Different strategies of electron cloud enhancement

1. Different strategies of electron cloud enhancement G. Iadarola , G. Rumolo SPSU-BD Study Group Meeting 12 October 2011

2. SPS scrubbing B. Goddard at LIU-SPS Coordination Meeting - 22 June 2011

3. Reference scenario • We consider the geometry of an MBB bending magnet with its average beta functions(<βx> = 33.85m <βy> = 71.87m) • All comparisons are carried out at injection energy E=26GeV assuming SEYmax= 1.5 and r.m.s. bunch length σz=0.2m. • We compare our results against the nominal 25ns beam i.e. bunch spacing bs=25ns, normalized emittanceεn=3μm and the following 4 batches filling pattern: 72 8 72 8 72 8 72 25ns buckets

4. Nominal 25ns beam 72 8 72 8 72 8 72 25ns buckets

5. Evaluating the scrubbing efficiency • In order to evaluate the scrubbing efficiency of the considered configurations we look at: • The scrubbing electron dose (number of e- with energy ≥20eV hitting the wall in one turn) • The distribution of the scrubbing current on the wall (since we want to scrub the same regions that are affected by electron cloud when the nominal beam is in the machine) • For the nominal 25ns beam we have: • 6.3e11 scrubbing e- per meter per turn

6. Scrubbing strategy 1 - 5ns bunch spacing • The idea is to extract from the PS in one turn (2.1 μs) the standard CNGS beam (2.4e13 protons) and immediately capture in the SPS 5ns buckets (this means approximately 418 bunches with intensity ~5.7e10 ppb). • It should be reasonable to inject two batches since the total charge in the SPS is the same of the standard CNGS beam. We assume normalized emittanceεn=6.5μm and the following filling pattern: 418 44 418 44 5ns buckets

7. Scrubbing strategy 1 - 5ns bunch spacing Beam intensity 5e10 ppb • We need two batches to scrub more efficiently than the 25ns nominal beam • With two batches the scrubbing dose is enhanced by a factor 4

8. Scrubbing strategy 1 - 5ns bunch spacing Electrons in this region receive the kick by the bunch passage, but do not reach the wall before the following bunch passage. • This beam scrubs very efficiently the central part of the chamber but practically does not scrub the regions involved by the nominal beam’s stripes

9. Scrubbing strategy 2 – Slip scrubbing 72 8 72 72 8 72 8 25ns buckets • The idea is to employ slip stacking in order to move the last two batches onto the first two

10. Scrubbing strategy 2 – Slip scrubbing 72 8 72 72 8 72 8 25ns buckets • The idea is to employ slip stacking in order to move the last two batches onto the first two

11. Scrubbing strategy 2 – Slip scrubbing 72 8 72 72 8 72 8 25ns buckets • The idea is to employ slip stacking in order to move the last two batches onto the first two

12. Scrubbing strategy 2 – Slip scrubbing 72 8 72 72 8 72 8 25ns buckets • The idea is to employ slip stacking in order to move the last two batches onto the first two

13. Scrubbing strategy 2 – Slip scrubbing 72 8 72 72 8 72 8 25ns buckets • The idea is to employ slip stacking in order to move the last two batches onto the first two

14. Scrubbing strategy 2 – Slip scrubbing 72 8 72 72 8 72 8 25ns buckets • The idea is to employ slip stacking in order to move the last two batches onto the first two

15. Scrubbing strategy 2 – Slip scrubbing 72 8 72 72 8 72 8 25ns buckets • The idea is to employ slip stacking in order to move the last two batches onto the first two • With the SPS RF system we can obtain two configurations: • (10+15)ns • (5+20)ns

16. Scrubbing strategy 2 – Slip scrubbing

17. Scrubbing strategy 2 – Slip scrubbing

18. Scrubbing strategy 2 – Slip scrubbing • The (10+15)ns configuration is much more efficient than (5+20)ns • With two(10+15)ns batches the scrubbing dose is enhanced by a factor 5

19. Scrubbing strategy 2 – Slip scrubbing • The (10+15)ns beam efficiently scrubs the entire region that is interested by the nominal beam while the (5+20)ns scrubs only the central region (similarly to the 5ns beam)

20. Scrubbing strategy 2 – Slip scrubbing In order to understand why (10+15)ns in much more efficient than (5+20)ns,let us consider the following quantity: Normalized e- number growth rate [s-1] • (10+15)ns • (5+20)ns

21. Scrubbing strategy 3 – Presence of 5-10% coast. beam 72 8 72 8 72 8 72 25ns buckets • The idea is to have 5-10% of uncaptured beam in order to enhance electron cloud effect. • This study is motivated by some observation from past MDs with one 25ns batch, namely: • A strong enhancement of the electron cloud is observed when the coasting fraction fills the entire machine but not when a gap is present in the coasting part • After the injection of a second batch, which cleans the uncaptured beam, a reduction on the electron cloud signal from the strip monitor is observed

