1. The PEP-II Interaction e+e- Factories Workshop
Region Upgrade
M. Sullivan
for the
International Committee for Future Accelerators
e+e- Factories Workshop
at SLAC
October 13-16, 2003 1

2. Outline Present design Upgrade parameters Why upgrade the IR
IR Upgrade Proposal #1
Crossing angle
SC QD1?
IR Upgrade Proposal #2
Beta functions
SR backgrounds
Summary
Outline
Design team:
M. Biagini
J. Seeman
M. Sullivan
with help from
M. Donald
S. Ecklund
U. Wienands 2

3. Machine Parameters that are Important for the IR
PEP-II KEKB
LER energy GeV
HER energy GeV
LER current A
HER current A
 y* mm
x* cm
X emittance nm-rad
Estimated sy* mm
Bunch spacing m
Number of bunches
Collision angle head-on 11 mrads
Beam pipe radius cm
Luminosity 6.6 1033 cm-2 sec-1 3

4. 4

5. 5

6. PEP-II Proposed Upgrade Plans
Now Upgrade
LER energy GeV
HER energy GeV
LER current A
HER current A
y* mm
x* cm
X emittance nm-rad
Estimated sy* mm
Bunch spacing m
Number of bunches
Collision angle head-on head-on 0 mrads
Beam pipe radius cm
Luminosity 6.6  1034 cm-2 sec-1 6

7. Why Upgrade (Initial motivations)
Lower the vertical beta function to 5-6 mm
Keep maximum betas low
Lower the amount of SR generated near the IP
Lower the beam-beam effect from the parasitic crossings
Possibly get enough separation to allow filling every RF bucket 7

8. Upgrade proposal #1
Replace the last 20 cm of each B1 magnet with quadrupole field (50% stronger than QD1 field)
Introduce a crossing angle to recover beam trajectories so we don’t have to redesign or move any magnets. We need a certain amount of separation at QF2.
Minimal hardware change
Relatively easy change to make
Moves the focusing closer to the IP 8

9. 9

10. Crossing angle and parasitic crossings
Recently Yunhai Cai has simulated a crossing angle in his beam-beam code and confirms Ohmi’s beam-beam result that an x crossing angle results in a significant luminosity reduction for very high tune shifts when one is near the ½ integer
Parasitic crossings
The introduction of a crossing angle increases the beam separation at the parasitic crossings and thereby decreases the beam-beam tune shifts from these near collisions. The effects we have already seen in by2 bunch patterns from parasitic crossings would be greatly reduced.
For PEP-II, the parasitic crossings occur at:
0.63, 1.26, 1.89 and 2.52 m in the by2 bunch pattern and
0.945, 1.89 m in the by3 bunch pattern
See Y. Cai’s and K. Ohmi’s talks on beam-beam simulations
See M. Biagini’s talk on parasitic crossings 10

11. Alternative proposal SC QD1?
Super-conducting QD1 magnets
Much more flexible
Could shield detector solenoid
Could have a skew quad winding
Could have dipole trims
Complete overhaul of hardware
No support tube
Cantilever supports from each end
Magic vacuum flange
Would still need B1 magnets or SC dipoles
Would need to add shielding around beam pipe 11

12. Alternative proposal #2
Stronger PM QD1 magnets
Use higher strength permanent magnet material
Move radial ion pump from behind B1 to behind QD1
Put 3 higher strength magnet slices in place of the pump
Remove 3 slices from the back of QD1
Keeps beam collision head-on since B1s are the same
Closer match to present machine
Minimal hardware change
Higher strength material has a higher temperature coefficient
Higher strength material has lower radiation resistance 12

13. Proposal #2 – Stronger QD1 closer to the IP13

14. Beta functions beta x* (cm) beta y* (mm) beta x max beta y max
Low-energy beam
Present design
With upgrade #1
With upgrade #2
High-energy beam 14

15. SR backgrounds
The present LER SR mask (multi-tipped mask) design should be adequate for shielding the detector Be beam pipe from the SR fans. The present design constraint of not allowing any photons to strike a surface where they can one-bounce to the Be beam pipe will be preserved. All this must be verified.
Any new IR design that maintains the present beam orbits to within a couple of mm should make the present masking design adequate from a background point of view.
The very high beam current of the LER means that we need to recheck to make sure that back-scattered photons from the downstream crotch chamber do not generate backgrounds by striking the detector beam pipe 15

16. SR fans and power
The proposed beam currents will generate a large amount of SR in the IR
The HER beam elements were designed for 2A so most of the HER parts should be OK. There is some question about the High-Power Downstream Dump current limit.
There are 2 vacuum chambers that see the LER SR power that need to be looked at more closely
The LER downstream crotch chamber that sees B1 radiation
The upstream LER SR mask for the Be beam pipe. It sees upstream QD1 radiation 16

17. 17

18. 18

19. SR fans and power (cont.)
The present power levels on the multi-tipped LER SR mask are about 30 W/mm at 2.1A beam current.
This goes to 50 W/mm at 3.6A and 65 W/mm for 4.5 A of LER beam.
An ANSYS model has been built to study this mask in more detail.
The crotch chamber design allowed for overlapping B1 radiation fans. This can only happen when the detector solenoid is off, so we should have some margin here. 19

20. ANSYS Model of the B1 Mask
The power levels on these tips is fairly high A
50 3.6A
65 4.5A 20

21. Summary
The initial upgrade proposal replaced the last 4 slices of the B1 magnets with quadrupole field. However, this replacement introduces a ± 3.3 mrad crossing angle at the IP. Recent beam-beam simulations indicate a luminosity reduction for beams with even a small crossing angle.
An alternative proposal is to strengthen the IP end of QD1 and remove some the outboard slices. This moves the center of the magnet closer to the IP while maintaining the head-on collision and hence the peak luminosity. However, this design has parasitic crossing effects that become quite large at the low by* values.
There are several alternatives between these two proposals and a compromise solution may be the better design
We might try to test the crossing angle lumi loss prediction if there is a measurable loss at small enough angles (+/- 0.5 mrad) 21

22. Summary cont.
SR backgrounds look like they can be controlled based on the plan to maintain very similar beam orbits to the present design, but this needs to be thoroughly checked
At 4.5A LER beam current and 2.0A HER beam current the four permanent magnets in the support tube (QD1 and B1) generate a total of 156kW of SR power
A large fraction of this power (at least 2/3) escapes from the near IR and is absorbed far away from the detector. Even so, we must trace where this power strikes beam pipes and make sure all of the components can absorb this much SR power.
Most of the vacuum chambers in the IR can handle the higher beam currents
Several chambers will need to be replaced in order to improve the IR impedance and pumping
The most difficult vacuum chamber in the IR region is the chamber under the B1 magnet that absorbs power from the LEB 22