Beam-beam sensitivity to parasitic crossings & Xing angle

1. Beam-beam sensitivity to parasitic crossings & Xing angle • Goal: measure the luminosity degradation associated with • parasitic crossings • horizontal crossing angle • Principle • by-2 pattern: compare Lsp at minimum, nominal & maximum parasitic-xing separation ( = e- x-angle) with full L optimization at each setting  sensitivity to Xing angle + parasitic crossings • by-4 pattern: compare Lsp at minimum, 0, & maximum (achievable) Xing angles ( = e- x-angle) with full L optimization at each setting  sensitivity to Xing angle only • HEB only: measure impact (if any) of e- x-angle on e- beam properties W. Kozanecki, Y. Cai, W. Colocho, J. Seeman, M. Sullivan, J. Turner (with special thanks to Nate Lipkovitz & Cliff Blanchette)

2. + x Experimental aspects (I) • Horizontal separation @ parasitic crossings • XP(e-) more +ve DX(PC)  • nominal: DX(PC) = 3.22 mm @ z = +/- 63 cm • for XPmax(e-) = - 0.60 / + 0.85 mrad, DX  3.6 mm (+ 12%) / 2.7 mm (-17%)

3. Experimental aspects (II) • Quality/reproducibility of measurements • thermal / beam-beam effects  keep currents constant (total / per bunch) • sparsified by-2: 836 bunches, 1201/751 mA, 1.44/0.90 mA/b • by-4: 851 bunches, 1221/758 mA, 1.43/0.89 mA/b • trickle both beams • re-optimize Lsp at each XP(e-) setting • tunes • local & global skews (both rings) • PR02 LER sext bumps (HER always, LER most of the time) • y-angle, collision phase (most of the time)

4. I. Measure Lsp degradation associated with parasitic Xings + Xing angle Sparsified by-2 pattern, LER/HER = 1.4/0.9 mA/b • Setup • Set LER/HER YANG, SLM/interferometer light levels • In both LER & HER, optimize all local & global skews, PR02 SEXT bumps, SD2 bumps in LER Arcs 5 & 11, collision phase • Mini scan of XP(e -) (+- 0.3 mrad) to locate optimum e- angle (XPopt = 0 )

5. Lsp degradation with parasitic Xings + Xing angle (cont’d) • At XP = 0, + 850, - 600, - 300, + 300 mrad • Optimize LER+HER local & global skews, PR02 SEXT bumps • Optimize collision phase • Record tune spectra, gated camera data, Lsp & Ib+,- patterns along the train •  20% degradation at + 850mrad • Investigated correlated variations in tunes & e+/e- spot sizes: • no clear trend in LER/HER tune tracker readings (too few points compared to fluctuation size) • no clear trend in LER SLM/interferometer sizes (fluctuations) • definite trend in HER spot sizes

6. Lsp degradation with parasitic Xings + Xing angle (cont’d) • At XP = 0, +850, - 600, -300, + 300 mrad • Optimize LER+HER local & global skews, PR02 SEXT bumps • Optimize collision phase • Record tune spectra, gated camera data, Lsp & Ib+,- patterns along the train •  20% degradation at + 850mrad

7. II. Measure Lsp degradation associated with Xing angle only by-4 pattern, same LER/HER bunch currents • Setup • Skew quads/sext bumps already restored to XP=0 settings found in step I • Optimize tunes, collision phase (in case RF-transient is pattern-dependent) • Mini scan of XP(e -) to check optimum e- angle (before further optimiation)  optimum XP very different (more +ve!)

8. Lsp degradation with Xing angle only (cont’d) • Optimize Luminosity at XP = +550, +850, - 600, 0 mrad • note XP=0 is by definition the optimum e- angle found in the by-2 pattern • Even after optimization @ + 850 mrad, L is higher at somewhat smaller XP(e-), and then drops again. sy- displays a corresponding trend.

9. Lsp degradation with Xing angle only (cont’d) • Even after optimization @ - 600 mrad, Lsp is higher at larger XP(e-). sy- displays a corresponding trend.

