Beam-beam limits: MD proposal

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

2. MD goals • Characterize current dependence of beam-beam performance • specific luminosity • vertical & horizontal spot sizes in LER & HER (incl. SXM) • loss rates @ 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. Data: 27Jan 04 Current dependence of L & beam sizes: HEB scan By-2 pattern 1320 bunches 10 Apr 04 

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

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

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

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

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

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 SLM/interf./SXM @ 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 tuneson 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: repeat @ 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: repeat @ IH ~ 0.65 mA/b

11. Time estimate • Full-current optimization (BBR taking data) • QSB setup & reoptimization @ nominal Ibunch 105’ • injection setup 15’ • steer to gold 45’ • optics optimization 45’ • Zero-current point 15’ • HER current scan 105’ (210’) • 7 points @ nominal IL 7x15’ • repeat at intermediate IL 7x15’ • LER current scan 105’ (210’) • 7 points @ nominal IH 7x15’ • repeat at intermediate IH 7x15’ Total 330’ (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 fixed @ 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- fixed @ 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. Specific luminosity Run 4 Luminosity History e+ bunch current Luminosity e- bunch current

17. 29 Oct-29 Jan 29 Jan –30 Apr 30 Apr – 31 Jul Luminosity vs. I+ * I- 1 Oct 03-31 Jul 04