Post on 05-Apr-2018
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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
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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.