1. Beam-beam limits: MD proposal
W. Kozanecki
MD goals
Useful lessons from the Jan 2004 beam-beam experiment
Proposed operational procedure
Numbers/procedures to be clarified

2. MD goals Characterize current dependence of beam-beam performance
specific luminosity
vertical & horizontal spot sizes in LER & HER (incl. SXM)
loss collimators in PR12 (HER), PR04 (LER)
beam lifetimes
tune spectra
turn-to-turn centroid (& shape ?) stability, using GC turn sweeper
Determine the bunch-current combination that maximizes the luminosity
for given maximum total currents, this fixes the optimal number of bunches - provided that
the single-bunch behavior scales strictly with the # of bunches
the beam-beam backgrounds (& HOM heating) remain tolerable at full current

3. Current dependence of L & beam sizes: HEB scan
Data: 27Jan 04
Current dependence of L & beam sizes: HEB scan
By-2 pattern 1320 bunches
10 Apr 04 

4. HEB scan (continued)
Data: 27Jan 04

5. HEB scan (continued)
Data: 27Jan 04

6. Current dependence of L & beam sizes: LEB scan
Data: 27Jan 04
By-2 pattern 1320 bunches
10 Apr 04

7. LEB scan (continued)
Data: 27Jan 04

8. LEB scan (continued)
Data: 27Jan 04

9. Operational procedure: general guidelines
Perform experiment in quasi-single-bunch (QSB) mode:
only few, widely-spaced bunches
cleanly separates beam-beam from multibunch issues
minimizes total current
avoids thermal effects  orbit stability, no risk of overtemp trips
avoids RF/LFB trips at the highest bunch currents
choose # bunches so that enough light on lowest Ibunch
Optimize optics for high beam-beam parameters
start with stable machine in delivery (multibunch) mode
go to QSB mode (steer to gold?)
perform full optics optimization in QSB mode
@ nominal Ibunch (favor beam-beam, rather than optical, performance)
tunes, collsion phase, skews, sextupole bumps ( + Decker bumps?)
At each bunch-current setting
reset vertical IP angles in both rings
check SLM / SXM / interferometer settings
optimize tunes on L (trickle off so lifetime measurable ?)

10. Operational procedure: current scan pattern
Setup in QSB mode at nominal bunch currents
IL = 1.45 mA/b, IH = 0.90 mA/b (full optics optimization)
Take zero-current point
IL = 0.10 mA/b, IH = 0.06 mA/b (beam-beam negligible  measure zero-current spot sizes)
HER current scan: keep IL ~ constant, vary IH
IL ~ 1.45 mA/b (nominal)
Proposed sequence: IH = .1, .3, .5, .7, .8, .9, 1.0,….mA/b (finer steps at top, going ahap)
Optional: IL ~ 0.90 mA/b (to understand beam-beam evolution + check consistency)
LER current scan: keep IH ~ constant, vary IL
IH ~ 0.90 mA/b (nominal)
Proposed sequence: IL = .40, .70, 1.00, 1.30, 1.45, 1.50, 1.60,…mA/b Optional: IH ~ 0.65 mA/b

11. Time estimate Total 330’ (540) 6 h (9 h)
Full-current optimization (BBR taking data)
QSB setup & nominal Ibunch 105’
injection setup 15’
steer to gold 45’
optics optimization 45’
Zero-current point ’
HER current scan ’ (210’)
7 nominal IL 7x15’
repeat at intermediate IL 7x15’
LER current scan ’ (210’)
7 nominal IH 7x15’
repeat at intermediate IH 7x15’
Total ’ (540)
6 h (9 h)

12. Numbers/procedures to be clarified ahead of time
How do we handle the parasitic-crossing aspects of the experiment?
Min/max currents & # bunches ?
Minimum total current ? to get a signal on
LER/HER SLMs + interferometers
LER XSLM
HER GC (we need the LER interferometer!)
Maximum # bunches (QSB) for LFB/TFB to either stay off, or function properly ?
Maximum total current for thermal orbit drifts to remain negligible ?
Trickle - or not ? (lifetime msmst, BLM loss rates, GC expts)
Steer to gold before starting, so that actual, low-I orbits are close to high-current orbits we’ll have then ?
Optimum optical settings may not be the same at low & high Ibunch (bec. of beam-beam)  optimize optics (skews, bumps..)
at high (= normal operating) bunch current (favors beam-beam)?
or at the lowest current (favors optics) ?
Optics characterization beforehand, afterwards, or not at all ?
At lowest & highest bunch currents, compare Lsp in QSB & multibunch (full current) modes? (could be done as part of recovery)
Exercise SXM & GC turn-sweeper DAQ ahead of time

13. Spare slides

14. HER-current dependence of L & beam sizes: observations
In single-beam mode, the HER beam sizes are current-independent
When the HER current increases (with the LER nominal I+)
the specific luminosity first rises, then turns over.
Lsp reaches a broad maximum around 0.5 mA/b
the specific luminosity is comparable at the highest & lowest HER bunch currents
the dependence of the total luminosity on the beam-current product
exhibits only moderate saturation
is mostly limited by transverse losses (lifetime, beam-beam backgrounds)
the LEB blows up transversely with rising HER current
65% increase in x, 23% in y @ the SLM/interferometer (wrt single beam)
the evolution of the transverse e+ loss rate is consistent with blowup in both x & y, and confirms a significant Touschek contribution to the LEB lifetime
the HEB experiences both ‘LEB-induced’ & ‘self’ blowup
up to 0.8 mA/b, the x-size remains constant (4 % > single beam), then blows up by an additional ~ 4-7% (8-11% total blowup)
the y-size first decreases (50%  25% blowup, then back up to 40%).
Its HER-current dependence largely mirrors that of the specific luminosity
the evolution of the transverse e- loss rate is consistent with the blowup pattern above
Data: 27Jan 04

15. LEB-current dependence of L & beam sizes: observations
In single-beam mode, the LER beam sizes are current-independent
When the LER current increases (with I- nominal )
Lsp remains roughly constant, except at the highest LER current
a 5 % decrease in Lsp is observed for ib+ > 1.0 mA/b
the dependence of the total luminosity on the beam-current product
exhibits only moderate saturation; however, raising the LEB current gains little L
is limited by transverse losses (lifetime, beam-beam backgrounds)
the transverse LEB size
in x: remains constant in the horizontal plane, and is 60-70% larger than in LEB-only mode
in y: varies from 1.2 to 1.3 times its single-beam value as I+ increases
the evolution of the transverse e+ loss rate suggests moderate or no variation of the blowup level with increasing positron current
the transverse HEB size
in x: varies from 1.04 to 1.08 times its single-beam value as I+ increases
in y: rises rapidly with LEB current, up to 1.4 times its single-beam value
both horizontal & vertical loss rates rise sharply for ib+ > 1.2 mA/b
Data: 27Jan 04

16. Run 4 Luminosity History
e+ bunch current
Specific luminosity
Luminosity
e- bunch current

17. 29 Oct-29 Jan
Luminosity vs. I+ * I-
1 Oct Jul 04
29 Jan –30 Apr
30 Apr – 31 Jul