1. Orbit Bumps in PEP-II to Maximize Luminosity
Franz-Josef Decker Oct-2003
Effect of bumps:
x-y coupling
dispersion
emittance
Typical amount for tuning
Examples
“Strange” bumps
Unwanted bumps
Further oscillations which have to be fixed

2. LER orbit showing the oscillation of 9 mm near the interaction point (PR02).

3. Bumps in sextupoles which are 180º apart (LER)
Symmetric  x-y coupling:
Antisymmetric  dispersion:
Emittance blow-up due undesired coupling or dispersion wave (oscillation) between two “kicks”:
Typical tuning amount: 0.2 kG in skew quad for 3-5% L
(or offset in sextupole: 250 µm  800 kG/m = 0.2 kG)

4. Useful Formulas for Determining Sextupole Kicks G = x ·S B = x ·G,
G = x ·S
B = x ·G,
 [rad] = 0.3 B·l [T-m] / E [GeV].
Or to put it in perspective for LER:
A “closed” y-bump of y = 10 mm
needs a corrector strength B = kGm.
In a sextupole with S = 800 kG/m
the gradient change is G= 8 kG (40% of regular quad)
The induced kick corresponds to Bx = 0.08 kGm
This is bigger than to make the bump!

5. Orbits Due to Corrector Strengths (BPMs were reading wrong)
Injection Top-off and Fill from Scratch

6. Complicated bump around PR06 1032 is not closed by 30%
due to big absolute offset in sextupoles

7. PEP-II Region 2 Magnet Setup at one Side of the IP

8. List of more Injection Problems and some Solutions
Dispersion bump in y
closed in coupled region
(Kick due to skew quad has
to be closed with x-correctors)

9. Oscillation from skew quad in PR12 to SFs in PR08
1 mm in x caused 0.1 mm in y

10. Bumps at High Current
It has been seen that very big bumps in x might affect the beam size of the LER. This was only observed at high current in colliding beam mode. In all cases we tried to reduce some unexpected beam loss in a area with no skew quads or sextupoles (PR06 and injection region PR08), so there should not have been any tune or coupling change involved. The problem was that it had a big effect on the other beam, the HER got blown up when the LER beam was removed from the wall. In two cases the HER lost bunches and in one the beam got actually aborted due to radiation in the detector from HER.
A possible explanation that is speculative argues that moving the orbit might change something, which has to do with electron cloud and/or wakefields. A special experiment was conducted but only at low current and with a single ring, no non-linear problems were found.
(EPAC 2002)

11. Bumps in LER Increase Luminosity
It might be now possible to steer the orbit down
and then fix the loss in luminosity at high current with bumps.

12. Possible Explanations of “strange” bumps
Bump in e--cloud gradient (with solenoid) gives wakefield kick
and like linac bumps we can cancel them (initial idea).
2. “Short cut” in chicane changes offsets in sextupoles all around
circumference:
bump
Other observations:
Big bumps necessary: 4 mm
Optimum is “one-sided”:
Only high current effect
Less effective after going
to ½ integer tune (?) L x

13. Unwanted bumps due to GOF (Global Orbit Feedback)
Initially:
Up to 17 mm bump:
=> vacuum leak
2 mm offset change
in sextupole because
of BPM jump at PR04
4152
2mm  mm
Config restored
2 mm
4 hours

14. HER
Big global skew
Quad (3.5 kG)
causes oscillation
which goes
through IP
Fix on other side
would change IP
Tilt!