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W. Kozanecki PEP-II MAC Review, 9-11 Oct 03
Beam-beam: Experimental StatusBeam-beam: Experimental Status
Near ½ integer Near ½ integer ((++x/yx/y ~ 0.52/0.57) ~ 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
W. Kozanecki PEP-II MAC Review, 9-11 Oct 03
Interplay between eInterplay between e-- - cloud & beam-beam issues - 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 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 1+3). Solenoids now cover most of the beam pipe in all straights and arcs.
F.-J. Decker, et. al.,
PAC’01, ‘03
W. Kozanecki PEP-II MAC Review, 9-11 Oct 03
Interplay between eInterplay between e-- - cloud & beam-beam issues: impact - cloud & beam-beam issues: impact
Bunch pattern optimization: maximize IBunch pattern optimization: maximize Ibunchbunch , taking into account , taking into account
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) The severity of electron-cloud effects (for a fixed bunch pattern) has been steadily decreasing over the yearshas 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)
W. Kozanecki PEP-II MAC Review, 9-11 Oct 03
Interplay between eInterplay between e-- - cloud & beam-beam issues: - cloud & beam-beam issues: towards higher luminositiestowards higher luminosities
Impact of eImpact of e-- cloud may be more severe once higher currents force cloud may be more severe once higher currents force the use of a the use of a denser pattern (‘by-2’)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.
F.-J. Decker, et. al.,PAC’ 03
W. Kozanecki PEP-II MAC Review, 9-11 Oct 03
Beam-beam limit studiesBeam-beam limit studies
Experimental procedureExperimental 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 -x,y in-
& out-of-collision
DiagnosticsDiagnostics Fast luminosity monitor (e+e- e+e- )
Beam-beam performance summaryBeam-beam performance summaryJ. Seeman, Aug 03
W. Kozanecki PEP-II MAC Review, 9-11 Oct 03
Summary (I)Summary (I)
Electron-cloudElectron-cloud effects have been minimized by a combination of effects have been minimized by a combination of scrubbing, solenoid-suppression, and bunch-pattern optimizationscrubbing, solenoid-suppression, and bunch-pattern optimization
LuminosityLuminosity (& (& backgroundbackground!) optimization relies on a delicate !) optimization relies on a delicate balancebalance between the currents, tunes, beam-beam parameters and between the currents, tunes, beam-beam parameters and e-cloud effects as these parameters vary along each bunch train.e-cloud effects as these parameters vary along each bunch train.
Current-dependenceCurrent-dependence of the of the spot sizesspot sizes in collision (@ in collision (@ xx ~ 0.5) ~ 0.5)
LEB (HEB) sizes depend only on e- (e+) current
y(e-) grows by 30-70% wrt the e- single-beam size
30-40% is more typical of recent (stable) running : mostly ‘spontaneous’ ?
x(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
some tune measurements in collision are difficult (esp. LER x)
extracting (or limits thereon) from L and (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- 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 commissionedNew 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 PEP-II has recently achieved, near the ½ integer, beam-beam parameters parameters xx/ / yy of about of about .109 / .082.109 / .082 ( (.040 / .040.040 / .040)) in the LER (HER) in the LER (HER)
W. Kozanecki PEP-II MAC Review, 9-11 Oct 03
Spare slidesSpare slides
W. Kozanecki PEP-II MAC Review, 9-11 Oct 03
In contrast to ‘classical’ single-ring collider,In contrast to ‘classical’ single-ring collider,
+x,y -
x,y (and +x,y -
x,y only) => +x,y -
x,y
interpreting luminosity in terms of requires additional knowledge and/or assumptions on individual IP spot sizes
energy-transparency conditions
+x,y = -
x,y <===> I+b E
+ = I-b E
-
(provided +x,y = -
x,y , +
x,y = -x,y ,
+x,y = -
x,y...)
largely violated in PEP-II
best performance repeatedly achieved with I + / I
–~ 1.3 (more typically 1.7 – 2, not 2.9!)
W. Kozanecki PEP-II MAC Review, 9-11 Oct 03
Typical performance since Run 4 startupTypical performance since Run 4 startup
I+ x I- (mA-2)
Lu
min
osit
y (
b-1)
I+ x I- (mA-2)
Sp
ecif
ic lu
min
osit
y (
b-1m
A-2b
-2)
W. Kozanecki PEP-II MAC Review, 9-11 Oct 03
At high I At high I ++, e, e-- cloud strength varies along minitrain => cloud strength varies along minitrain => e+ beam size varies (long range + within train) => Luminosity varies
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
W. Kozanecki PEP-II MAC Review, 9-11 Oct 03
W. Kozanecki PEP-II MAC Review, 9-11 Oct 03
HER b-b limit (continued)
• 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