Sullivan IRvacuum

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IR Vacuum

M. Sullivan

MAC Review

Oct. 25-27, 2006

M. Sullivan

for the PEP-II team

Machine Advisory Committee Review

October 25-27, 2006

IR Vacuum Update


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IR Vacuum

M. Sullivan

MAC Review

Oct. 25-27, 2006

Outline

Brief recap of last Januarys 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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IR Vacuum

M. Sullivan

MAC Review

Oct. 25-27, 2006

History

We started to notice a large number of beam aborts from highradiation levels from the detector about mid December of lastyear. In retrospect, we have identified events of this type asearly as the beginning of Dec.

At that time, we were more concerned about the fast dI/dt LERaborts and some of us thought that perhaps these BaBaraborts were a new manifestation of the fast dI/dt aborts.

It wasnt until Monday, the day after New Years, that we

discovered we had very fast vacuum spikes somewhere nearthe detector on the LER upstream side whenever the detectoraborted the beam due to high radiation levels.


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IR Vacuum

M. Sullivan

MAC Review

Oct. 25-27, 2006

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 currentsthan colliding beams. Conclusion: either close by or inside sharedbeam pipe.

Insensitive to orbit (+/- 2mm in Y and +/-8 mm in X at the ends ofthe support tube)

Radiation levels from the LER beam are consistently higher thanradiation levels from the HER beam

Exhibits more bunch charge dependence than total current

dependence


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IR Vacuum

M. Sullivan

MAC Review

Oct. 25-27, 2006

Interaction Region Layout

LERHER

7039 gaugeSIG11LER 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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IR Vacuum

M. Sullivan

MAC Review

Oct. 25-27, 2006

NEG heating test

In order to try to localize the source of the vacuumspikes we tried heating up some of the NEG pumpsand recording the vacuum readings from thevarious gauges and pumps

We decided to heat up the two NEG pumps closestto 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 toregenerate the NEG pumps


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IR Vacuum

M. Sullivan

MAC Review

Oct. 25-27, 2006

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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IR Vacuum

M. Sullivan

MAC Review

Oct. 25-27, 2006

Pressure and Background Ratios

Forward Q2 NEG

Backward Q2 NEG

HERradiationevent

LERradiationevent


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IR Vacuum

M. Sullivan

MAC Review

Oct. 25-27, 2006

Conclusions of Last January

In December we started encountering, with increasing

frequency, beam aborts associated with high radiationlevels in the detector

It took us a while to understand that these aborts were anew problem and not an alternate manifestation of theinstabilities we were already trying to identify

We have identified the cause of these beam aborts to bedue to fast vacuum spikes in the IR.

All present information points to a region on the forwardside 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 tubeend bellows

The most likely initiation for the event is an arc


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IR Vacuum

M. Sullivan

MAC Review

Oct. 25-27, 2006

The Story Continued.

Throughout Jan.-Mar. we made several attempts to try to locate the source ofthese 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. Manydifferent analysis techniques were used: from timing differences to trackreconstruction to neutron counting rates

We also analyzed timing differences between gas signals from the variousgauges and pumps in the area There was an analysis of the shape of the gas pulse from the gauges and

pumps Im sure I have forgotten a few more


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IR V


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IR Vacuum

M. Sullivan

MAC Review

Oct. 25-27, 2006

Side view of BaBar

Backward

Q1/Q2bellows

Forward

Q1/Q2

bellows

Forward

Q2chamber

IR V


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IR Vacuum

M. Sullivan

MAC Review

Oct. 25-27, 2006

Heating up the

forward Q2 NEGLuminosity went down when weadded gas.

The decrease is noticeable at a

pressure of about 30 nTorr

Traced to the HER vertical spotsize increasing.

Luminosity restored as the

pressure goes down.

