Beam observations with different bunch spacings and synthesis G. Arduini – BE/ABP

1. Beam observations with different bunch spacings and synthesis G. Arduini – BE/ABP with input from: V. Baglin, H. Bartosik, O. Dominguez Sanchez De La Blanca, U. Iriso, J.M. Jimenez, K. Li, H. Maury, E. Métral, F. Roncarolo, G. Rumolo, B. Salvant, F. Zimmermann Acknowledgements: BI, Cryo, Injection Team, OP, RF

2. Outline Beam stability observations and expectations Summary of the observations Why a scrubbing run? Scrubbing run plan Summary and outlook Chamonix 2011 - Beam observations with different bunch spacings and overall synthesis

3. Beam stability at 450 GeV/c (50 ns) F. Roncarolo 12+4x24 – 1.85 ms spacing V: ~0.2 s rise time H: ~1 s rise time E. Métral E-cloud build-up over more trains instabilities, e blow-up (mostly V) Expected behaviour of instability driven by electron cloud in the arcs: coupled bunch instability in H-plane ( damper) single bunch instability in the V-plane Chamonix 2011 - Beam observations with different bunch spacings and overall synthesis

4. Beam stability at 450 GeV/c (50 ns) F. Roncarolo Single bunch instability can be stabilized by increasing chromaticity to very large values (up to 18 units)and by transverse emittance blow-up in the injectors (~3-3.5 mm @ SPS extr.) although blow-up is still observed. Effect of the chromaticity predicted by simulations (E. Benedetto, F. Zimmermann 2006) This is the way we have to run for scrubbing but not possible for physics due to the low lifetime and blow-up Chamonix 2011 - Beam observations with different bunch spacings and overall synthesis

5. Beam stability at 450 GeV/c (75 ns) High chromaticity allows stabilizing the beam but blow-up (mainly vertical) is still observed  Larger blow-up in the vertical plane compatible with instability and incoherent effects generated by e-cloud close to threshold Bunch population = 1.2×1011 p – batch spacing 1.005 ms F. Roncarolo Chamonix 2011 - Beam observations with different bunch spacings and overall synthesis

6. Instability thresholds E. Benedetto, F. Zimmermann (2006) • No single bunch e-cloud instability expected at 450 GeV/c (worst case) and 3.5-4 TeV/c if electron density <1011e-/m3 at the beam position. • Once the threshold electron density is reached the beam is unstable Chamonix 2011 - Beam observations with different bunch spacings and overall synthesis

7. Incoherent effects 450 GeV/c (for 30% of LHC field free and 70% of LHC dipole field) • Tune spread due to electron cloud pinching can reach 0.01 for 1011 e-/m3 at 450 GeV/c (worst case) at the beam position • At 75 ns (>600 bunches) the heat load measured in the arcs was ~10 mW/m (close to resolution limit)  AVERAGE e- density of 6 x 1010 e-/m3 assuming an average e- energy of 100 eV and ~800 bunches • 75 ns beam is at the limit of single bunch stability/incoherent effects even for low cryo activity in the arcs K. Li, G. Rumolo Chamonix 2011 - Beam observations with different bunch spacings and overall synthesis

8. Summary of the observations Reduced vacuum activity with 75 ns spacing as compared to 50 ns but important pressure rise observed for large number of bunches also for 75 ns  need scrubbing to ramp and collide more than ~200-300 bunches with 75 ns spacing with no significant pressure rise Low heat load (close to detection limit) due to e-cloud in the beam screens for 75 ns beam ( slow scrubbing in the arcs) Typical signature of ECI observed with 50 ns. For 75 ns beam blow-up is visible correlated to coherent and incoherent effects close to ECI threshold leading to low lifetime and losses  scrubbing Comparison of 50ns vacuum and heat load during the ramp and at 3.5 TeV before and after scrubbing at 450 GeVshows clear improvement in the straight sections as mentioned in the previous talk but we never achieved so far 100 mW/m during the scrubbing period with 50 ns nor with 75 ns (max. ~40 mW/m) Chamonix 2011 - Beam observations with different bunch spacings and overall synthesis

