Crossing angle θx(mrad)=±11 relative to the detector solenoid axis
Beam comes to IP from outer ring then goes into inner ring
1st S.C. magnet has combined anti-solenoid(ASOL) and
bending coil(SCB), the latter is used in synchrotron radiation mode
Both electron and positron are off the axis of SCQ
All S.C. coils are in one cryostat
ISPB is a septum type magnet, it will bend the beam in inner ring
OQ1, IQ1, OQ2, IQ2 are special magnets with dual aperture, beam
goes through their axes.

11. IR beam line layout(continued)
βy max<120m(βy *=1.5cm)180m(βy *=1cm), βx max<6m in s=0~1.9m

12. IR Magnets(Superconducting)
Anti-solenoid ASOL with integral strength 1.42 T·m, 0.6m away from IP, it will be extended onto SCQ to cancel the effect of the solenoid fringe field
SCB placed at the same location as ASOL with integral strength 0.26 T·m
SCQ with strength 17.6T/m and effective length 0.4m, 1.15m away from IP

16. IR special magnets(Normal)
D to IP
(m) L
D between e±
BSC(H×V)
strength
entry
exit
(mm)
ISPB
2.25
0.4 78
108
72×72
90×64
0.6T
OQ1
2.90
131
167
101×59
106×58
12T/m
IQ1
3.55
0.5
190
235
129×47
136×42
OQ2
4.55
281
326
82×72
78×73
IQ2
5.55
372
417
106×38
101×35

17. Background issue and consideration of new design
Since the beam coming from outer ring is off axis in the SCQ, this will lead to synchrotron radiation. From the previous plot, the minimum distance between beryllium pipe and the trajectory of the photon is only 7.2mm. This situation must be improved.

18. 7.2

19. Three improvement methods
Increase the radius of the beryllium pipe
Add a bending coil on SCQ to cancel(or partially) the bending field due to the off axis for the beam in outer ring, this will lead to BSC increasing from Φ120 to Φ140
Place SCQ to the center of outer ring beam orbit, this will lead to the equivalent BSC increasing from Φ120 to Φ170

20. Before doing the improvement, we need to know
What is the acceptable level of the synchrotron radiation?
The more detail boundary conditions in IR
The acceptable heating power of each IR component

21. Summary IR design is very preliminary
Due to the background issues we must do more detail IR design
Many items are not taken into account such as background from the loss particle, vacuum, beam diagnostics, …