1. Beam-beam Experience at PEP-II
W. Kozanecki, CEA-Saclay
Introduction: PEP-II collision parameters & recent performance
Interplay between e- - cloud & beam-beam issues
Beam-beam limit studies
Summary

2. PEP-II Collision Parameters
J. Seeman, Jun 03
PEP-II Collision Parameters
IP Parameter Design Peak performance (Jun 03)
C-M energy (GeV) (e+: 3.1 ; e-: 9.0)
Crossing angle (mrad) < 1.0
Luminosity (x 1033/cm2/s)
Number of bunches
LER current (mA, e+)
HER current (mA, e-)
LER/HER current ratio 2.9/1 1.3/1
by*/bx* (cm/cm) / / 40+, 1.2 / 28-
Emittance (nm-rad) (y/x) / / 30+, 1.8 / 49-
IP rms beam size sy/sx (mm) / / 113
LER tunes (x/y) / / 36.57
HER tunes (x/y) / / 23.62
Beam-beam parameter (vertical +/-) / 0.040
Beam-beam parameter (horizontal +/-) / 0.040

3. Typical recent performance (May-Jun 03)
Luminosity (mb-1)
I+ x I- (mA-2)
Specific luminosity (mb-1mA-2b-2)
I+ x I- (mA-2)
Near ½ integer (n+x/y ~ 0.52/0.57)
e- y-size  with  e+ current
e+ x-size  with  e- current
Total luminosity: some tune-shift saturation, but potential for more L
Specific luminosity scales (primarily) with e+ current

4. Interplay between e- - cloud & beam-beam issues
F.-J. Decker, et. al., PAC’01, ‘03
Interplay between e- - cloud & beam-beam issues
Bunch-by-bunch luminosity versus position along the whole train. Pattern: ‘by-4’ (8.4 ns spacing) with 7 additional big gaps, July 2000.
The first bunches of each mini-train have a high luminosity, which drops to 40 % of its initial value at the end of the longest train. The long gaps clear the electron cloud, which slowly builds up again over along the mini-train. Solenoids had been installed in part of the straights only.
Standard luminosity pattern in spring Pattern: ‘by-3’ (6.3 ns spacing)
Mini-trains of 10 and 11 bunches are alternating. There is an ion gap of about 3%. In this pattern each mini-train has constant luminosity (except for bunches 1+3). Solenoids now cover most of the beam pipe in all straights and arcs.

5. Sensitivity to pattern details...
“by-3’ pattern (6.3 ns spacing),
trains with minigaps
(6 Oct 03)
Solenoids cover most of the beam pipe in all straights and arcs.
Straight “by-3’ pattern (6.3 ns spacing),
no minigaps
(Oct 03)
Solenoids cover most of the beam pipe in all straights and arcs.

6. Interplay between e- - cloud & beam-beam issues: impact so far...
Bunch pattern optimization: maximize Ibunch , taking into account
total-current budget (RF power, beam-heating problems)
minitrain spacing (larger minigaps => better e- cloud suppression)
minitrain length (shorter minitrains => less e- cloud buildup)
# of minitrains (fewer minitrains => fewer ‘fragile’ bunches)
need for current ramps at start of train (and/or minitrains)
The severity of electron-cloud effects (for a fixed bunch pattern) has been steadily decreasing over the years
Low-field (25-35 G) solenoids now cover most of the accessible beam-pipe sections. This system was upgraded this summer ( up to 3x higher field in the straights + cover remaining short sections)
Vacuum-pipe scrubbing appears to have played a significant role
Some e– cloud effects are no longer apparent
single-beam e+ blowup at high I+ no longer observed (but what once I+  ?)
In typical recent running, only 1st (few) bucket(s) in each minitrain affected by electron cloud (if any)

7. F.-J. Decker, et. al.,PAC’ 03
Interplay between e- - cloud & beam-beam issues: towards higher luminosities
Impact of e- cloud may be more severe once higher currents force the use of a denser pattern (‘by-2’)
e- cloud
Pattern: ‘by-2’ (4.2 ns spacing), long trains (Feb 03)
20-25% luminosity loss after the first 5-10 bunches in each train
Pattern: ‘by-2’ (4.2 ns spacing), short trains [ ] (Apr 03)
The first buckets of the duplets have the same high L, while the L of the second ones drops about 50% over 320 ns. It remains constant for several hundred ns, then slowly grows to nearly 75% before it starts dropping again.