9. Aim of the scrubbing run • Limit the electron cloud density at the beam centre to less than 1010 e-/m3 for 75 ns beam operation to: • Remain well below the threshold for the onset of the electron cloud single bunch instability • Limit the tune spread which is at the origin of the incoherent blow-up • At the end of the scrubbing run we should aim at getting heat loads in the arcs below or close to detection limits for 50 ns beam to stay below these densities for 75 ns operation • SEY < 1.7 - 1.8are required from simulations in the dipoles and in the field free regions for a 75 ns beam at 450 GeV/c and at 3.5-4 TeV/c. • The above values are within reach within ~1 week of scrubbing (see JM Jimenez presentation) even assuming an efficiency of ~30% although based on an optimistic electron dose rate not assuming reduction of the flux of electrons in the last part of the scrubbing Chamonix 2011 - Beam observations with different bunch spacings and overall synthesis

10. Why scrubbing? (SPS experience) Scrubbing 2002 Reduction of the V-emittance blow-up along a 15 s injection plateau BUT still blow-up at injection for the beam that is used for scrubbing Chamonix 2011 - Beam observations with different bunch spacings and overall synthesis

11. Why scrubbing with 50 ns for operation at 75 ns? (SPS experience) • An electron-cloud free environment cannot be achieved efficiently for the beam that is used for scrubbing as the cleaning become less and less efficient as the SEY reduces  scrubbing with tighter spacing than those used in operation  50 ns scrubbing for operation with 75 ns M. Taborelli StSt (cond.) StSt a-C StSt StSt a-C Chamonix 2011 - Beam observations with different bunch spacings and overall synthesis

12. Scrubbing run requirements • Pre-conditions for the start of the scrubbing run: • In the injectors: • 50 ns beam with up to 1.5x1011 p/bunch with emittances in the range 2-3.5 mm • 75 ns beam with up to 1.2x1011 p/bunch with emittances in the range 1.5-3.5 mm • In the LHC: • Injection of 4x36 bunches with 50 ns spacing (up to nominal transverse emittance at least) must be set-up in advance (3-4 days work) • Machine protection set-up for high intensity at 450 GeV/c (up to 1404 bunches) • RF should be conditioned for operation at high intensity • Transverse feedback set-up for high intensity operation with 75 and 50 ns beams • During the scrubbing run: • Solenoids (experimental and anti e-cloud) should be OFF • Vacuum interlock levels should be temporarily increased to 2x10-6 mbar where and when needed and compatibly with machine and experiment protection requirements Chamonix 2011 - Beam observations with different bunch spacings and overall synthesis

13. Scrubbing run with 50 ns beam • First half of the scrubbing run (Day 1-2): • Inject trains of up to 4x36 bunches (1.3-1.5x1011 p/bunch assuming operation with 75 ns at 1.2x1011 p/bunch to allow for current decay) with nominal emittance up to >1000 bunches compatibly with vacuum rise, heat loads and beam stability. • Monitoring of the heat load on the beam screen and RF stable phase (µ energy loss) as well as vacuum evolution to follow-up the scrubbing progress • Large chromaticity and nominal emittances will be required to stabilize the beams  expect low lifetime as observed during the scrubbing • In the middle of the run (Day 3-4): • Evaluation of the sensitivity to orbit, radial position, energy • collapse the separation bumps, crossing angles with circulating beam with high intensity circulating single beam at 450 GeV  main unknown is the localization of the scrubbing in dipole field regions and possibly “scrub” in these conditions • Ramp with 50 ns beam (few hundred bunches according to progress) Chamonix 2011 - Beam observations with different bunch spacings and overall synthesis

14. Scrubbing run with 50 ns beam • Second half of the scrubbing run (Day 5-7): • Reduction of the transverse emittance to maximize the energy of the electrons and reduce chromaticity (if possible) to maximize lifetime • End of the scrubbing run (Day 8): • ramp with a 50 ns beam with few hundred bunches to assess effectiveness of the scrubbing and provide input for requirements for operation at 50 ns • Start physics with 75 ns with fast ramp-up in intensity to 900 bunches Chamonix 2011 - Beam observations with different bunch spacings and overall synthesis