22. Scrubbing strategy 3 – Presence of 5-10% coast. beam • The observed behavior is reproduced in simulation if we consider a situation for which one batch is not sufficient to reach saturation (e.g. 25ns nominal beam, 1batch, SEYmax= 1.3) • A memory effect can be observed among different turns due to the electrons trapped by the coasting fraction • The presence of a gap in the coast cleans this memory effect

23. Scrubbing strategy 3 – Presence of 5-10% coast. beam • Since the electron number settles alter a few μs after the last bunch we can simulate a shorter machine length in order to have the effect of several turns avoiding huge simulation times • A sort of regime is reached after five turns • The number of e- hitting the wall in on turn is enhanced by a factor 2000 with respect to simple 1 batch situation and by a factor 30 against 2 batches

24. Scrubbing strategy 3 – Presence of 5-10% coast. beam • Let us consider a realistic scrubbing scenario (4 batches, SEYmax= 1.5) • Saturation is reached within the injected batches, no multi-turn effect is observed • Only the contribution of the first batch is enhanced (the scrubbing dose does not increase more than 30%)

25. Scrubbing strategy 4 – PS bunch splitting deregulation 72 8 72 8 72 8 72 25ns buckets • The idea is to introduce a deliberate deregulation in the PS splitting process in order to have an odd-even modulation in bunch intensity.

26. Scrubbing strategy 4 – PS bunch splitting deregulation • Thescrubbing dose systematically decreases when the odd-even modulation is increased • In particular, the slope during the build-up phase decreases and this can give an indication to understand this behavior…

27. Scrubbing strategy 4 – PS bunch splitting deregulation Prevalent absorption interval Prevalent emission interval Bunch passage The normalized contribution of the n-th bunch passage to the electron cloud is given by: n-th beam period

28. Scrubbing strategy 4 – PS bunch splitting deregulation Prevalent absorption interval Prevalent emission interval Bunch passage The normalized contribution of the n-th bunch passage to the electron cloud is given by: Δn is proportional to the slope of the e- number curve in log scale, since: n-th beam period

29. Scrubbing strategy 4 – PS bunch splitting deregulation • We run several simulations with different beam intensities in order to study the dependence of Δn vs intensity. We found a non monotonic behavior:

30. Scrubbing strategy 4 – PS bunch splitting deregulation This behavior can be understood if we look at the energy spectrum of the e- impacting on the wall:

31. Scrubbing strategy 4 – PS bunch splitting deregulation We can try to estimate the growth rate of the number of e- from the energy spectrum using the following formula: • The non monotonic behavior of the electrons growth rate is the effect of a match/mismatch between the energy spectrum of the electrons and the shape of the SEY curve

32. Scrubbing strategy 4 – PS bunch splitting deregulation Let’s look to Δn behavior when we increase the odd/even deregulation: Nominal intensity

33. Scrubbing strategy 4 – PS bunch splitting deregulation Let’s look to Δn behavior when we increase the odd/even deregulation: Involved bunch intensities

34. Scrubbing strategy 4 – PS bunch splitting deregulation Let’s look to Δn behavior when we increase the odd/even deregulation: Involved bunch intensities

35. Scrubbing strategy 4 – PS bunch splitting deregulation Let’s look to Δn behavior when we increase the odd/even deregulation: Involved bunch intensities

36. Scrubbing strategy 4 – PS bunch splitting deregulation Let’s look to Δn behavior when we increase the odd/even deregulation: Involved bunch intensities

37. Scrubbing strategy 4 – PS bunch splitting deregulation Let’s look to Δn behavior when we increase the odd/even deregulation: Involved bunch intensities

38. Conclusions We have investigated several strategies for the enhancement of the electron cloud in the SPS. Our conclusions are summarized in the following table:

39. Thanks for your attention!

40. ….

41. Conclusions • We have investigated several strategy for the enhancement of the electron cloud in the SPS. • 5ns beam • We need to inject at list two batches • In this case the scrubbing dose is enhanced by a factor 4 • Only the central region of the pipe is scrubbed efficiently • Slip scrubbing • (10+15)ns much more efficient than (5+20)ns • In this case the scrubbing dose is enhanced by a factor 5 • The region affected by electron cloud for the nominal beam is scrubbed efficiently • Presence of 5-10% of uncaptured beam • Can lead to a significant enhancement when there is not a strong multipacting • In our scrubbing scenario does not give more than 30% enhancement • Odd-even bunch intensity modulation • No electron cloud enhancement is observed

42. Scrubbing strategy 1 - 5ns bunch spacing

43. Slip sacking Slip stacking seems to be very promising and in particular (10+15)ns in much more efficient than (5+20)ns. To understand why let’s look at the quantity: Normalized e- number growth rate [s-1] (10+15)ns at saturation: all high the high energy impacts, due to the first bunch of the doublet, happen before the passage of the second bunch.

44. Slip sacking Slip stacking seems to be very promising and in particular (10+15)ns in much more efficient than (5+20)ns. To understand why let’s look at the quantity: Normalized e- number growth rate [s-1] (5+20)ns at saturation: part of the high energy impacts, due to the first bunch of the doublet, are avoided because of the passage of the second bunch.

45. Scrubbing strategy 4 – Presence of 5-10% coast. beam