M. Sullivan for the PEP-II team Machine Advisory Committee Review October 25-27, 2006

1. IR Vacuum Update M. Sullivan for the PEP-II team Machine Advisory Committee Review October 25-27, 2006

2. Outline • Brief recap of last January’s presentation • History • General Characteristics • NEG heating tests • Last January Conclusions • Attempts to pin down the source • More NEG heating tests • Software • Hardware • The Answer • What it was • Present fix • Future fix

3. History • We started to notice a large number of beam aborts from high radiation levels from the detector about mid December of last year. In retrospect, we have identified events of this type as early as the beginning of Dec. • At that time, we were more concerned about the fast dI/dt LER aborts and some of us thought that perhaps these BaBar aborts were a new manifestation of the fast dI/dt aborts. • It wasn’t until Monday, the day after New Years, that we discovered we had very fast vacuum spikes somewhere near the detector on the LER upstream side whenever the detector aborted the beam due to high radiation levels.

4. Some Characteristics • Very fast high pressure spikes. Difficult to see with 6 min history buffers. • Seen in single ring for each beam but at much higher currents than colliding beams. Conclusion: either close by or inside shared beam pipe. • Insensitive to orbit (+/- 2mm in Y and +/-8 mm in X at the ends of the support tube) • Radiation levels from the LER beam are consistently higher than radiation levels from the HER beam • Exhibits more bunch charge dependence than total current dependence

5. Interaction Region Layout Detector Forward Q2 NEG Support tube end bellows 8020 pump 2187 gauge LER HER SIG11 LER sensitive 7043 pump 3027 gauge 7039 gauge 3027 pump 3044 pump Backward Q2 NEG LER frangible link NEG pump

6. NEG heating test • In order to try to localize the source of the vacuum spikes we tried heating up some of the NEG pumps and recording the vacuum readings from the various gauges and pumps • We decided to heat up the two NEG pumps closest to the IP. These are the two NEGs just outboard of the ends of the support tube • We did this without beam and we did not try to regenerate the NEG pumps

7. List of Vacuum spikes The list of vacuum aborts that we had started in January grew to over 200 entries by the 3rd week of March.

8. Pressure and Background Ratios LER radiation event Forward Q2 NEG HER radiation event Backward Q2 NEG

9. Conclusions of Last January • In December we started encountering, with increasing frequency, beam aborts associated with high radiation levels in the detector • It took us a while to understand that these aborts were a new problem and not an alternate manifestation of the instabilities we were already trying to identify • We have identified the cause of these beam aborts to be due to fast vacuum spikes in the IR. • All present information points to a region on the forward side of the detector and probably in the LER beam pipe or in the shared beam pipe. • The most likely location is the forward side support tube end bellows • The most likely initiation for the event is an arc

10. The Story Continued…. • Throughout Jan.-Mar. we made several attempts to try to locate the source of these vacuum spikes • More NEG heating tests with and without beam (the IR vacuum model has been greatly improved) • We first took out a NEG pump and then later replaced a chamber in the incoming LER beam pipe • RGA readings indicated a large burst of nitrogen when we had a vacuum spike • We installed the ability to remotely make a gas burst into the beam pipe. This told us we did not have an air leak. • The BaBar detector collaboration used the events taken by the detector when these vacuum spikes occurred to try to locate the source. Many different analysis techniques were used: from timing differences to track reconstruction to neutron counting rates… • We also analyzed timing differences between gas signals from the various gauges and pumps in the area • There was an analysis of the shape of the gas pulse from the gauges and pumps • I’m sure I have forgotten a few more…

11. Average of 8 events seen in March normalized to 2187 gauge Detector Forward Q2 NEG Support tube end bellows 10.7 100 LER HER 939 0.61 10.6 0 0.95 Backward Q2 NEG LER frangible link NEG pump

12. Side view of BaBar Forward Q1/Q2 bellows Backward Q1/Q2 bellows Forward Q2 chamber

13. Heating up the forward Q2 NEG Luminosity went down when we added gas. The decrease is noticeable at a pressure of about 30 nTorr Traced to the HER vertical spot size increasing. Luminosity restored as the pressure goes down. This told us that the HER was sensitive to relatively small gas bursts

14. BaBar Analyses from Brian’s Talk

15. The Answer • In late February we were looking at the Q1/Q2 bellows with a bore scope and trying to see if any of the tiles had come loose when it suddenly became clear that we had incorrectly designed the RF seals that are next to the tiles. The seals were touching the tiles instead of touching the Cu under the tiles. • With this knowledge we made new RF seals and prepared for a ten day access starting on March 19

16. Forward Q1/Q2 bellows section Q1 side of bellows

17. Close up of damage to the tiles

18. RF seal – note the bolt head RF finger seen in borescope videos

19. Close up of RF seal

20. Another dark spot on another finger

21. Dark spot corresponds to a pit on the tile

22. Why was the failure located here? • No real answers • Something to do with the corners of the tiles? • Something to do with the edges of the RF fingers? • Perhaps a weak point in the tile? A crack?

23. Picture of bellows before installation in 2002 This picture is a miracle!! The correct bellows section in the correct orientation!!! Thank you Scott!!

24. RF seal repositioned onto bellows Tile damage is here

25. Cu posts not brazed to the tile

26. Present Fix • We designed and built a set new RF seals so that they engage the Cu underneath the tiles • We took out the bellows section from the other side of the detector and placed it in the location of the damaged bellows section • We then reinstalled one of the Mk I bellows on the backward side of the detector. This side collects about half as much power as the forward side • All locations had new RF seals in stalled

27. 3/6 - New RF Seal Plate submitted to MFD 3/3 - Prototype RF Seal made using existing SPEAR3 RF seal die and another existing die. New Q1/Q2 Bellows RF Seal

28. New RF seal – Compound J seal

29. New RF seal – Side away from Tiles

30. New RF seal – Close up of tile side

31. Improved Design • Current bellows in the forward side (MkII) can absorb 5-10 kW and is currently absorbing about 7 kW • When we go to higher currents and shorter bunches we will absorb even more power • Presently building a new design bellows (MkIII) that will absorb less direct power from the beam • Plan to install the new design this down time

32. Q1/Q2 Blws – New Design MkIII • New concept developed • based on best information available. • Maximum Tile/slot length • ~2.4” • Absorbing tiles are open to the convolutions • No additional tile sets needed in bellows cavity. • HER Arc Style Bellows • Spring • Stub • RF shield • Possibly reduce further the travel and offset requirements to increase length.

33. Summary • We finally tracked down the problem to a design flaw in how the RF seal was engaged at the edge of the absorber tiles in the Q1/Q2 bellows section • An arc track had developed on the surface of the tile. This explains why the beam current threshold came down initially and then stabilized. • Once the problem was figured out, new RF seals were made and installed • Since the repair, we have had NO unusual abort causing vacuum activity in this area • We would like to thank all of the people who helped us with this tough problem – BaBar collaborators, engineers, technicians, machine shops, experts from other groups, etc.