8. Towards higher luminosities (2)
parasitic crossings do matter!
e- cloud
1 parasitic minitrain edges (instead of 2)

9. Beam-beam limit studies
Experimental procedure
Fix one beam current (typically similar to physics conditions)
Vary the current of the other beam from 0 to maximum possible; at each setting, optimize luminosity on tunes
Measure L/bunch, specific luminosity Lsp, individual beam sizes s-x,y in- & out-of-collision
Diagnostics
Fast luminosity monitor (e+e-  e+e- g)
Horizontal beam sizes: synchrotron-light monitor (SLM)
Vertical beam sizes: SR-light interferometer

10. W.K., PEP-II Performance Workshop, Dec 00
HER b-b high e- current: I- =625 mA, I+ = mA, 597 bunches
Collisions: LEB blowup in both x & y
(N.B.: y blowup instrumental?)
Single-beam: LEB blowup minimal
LSP drops by ~ 25-30%
LEB blowup in collision >> single-beam blowup, by an amount that depends on its own current => LSP drops!
HER:SLM x & y size in collision flat with I+
No indication of b-b induced HER blowup (in this data set)

11. LER b-b limit @ high e+ current: I+ =1070 mA, I- =200-630 mA, 597 bunches
LSP drops by < 5%
LER:SLM x & y size in collision >> single-beam, flat with electron current (not “naive” b-b!)
HER: I- dep. of x, y SLM size in collision, consistent with single-beam behavior (flat)
LER b-b low e+ current: I+ = 90 mA, I- = mA, 829 bunches

12. Typical current-dependence of specific luminosity & beam sizes, Jan-Apr 03
6 Feb 03 data

13. Wandering in tune space...
LER
 Apr 03:
nx = .64
ny = .56
since May 1, 2003:
nx = .52
ny = .57
HER
nx = .57
ny = .64
ny = .62
The values quoted here are nominal, unshifted tunes

14. “Before” ½ integer vs. “after”: LER beam sizes
LER x-size (28 Apr)
+11 %
LER current (e+) [mA]
% wrt sx ( single-beam)
+ 8 %
LER x-size (2 May)
HER current (e+) [mA]
LER y-size (2 May)
+ 12 %
HER current (e+) [mA]
LER y-size (28 Apr)
+7 %
LER current (e+) [mA]

15. “Before” ½ integer vs. “after”: HER beam sizes & Luminosity
HER y-size (28 Apr)
HER y-size (2 May)
+ 39 %
LER current (e+) [mA]
LER current (e+) [mA]
L/bunch (28 Apr)
L/bunch (2 May)
Product of bunch currents [(mA2/b)2]
Product of bunch currents [(mA2/b)2]

16. Original tunes (n+x/y ~ 0.64/0.56)
LSP (2 May 03)
Msrd: - 25 %
Predctd: -25%
Spec. luminosity [1030 cm-2 s-1 mA-2/b]
LER current (e+) [mA]
Original tunes (n+x/y ~ 0.64/0.56)
e- size ~ independent of e+ current
e+ size ~  with  e+ current (mostly x: proximity to 2/3?)
Near ½ integer (n+x/y ~ 0.52/0.57)
e- size  with  e+ current
e+ size  with  e- current
Specific luminosity
scales (primarily) with e+ current
=> HEB the ‘weaker’ beam (at that time...)
Total luminosity
some tune-shift saturation (prob. HEB), but potential for more luminosity!
Ibunch(e-) x Ibunch(e+) [(mA/b)2]
L / bunch (1030 cm- 2s-1 b-1)
Luminosity/bunch (2 May 03)

17. Impact of horizontal-tune change on IP spot size
M. Baak, Sep 03
Measurement of luminous-region size by BaBar (dimuons & Bhabha’s)
1/sL = sqrt (1/s+2 + 1/s-2)
nx  0.5: Reduction of horizontal spot size from ~ 100 m to ~ 70 m.
No changes observed in longitudinal spot size.
before April 28th
after April 28th
nx ~ 0.5
simulation (high I, well-chosen v+x )
(skipped these runs)

