Beam-beam: Experimental Status

1
Beam-beam Experimental Status
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
IP Parameter
Design Peak performance (Jun 03) C-M
energy (GeV) (e 3.1 e- 9.0)
10.58 10.58 Crossing angle (mrad) 0.0 lt
1.0 Luminosity (x 1033/cm2/s) 3.00 6.57
Number of bunches 1658 1034 LER current (mA,
e) 2146 1550 HER current (mA, e-)
750 1175 LER/HER current ratio
2.9/1 1.3/1 by/bx (cm/cm)
1.5 / 50 1.2
/ 40, 1.2 / 28- Emittance (nm-rad) (y/x)
1.5 / 49
1.8 / 30, 1.8 / 49- IP rms beam size
sy/sx (mm) 4.7 / 157
4.6 / 113 LER tunes
(x/y) 38.64 /
36.57 38.52 / 36.57 HER tunes
(x/y) 24.62 /
23.64 24.52 / 23.62 Beam-beam
parameter (vertical /-) 0.03
0.082 / 0.040 Beam-beam parameter
(horizontal /-) 0.03
0.109 / 0.040
3
Typical recent performance (May-Jun 03)

• Near ½ integer (nx/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., PAC01, 03
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 2003
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 13). Solenoids now cover most of the
beam pipe in all straights and arcs.
5
Interplay between e- - cloud beam-beam issues
impact

• Bunch pattern optimization maximize Ibunch ,
  taking into account
• total-current budget (RF power, beam-heating
  problems)
• minitrain spacing (larger minigaps gt better e-
  cloud suppression)
• minitrain length (shorter minitrains gt less e-
  cloud buildup)
• of minitrains (fewer minitrains gt 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)

6
Interplay between e- - cloud beam-beam issues
towards higher luminosities
F.-J. Decker, et. al.,PAC 03

• 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
2-4-2-4.-.. (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.
7
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 (ee- ? ee- g)
• Horizontal beam sizes synchrotron-light monitor
  (SLM)
• Vertical beam sizes SR-light interferometer

8
W.K., PEP-II Performance Workshop, Dec 00
HER b-b limit _at_ high e- current I- 625 mA, I
100-1400 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 gtgt single-beam blowup,
  by an amount that depends on its own current gt
  LSP drops!
• HERSLM x y size in collision flat with I

No indication of b-b induced HER blowup (in
this data set)
9
LER b-b limit _at_ high e current I 1070 mA, I-
200-630 mA, 597 bunches
LSP drops by lt 5

• LERSLM x y size in collision gtgt 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 limit _at_ low e current I 90 mA, I-
20-750 mA, 829 bunches
10
6 Feb 03 data
Typical current-dependence of specific luminosity
beam sizes, Jan-Apr 03
11
Wandering in tune space...

• LER
• ? Apr 03
• nx .64
• ny .56
• since May 1, 2003
• nx .52
• ny .57
• HER
• ? Apr 03
• nx .57
• ny .64
• since May 1, 2003
• nx .52
• ny .62

The values quoted here are nominal, unshifted
tunes
12
Before ½ integer vs. after LER beam sizes
LER y-size (28 Apr)
7
LER current (e) mA
13
Before ½ integer vs. after HER beam sizes
Luminosity
LER current (e) mA
LER current (e) mA
Product of bunch currents (mA2/b)2
Product of bunch currents (mA2/b)2
14

• Original tunes (nx/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 (nx/y 0.52/0.57)
• e- size ? with ? e current
• e size ? with ? e- current
• Specific luminosity
• scales (primarily) with e current
• gt HEB the weaker beam (at that time...)
• Total luminosity
• some tune-shift saturation (prob. HEB), but
  potential for more luminosity!

Luminosity/bunch (2 May 03)
15
Dynamic-b effect?
M. Baak, Sep 03

• Measurement of luminous-region size by BaBar
  (dimuons Bhabhas)
• 1/sL sqrt (1/s2 1/s-2)
• Reduction of horizontal spot size from 100 ?m
  to 70 ?m.
• No changes observed in longitudinal spot size.

(skipped these runs)
16
Tune spectra in collision
Old tunes
nx 0.5 (June 03)
?
nx
ny
n-x
n-y
17
y-tunes in collision, vs. e- bunch current (LEB
trickle, 12 Jun 03)
HER bunch current (e-) mA/b
HER y-tune not fully compensated
HER bunch current (e-) mA/b
18
Determination of beam-beam parameters from
luminosity spot-size data
19
Measured beam-beam parameters
20
Beam-beam performance summary
J. Seeman, Aug 03
21
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
  (_at_ 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 gt 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 Bhabhas)
• 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

22
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)

23
Spare slides
24

• In contrast to classical single-ring collider,
• ex,y ? e-x,y (and bx,y ? b-x,y only) gt
  sx,y ? s-x,y
• interpreting luminosity in terms of x requires
  additional knowledge and/or assumptions on
  individual IP spot sizes
• energy-transparency conditions
• xx,y x-x,y ltgt Ib E I-b E-
• (provided bx,y b-x,y , ex,y e-x,y , nx,y
  n-x,y...)
• largely violated in PEP-II
• best performance repeatedly achieved with I /
  I 1.3 (more typically 1.7 2, not
  2.9!)

25
Typical performance since Run 4 startup
26
Interplay between e- - cloud beam-beam issues
(2)

• At high I , e- cloud strength varies along
  minitrain gt
• e beam size varies (long range within train)
  gt Luminosity varies
• e- beam-beam tune-shift varies (gt e- beam size
  may vary ??)
• tunes optimized on the average only gt
• 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
27
Beam-beam flip-flop
R. Holtzapple, et. al., SLAC-PUB-9238 (2001 data)

• 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.

28
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

29
Beam-beam flip-flop (ctd)

• 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
30
(No Transcript)
31
By-5 - 1/50th, 597 bunches , by 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 gtgt single-beam blowup,
  by an amount that depends on its own current gt
  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)
32
LER b-b limit _at_ 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

33
LER b-b limit _at_high e current I 1070 mA, I-
200-630 mA, 597 bunches
LSP drops by lt 5

• LERSLM x y size in collision gtgt 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)

34
6 Feb 03 data
Comparison of individual beam sizes with
luminous-region measurements
35

• Very little HEB current-dependence of either LER
  or HER spot size
• LEB current-dependence for 1.1 lt I lt 1.85 A,
• LER x-size 20
• y- 10-15
• HER x-size 6 (but large spread)
• y- 10 (but large
  spread may be consistent with 0)
• Lsp - 30 (measured)
• - 21
  (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)
36
Specific luminosity vs. beam currents
I
I (mA)
I- (mA)
LSP
I- (mA)
37
LEB size vs beam currents
LER x-size
LER y-size
38
Before ½ integer vs. after LER beam sizes
LER x-size (28 Apr)
11
LER y-size (28 Apr)
7
39
y-tunes in collision, vs. e- bunch current (LEB
trickle, 12 Jun 03)
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
40
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
41
Current-dependence of measured SL spot sizes
42
Current-dependence of estimated IP spot sizes
43
Comparison of fitted measured L LSP
44
But it does not always work so well...
5-6 Jun
3 May