10. Lsp degradation with Xing angle only (cont’d) • Similar effect seen in previousXing-angle MD (by-4 pattern, 11 May 04)

11. Lsp degradation with Xing angle only (cont’d) • Optimize specific luminosity at XP = +550, +850, - 600, 0 mrad • note XP = “0” is by definition the optimum e- angle found in the by-2 pattern • Lsp > 4.1 @ XP = “0” • 7% degradation at + 850mrad

12. Without parasitic Xings (by-4) Lsp exhibits a parabolic dependence on XP(e-) With parasitic Xings (by-2) the peak Lsp is ~ 5% lower (@ nominal PC separation) than in the by-4 pattern the larger XP(e-), the steeper the Lsp degradation The optimum e- x angle is ~ 0.2 mrad more -ve in the by-2 pattern ( weaker PC effects) This suggests that in the presence of parasitic Xings, the optimum e- angle is a compromise between Xing-angle & PC-inducedluminosity degradation Lsp dependence on Xing angle & PC separation: experimental summary

13. Simulation neglects Xing-angle effects Lsp dependence on Xing angle & PC separation: data vs. simulations

14. Related topics... • Parasitic crossings • how do the Pacman bunches fare? • what is happening in the long minitrain? • Crossing angle (w/o PC) • why do the HER optics vary (or appear to vary) with electron x-angle, even though there are non-linear elements inside the XP bump?

15. Parasitic crossings: how do the Pacman bunches fare? Sparsified by-2 pattern

16. Parasitic crossings: the drooooping minitrain

17. e- x-angle response of Lsp &HER beam sizes in collision Collisions, by-4 Collisions, by-4 Collisions, by-4 No optimization during scan

18. HEB x size (e- only, by-2) HEB y size (e- only, by-2) e- x-angle response of HER beam sizes Collisions, by-4 Collisions, by-4

19. HEB x tune (e- only, by-2) HEB y tune (e- only, by-2) e- x-angle response of HER tunes Collisions, by-4 Collisions, by-4

20. Summary (in words...) • In the by-4 pattern (where parasitic-crossing ing effects are expected to be negligible) • The specific luminosity exhibits a roughly parabolic dependence on the horizontal e- angle (after reoptimization @ each angle). It degrades by ~ 6-7 % for an e- x-angle of ~ 650 mrad above the optimum. • At the same angle, the simulation predicts a 3% degradation only. More generally, the crossing-angle dependence of the luminosity is significantly steeper in the data than in the simulation. • Systematic variations of the e- horizontal beam size and vertical tune, observed in e- x-angle scans recorded in collision, are also apparent, and of comparable magnitude, when varying the horizontal e- angle in single-beam mode. The large variations in vertical HEB spot size, observed in collision only, are strongly correlated with Lsp variations and clearly of beam-beam origin. • Whether the horizontal spot size variation could be associated to image motion on the SLM screen remaisn to be verified. But it is unlikely, because the x-angle bump is reasonably well closed. • Even though the e- horizontal-angle bump spans only linear optical elements (apart from the solenoid), the observed tune variation suggests the presence of significant non-linear fields in that region of the HER.

21. Summary (more words...) • In the presence of parasitic crossings (sparsified by-2 pattern) • The peak specific luminosity is ~ 5% lower (@ nominal PC separation) than in the by-4 pattern, where parasitic crossings should be negligible; the more positive the e- x-angle, the steeper the additional luminosity degradation. • The optimum e- x-angle is ~ 200 mrad more negative (i.e.  weaker PC effects) in the by-2 pattern, than in the by-4 pattern. This suggests that in the presence of parasitic crossings, the optimum e- angle is a compromise between Xing-angle & PC-induced luminosity degradation. • The dependence of the PC-associated luminosity degradation on e- angle (i.e. on horizontal PC separation) is consistent with, and slightly weaker than, that predicted by beam-beam simulations. • “Pacman” bunches exhibit a luminosity degradation that varies from 20-25% (wrt to other minitrain bunches) near the optimum e- angle, to 10-15% at large positive angle (850 mrad). This effect is not understood and requires further study.

22. Summary (in pictures)

23. Appendix: documentation & data sets • PEP-II e-log • collision data: dedicated MD, 1 Jul 04, day + swing + early owl shifts • HEB-only data: opportunistic MD, 14 Jul 04, swing shift • Data sets • collision data: PHYSICS4_DATA:[pep2.char.1Jul04] • L, currents, beam sizes, tunes, quads & bumps: lumtun_*_1Jul.dat • bunch-by-bunch data: XP*_BICDATA.MAT • gated camera: gacam_*_1Jul,dat • single-beam data: PHYSICS4_DATA:[pep2.ip.witold.smr04B]lumt_herxpcall_2_14Jul • orbit fit set to PR02 BPMS 7052-8012 (HIPP) throughout