m sullivan for the machine advisory committee review january 18 20 2006
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IR Vacuum. M. Sullivan for the Machine Advisory Committee Review January 18-20, 2006. Outline. History A Selected Event General Characteristics NEG heating test Vacuum model Last Minute Update Conclusions. History.

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m sullivan for the machine advisory committee review january 18 20 2006
IR Vacuum

M. Sullivan

for the

Machine Advisory Committee Review

January 18-20, 2006

  • History
  • A Selected Event
  • General Characteristics
  • NEG heating test
  • Vacuum model
  • Last Minute Update
  • Conclusions
  • We started to notice a large number of beam aborts from high radiation levels from the detector the week before the holiday break.
  • At that time, we were more concerned about the fast dI/dt LER aborts and thought perhaps these BaBar aborts were some sort of new manifestation of the fast dI/dt aborts.
  • On Christmas eve we discovered a vacuum leak in the HER and that appeared to generate instabilities in the HER. So we thought that maybe this was the cause of the aborts with high radiation levels in the detector.
  • It wasn’t until Monday, the day after New Years, that we discovered we had very fast vacuum spikes somewhere near the detector whenever the detector aborted the beam due to high radiation levels.
  • Closer inspection revealed that the detector was seeing high radiation levels from the incoming LER beam whenever these vacuum spikes occurred.
aborts with babar ler radiation
Aborts with BaBar LER Radiation

IP vacuum spikes found

HER vacuum leak found

HER leak fixed

interaction region layout
Interaction Region Layout


Forward Q2 NEG

Support tube end bellows

8020 pump

2187 gauge




LER sensitive

7043 pump

3027 gauge

7039 gauge

3027 pump

3044 pump

Backward Q2 NEG

LER frangible link

NEG pump

a selected event
A Selected Event

HER current

LER current

Pressure at 3037 goes up to ~300 ntorr and is back down to <1 ntorr in ~30 sec

2187 gauge

3027 pump

3027 gauge

selected event cont
Selected Event (cont.)


7039 gauge

8020 pump

some characteristics
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.

Relatively 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

Chicken or Egg. There was some concern that beam instabilities from some other source in the ring were making the beam jump around enough to generate these vacuum spikes in the IR. This was tested by inducing beam instabilities (both single ring and in collision) and looking for vacuum activity in the IR. No activity was seen.

neg heating test
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
q2 neg heating no beams
Q2 NEG heating (no beams)

Backward Q2 NEG

Forward Q2 NEG

The 3027 pump wakes up when the pressure gets high enough. There may be some wake up pressure for gauge 3027?

list of vacuum spikes
List of Vacuum spikes

We started a list of vacuum spikes over the last few days characterizing each event by writing down the peak pressures and peak detector background values as well as the beam currents.

pressure and background ratios
Pressure and Background Ratios

LER radiation event

Forward Q2 NEG

HER radiation event

Backward Q2 NEG

Vacuum model
  • We have a vacuum model for the Interaction Region straight
  • There are two major uncertainties in the model: the actual pumping speed of the various NEG pumps and the outgassing due to synchrotron radiation.
  • The uncertainty in the SR outgassing comes from not knowing what the photon desorption coefficient () is (next page). Perhaps the SR outgassing is not much more than the thermal outgassing rate after as many Amp-hrs as PEP-II has.
  • The IR is covered with NEG pumping (see previous diagram) so the NEG pumping speed dominates the vacuum pressure calculations
  • We are using the Q2 NEG heating data to come up with a consistent set of pumping speeds for the various NEG pumps in the IR. The model should then be able to point to the most likely source for the vacuum spikes
what is the value of
What is the value of ?

We have 27,000 Amp-hrs in the HER and 42,000 Amp-hrs in the LER. 60,000 Amp-hrs in the IR?

Last Minute Update

Yesterday we opened up the beam pipe about 15 m from the IP in the upstream LER

We then ran a borescope all the way in to the IP bellows (25 cm from the IP)

The IP bellows and of what we saw of the Q1/Q2 bellows on this side of the detector looked fine

We have not yet seen anything that is a “smoking gun”

We did observe that one RF seal between 2 flanges looks “different” from any other RF seal we saw

The location of this RF seal is in the right area

We have not yet seen the complete Q1/Q2 bellows but everything looked fine of what we did see

Time of this information 15:00 Tuesday

q2 bellows section
Q2 Bellows section

Bellows are fully compressed in pictures

  • 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
  • Yesterday we opened up the IR vacuum system and using a borescope looked at the forward side LER beam pipe