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

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IR VacuumM. Sullivan

MAC ReviewOct. 25-27, 2006

M. Sullivan

for the PEP-II team

Machine Advisory Committee ReviewOctober 25-27, 2006

IR Vacuum Update

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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

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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.

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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

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Interaction Region Layout

LER HER7039 gauge

SIG11LER sensitive

2187 gauge

3044 pump

LER frangible link

Detector

7043 pump

8020 pump

3027 gauge

Support tube end bellows

3027 pump

NEG pump

Backward Q2 NEG

Forward Q2 NEG

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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

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

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Pressure and Background Ratios

Forward Q2 NEG

Backward Q2 NEG

HER radiation event

LER radiation event

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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

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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…

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Average of 8 events seen in March

normalized to 2187 gauge

LER HER

10.6

100

LER frangible link

Detector

0.61

10.7

939

Support tube end bellows

NEG pump

Backward Q2 NEG

Forward Q2 NEG

0 0.95

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Side view of BaBarBackward Q1/Q2 bellows

Forward Q1/Q2 bellows

Forward Q2 chamber

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Heating up the forward Q2 NEGLuminosity 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

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BaBar Analyses from Brian’s Talk

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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

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Forward Q1/Q2 bellows section

Q1 side of bellows

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Close up of damage to the tiles

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RF seal – note the bolt head

RF finger seen in borescope videos

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Close up of RF seal

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Another dark spot on another finger

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Dark spot corresponds to a pit on the tile

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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?

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Picture of bellows before installation in 2002

This picture is a miracle!! The correct bellows section in the correct orientation!!!Thank you Scott!!

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RF seal repositioned onto bellows

Tile damage is here

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Cu posts not brazed to the tile

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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

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New Q1/Q2 Bellows RF Seal

• 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.

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New RF seal – Compound J seal

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New RF seal – Side away from Tiles

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New RF seal – Close up of tile side

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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

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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 shieldPossibly reduce

further the travel and offset requirements to increase length.

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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.

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

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