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

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

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 

HEB scan (continued) Data: 27Jan 04

HEB scan (continued) Data: 27Jan 04

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

LEB scan (continued) Data: 27Jan 04

LEB scan (continued) Data: 27Jan 04

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 tunes on L (trickle off so lifetime measurable ?)

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

Time estimate Total 330’ (540) 6 h (9 h) 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)

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

Spare slides

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

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

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

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