This told us that the HER wassensitive to relatively smallgas bursts

IR V


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IR Vacuum

M. Sullivan

MAC Review

Oct. 25-27, 2006

BaBar Analyses from Brians Talk

IR Vacuum


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IR Vacuum

M. Sullivan

MAC Review

Oct. 25-27, 2006

The Answer

In late February we were looking at theQ1/Q2 bellows with a bore scope and tryingto see if any of the tiles had come loosewhen it suddenly became clear that we hadincorrectly designed the RF seals that arenext to the tiles. The seals were touchingthe tiles instead of touching the Cu underthe tiles.

With this knowledge we made new RF sealsand prepared for a ten day access startingon March 19

IR Vacuum


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IR Vacuum

M. Sullivan

MAC Review

Oct. 25-27, 2006

Forward Q1/Q2 bellows section

Q1 side of

bellows

IR Vacuum


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IR Vacuum

M. Sullivan

MAC Review

Oct. 25-27, 2006

Close up of damage to the tiles

IR Vacuum


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IR Vacuum

M. Sullivan

MAC Review

Oct. 25-27, 2006

RF seal note the bolt head

RF finger

seen inborescopevideos

IR Vacuum


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IR Vacuum

M. Sullivan

MAC Review

Oct. 25-27, 2006

Close up of RF seal

IR Vacuum


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IR Vacuum

M. Sullivan

MAC Review

Oct. 25-27, 2006

Another dark spot on another finger

IR Vacuum


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IR Vacuum

M. Sullivan

MAC Review

Oct. 25-27, 2006

Dark spot corresponds to a pit on the tile

IR Vacuum


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M. Sullivan

MAC Review

Oct. 25-27, 2006

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?

IR Vacuum


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M. Sullivan

MAC Review

Oct. 25-27, 2006

Picture of bellows before installation in 2002

This picture is a

miracle!!

The correct

bellows section in

the correct

orientation!!!

Thank you Scott!!

IR Vacuum


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M. Sullivan

MAC Review

Oct. 25-27, 2006

RF seal repositioned onto bellows

Tile damageis here

IR Vacuum


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M. Sullivan

MAC Review

Oct. 25-27, 2006

Cu posts not brazed to the tile

IR Vacuum


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M. Sullivan

MAC Review

Oct. 25-27, 2006

Present Fix

We designed and built a set new RF seals so thatthey 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 ofthe damaged bellows section

We then reinstalled one of the Mk I bellows on the

backward side of the detector. This side collectsabout half as much power as the forward side

All locations had new RF seals in stalled

IR Vacuum


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M. Sullivan

MAC Review

Oct. 25-27, 2006

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.

IR Vacuum

S


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M. Sullivan

MAC Review

Oct. 25-27, 2006

New RF seal Compound J seal

IR Vacuum

M S lli


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M. Sullivan

MAC Review

Oct. 25-27, 2006

New RF seal Side away from Tiles

IR Vacuum

M S lli


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M. Sullivan

MAC Review

Oct. 25-27, 2006

New RF seal Close up of tile side

IR Vacuum

M Sullivan


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M. Sullivan

MAC Review

Oct. 25-27, 2006

Improved Design

Current bellows in the forward side (MkII)can absorb 5-10 kW and is currentlyabsorbing about 7 kW

When we go to higher currents and shorterbunches 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

IR Vacuum

M Sullivan


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M. Sullivan

MAC Review

Oct. 25-27, 2006

Q1/Q2 Blws New Design MkIII

New concept

developed based on best

informationavailable.

MaximumTile/slot length

~2.4

Absorbing tilesare open to theconvolutions

No additionaltile setsneeded inbellowscavity.

HER Arc StyleBellows

Spring

Stub

RF shield

Possibly reduce

further the travel and

offset requirements to

increase length.

IR Vacuum

M Sullivan


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M. Sullivan

MAC R i

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 theQ1/Q2 bellows section

An arc track had developed on the surface of the tile. Thisexplains why the beam current threshold came down initiallyand then stabilized.

Once the problem was figured out, new RF seals were madeand installed

Since the repair, we have had NO unusual abort causingvacuum activity in this area

We would like to thank all of the people who helped us with thistough problem BaBar collaborators, engineers, technicians,machine shops, experts from other groups, etc.

Documents

Sullivan IRvacuum