15. Summary and outlook • SPS experience + LHC observations  scrubbing with 50 ns beams at 450 GeVshould allow: • operation with 75 ns below multipacting threshold • Suppression of electron-cloud related coherent and incoherent effects • Unwanted pressure rise in uncoated areas • Allow machine development with 50 ns beams with large number of bunches during the year and evaluation of next step (50 ns operation) • This imply having a period of at least 7 days of scrubbing + 1 day for validation and scrubbing result evaluation. • Simulation effort (build-up, instability thresholds, tune spread) for the LHC has been re-started (but takes time!) to provide best fit to the data and complete simulations to investigate parametric dependancies(energy, bunch length, 50 ns limitations) and provide additional input for the scrubbing run. Chamonix 2011 - Beam observations with different bunch spacings and overall synthesis

16. Spare slides Chamonix 2011 - Beam observations with different bunch spacings and overall synthesis

17. Chamonix 2011 - Beam observations with different bunch spacings and overall synthesis

18. Simulations (e-cloud build-up) • Preliminary simulation results indicate that the observations with 50 ns beam for the pressure rises in the uncoated field free regions are consistent with SEY>2.2-2.5 and r~0.5 assuming emax of 230 eV and initial pressure of 10-9 mbar. O. Dominguez, G. Rumolo, F. Zimmermann Chamonix 2011 - Beam observations with different bunch spacings and overall synthesis

19. 75 ns – 824 bunches • 75 ns is certainly better than 50 ns but scrubbing will be required in order to ramp with 936 bunches also taken into account that the nominal scheme assumes trains of up to 96 bunches (here maximum train length is 48 bunches) V. Baglin, G. Bregliozzi, G. Lanza

20. 75 ns beam and vacuum at 450 GeV Chamonix 2011 - Beam observations with different bunch spacings and overall synthesis

21. Cryogenics: 75 ns vs. 50 ns 75 ns up to 824 bunches (Beam 1). ~10 mW/m heat load (close to detection limit)  AVERAGE density ~6x1010 e-/m3 50 ns up to 444 bunches (Beam 1)  heat load due to e-cloud: ~ 40 mW/m  AVERAGE density ~5x1011 e-/m3 Visible e-cloud activity ~ 40 mW/m on beam1 L. Tavian Evian Workshop - 50 and 75 ns operation

22. Beam stability at 450 GeV/c (75 ns) Q’H,V=14 Q’H,V=24 B. Salvant Observed oscillation (coupled-bunch mode) could be related to sources other than e-cloud as not only the tail of the batch is oscillating  to disentangle need to test operation with low chromaticity in the H-plane. Chamonix 2011 - Beam observations with different bunch spacings and overall synthesis

23. Instability thresholds K. Li, G. Rumolo PRELIMINARY 450 GeV/c field free 450 GeV/c dipole 1010 [e-/m3] 1010 [e-/m3] • No single bunch e-cloud instability expected at 450 GeV/c (worst case) and 3.5-4 TeV/c if electron density <1011 e-/m3 at the beam position. • Once the threshold electron density is reached the beam is unstable • If dominated by: • e-cloud in field free regions  single bunch instability (SBI) in both planes, • e-cloud in dipole field regions  SBI in V-plane only Chamonix 2011 - Beam observations with different bunch spacings and overall synthesis

24. Ongoing simulations work • If dominated by • e-cloud in field free regions  single bunch instability (SBI) in both planes, • e-cloud in dipole field regions  SBI in V-plane only 4 TeV/c dipole 4 TeV/c field free K. Li, G. Rumolo PRELIMINARY Chamonix 2011 - Beam observations with different bunch spacings and overall synthesis

25. at re=6x1011m-3 instability suppressed for Q’>15 similar to LHC slow emittance growth below threshold LHC injection, Nb=1.15x1011, re=6x1011 m-3 E. Benedetto, Ph.D. Thesis, Politecnicodi Torino, 2006