18. Tune spectra in collision
Old tunes
nx ~ 0.5 (June 03) ?
n+x
n+y
n-x
n-y

19. y-tunes in collision, vs. e- bunch current (LEB trickle, 12 Jun 03)
LEB x-size
+ 64 % wrt sx ( single-beam)
HER bunch current (e-) [mA/b]
LER y-tune: b-b tune shift!
Dn+ ~ .004 for 0.81 < I- < 1.07 A
HER bunch current (e-) [mA/b]
HER y-tune: not fully compensated
HER bunch current (e-) [mA/b]

20. Determination of beam-beam parameters from luminosity & spot-size data

21. “Measured” beam-beam parameters

22. Beam-beam performance summary
J. Seeman, Aug 03
Beam-beam performance summary

23. Summary (I)
Electron-cloud effects have been minimized by a combination of scrubbing, solenoid-suppression, and bunch-pattern optimization
Luminosity (& background!) optimization relies on a delicate balance between the currents, tunes, beam-beam parameters and e-cloud effects as these parameters vary along each bunch train.
Current-dependence of the spot sizes in collision nx ~ 0.5)
LEB (HEB) sizes depend only on e- (e+) current
sy(e-) grows by 30-70% wrt the e- single-beam size
30-40% is more typical of recent (stable) running : mostly ‘spontaneous’ ?
sx(e+) grows by a factors of 1.5 to > 2 wrt the e+ single-beam size
50-60% is more typical of recent (stable) running: ‘spontaneous’ or effective optimum?
offline analysis of luminous region size (using dimuons & Bhabh’as)
corroborates the expected reduction in IP x-spot size (  simulation?)
...but the observed variations of IP spot size are not always consistent with the measured current-dependence of individual e+ and e- beam sizes

24. Summary (II) Beam-beam parameter measurements
some tune measurements in collision are difficult (esp. LER nx)
extracting x (or limits thereon) from L and s(SL)+,- x,y promising, but...
it needs better control of SLM/interferometer systematics
it would greatly benefit from the ability to ‘translate’ SL sizes into IP sizes
the interpretation/analysis is complicated by dynamic-b effects.
1 measurement of vertical LEB beam-beam tune-shift vs. HER current! but interpretation not straightforward..
can we gain more from such parasitic monitoring?
New diagnostics being commissioned
gated cameras in HER & LER
on-line measurements of IP positions & spot sizes by BaBar
gated tune monitor
PEP-II has recently achieved, near the ½ integer, beam-beam parameters xx/ xy of about .109 / .082 (.040 / .040) in the LER (HER)

25. Spare slides

26. In contrast to ‘classical’ single-ring collider,
e+x,y  e-x,y (and b+x,y  b-x,y only) => s+x,y  s-x,y
interpreting luminosity in terms of x requires additional knowledge and/or assumptions on individual IP spot sizes
energy-transparency conditions
x+x,y = x-x,y <===> I+b E+ = I-b E-
(provided b+x,y = b-x,y , e+x,y = e-x,y , n+x,y = n-x,y...)
largely violated in PEP-II
best performance repeatedly achieved with I + / I –~ (more typically 1.7 – 2, not 2.9!)

27. Typical performance since Run 4 startup
I+ x I- (mA-2)
Specific luminosity (mb-1mA-2b-2)
I+ x I- (mA-2)
Luminosity (mb-1)

28. Interplay between e- - cloud & beam-beam issues (2)
At high I +, e- cloud strength varies along minitrain =>
e+ beam size varies (long range + within train) => Luminosity varies
e- beam-beam tune-shift varies (=> e- beam size may vary ??)
tunes optimized on the average only =>
slight L loss
‘raining’ buckets (rapid loss of charge, background spikes, flip-flop)
electron-cloud enhanced beam-beam blowup of the e+ beam
Gated-camera measurements
(2001 data, 4-by-22 pattern)
R. Holtzapple, PEP-II Performance Workshop, Jan 2002

29. R. Holtzapple, et. al., SLAC-PUB-9238 (2001 data)
Beam-beam flip-flop
Near the top of a fill:
Several LER bunches have reduced beam size (~30% reduction in beam size)
Several bunches - typically near the head of the train - have ~ ½ L
2 states of low-luminosity bunches: they have either reduced LER current or reduced LER beam size
Ib+ = low; Ib- = average
Ib+, Ib- = average; sx, y+ = small. Small LER bunches + low L implies transverse blow-up of the e- beam, since bunch currents are not particularly low for these.

30. Beam-beam flip-flop (2)
Single Bunch Transition to low-luminosity state
Initially at average beam size
Luminosity dropped when rings were filled
The bunch experiences a sudden change in luminosity/beam size
This bunch went from one unstable state (low luminosity / small x, y) to the other unstable state (short lifetime)
Transition between states is fast (~0.5 sec.)
Horizontal beam size oscillation accompanies the transition

31. Beam-beam flip-flop (c’td)
Possible Explanation of Beam Size Flip-Flop Dynamics
The LER bunches at the front of the train have a smaller transverse beam size (lower electron cloud density). These small (strong) LER bunches blow-up the HER bunches.
The resulting tune shift for the these LER bunches is smaller than “normal”, and as a result, they have a horizontal tune located near a resonance which gives them a shorter lifetime.
The LER bunches lose charge. Eventually the HER becomes strong enough to flop itself, and the LER bunch, back to “normal” size.
To confirm this theory, a 2nd gated camera has been installed in the HER and is being commissioned.
Start of the store (high I): L/bunch (almost) uniform throughout each train.
End of the store (low I): single-bunch L dropouts are prevalent in the 1st few trains

33. By-5 - 1/50th, 597 bunches , b*y = 1.25, July 00
HER b-b limit (continued)
Collisions: LEB blowup in both x & y
(N.B.: y blowup instrumental??)
Single-beam or separated: LEB blowup minimal
LEB blowup in collision >> single-beam blowup, by an amount that depends on its own current => LSP drops!
LEB blowup with both beams present but out of collision, is similar to single-beam blowup
No indication of b-b induced HER blowup (in this data set)

34. LER b-b limit @ low e+ current: I+ = 90 mA, I- = 20-750 mA, 829 bunches
LSP drops by ~ 10-15%
LER: definite, but moderate, increase in both x,y SLM sizes with electron current (as expected)
HER: I- dep. of x, y SLM size in collision, consistent with single-beam behavior

35. LER b-b limit @high e+ current: I+ =1070 mA, I- =200-630 mA, 597 bunches
LSP drops by < 5%
LER:SLM x & y size in collision >> separated, but both are flat with electron current (not “naive” b-b!)
HER: I- dep. of x, y SLM size in collision, consistent with single-beam behavior (flat)

36. Comparison of individual beam sizes with luminous-region measurements
6 Feb 03 data

37. Lsp =2.9 predicted from HER/LER beam size relative variation
Very little HEB current-dependence of either LER or HER spot size
LEB current-dependence : for 1.1 < I+ < 1.85 A,
LER x-size: + 20%
y : %
HER x-size: % (but: large spread)
y : + 10% (but: large spread; may be consistent with ~0)
Lsp : - 30% (measured)
: % (predicted from spot size variations)
Lsp
Lsp =2.9 predicted from HER/LER beam size relative variation
5 Apr 03 data
LER bunch current (mA/bunch)

38. Specific luminosity vs. beam currents
LSP
Msrd: - 25 %
Predctd: -25% I+
I+ (mA)
I- (mA)
LSP
I- (mA)

39. LEB size vs beam currents
LER x-size
LER y-size
+ 8 %
LER x-size
LER y-size
+ 12 %

40. “Before” ½ integer vs. “after”: LER beam sizes
+ 8 %
LER x-size (2 May)
LER x-size (28 Apr)
+11 %
LER y-size (2 May)
+ 12 %
LER y-size (28 Apr)
+7 %

41. y-tunes in collision, vs. e- bunch current (LEB trickle, 12 Jun 03)
LER y-tune: b-b tune shift!
Dn+ ~ .004 for 0.81 < I- < 1.07 A
LER y-tune knob
HER bunch current (e-) [mA/b]
HER bunch current (e-) [mA/b]
HER y-tune knob
HER y-tune: not fully compensated
HER bunch current (e-) [mA/b]
HER bunch current (e-) [mA/b]

42. Luminous spot sizes vs. beam currents
M. Baak, Sep 03
No apparent dependence of the horizontal luminous size on LER or HER currents
No apparent dependence of the longitudinal luminous size on LER current.
after April 28th nx ~ 0.5

43. Current-dependence of measured SL spot sizes

44. Current-dependence of estimated IP spot sizes

45. Comparison of fitted & measured L & LSP

46. Comparison of fitted & measured L & LSP

47. But it does not always work so well...
3 May
5-6 Jun