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Re: Current Events Discussion

Posted: Sat Oct 02, 2010 10:11 am
by terryburton
At the moment the machine operators believe the UFOs to be the result of dislodged dust falling through the beam. They hope that this exists as an artefact of the machine construction rather than a systematic problem and that it will be burned away by continual use so that the number of UFO events decreases with time (relative to beam energy).

Re: Current Events Discussion

Posted: Sat Oct 02, 2010 3:16 pm
by josch222
Is it possible that there is still some soot in the beam pipes or magnets, remaining from
the incident at September 19 2008 in Sector 3-4?
AFAIR they replaced a lot of magnets but they repaired and/or cleaned some as well.
As I read about this procedure I wondered how they did this, after all there is the beam shield and I doubt they were able to get behind it. Maybe there is some fluff from the
cleaning rags left :-).
Some days ago they had some losses while moving the roman pots despite they were
far away from the beam. Could this be related to the UFOs? Falling dust due to some
vibration from the roman pots motors?

I already feel sorry about the poor guy who is sent on a 27km walk to knock on all the pipes and magnets to shake the dust down ;-)

Re: Current Events Discussion

Posted: Sat Oct 02, 2010 9:38 pm
by terryburton
The incidence of the UFOs does not appear to be concentrated at the location of the events of September 2008.

Source is three weeks old: http://cdsweb.cern.ch/record/1294051 (at 35 mins)

Re: Current Events Discussion

Posted: Sun Oct 03, 2010 7:10 pm
by Tau
If you don't want to watch the 40 minute video, you can see just the slides at
http://indico.cern.ch/getFile.py/access ... fId=103500
The information about the dust is on slide 20.

Re: Current Events Discussion

Posted: Sun Oct 03, 2010 7:56 pm
by PhilG
There was a lot more info about the UFOs on Myers' recent talk http://indico.cern.ch/conferenceDisplay ... fId=105780 slides 22-27 but see the video for the full story.

Re: Current Events Discussion

Posted: Mon Oct 04, 2010 1:05 am
by josch222
Thanks for the references,
nice to see that someone in the audience at Myers speech had a similar assumption.
Myers explanation sounds very reasonable but I don't understand why they didn't
cleaned the beam shields before they assembled them (or burred them before packing),
it seems to be a known problem in accelerators.

Even the asymmetric shape (slow rise and faster decay) of the losses is explained
if the debris is vaporized by the beam.
But one thing is a bit strange:
If I calculated correctly, a small falling piece needs about 1ms to pass the beam (1mm)
if it had 50mm of free fall. One single bunch needs only 90┬Ás for one revolution.
That is enough time for every piece to get hit if it falls through the beams way.
So why is the rate of this events intensity dependent (and so well correlated)?
I can only think of two options:
1. The area used by the beam increases with intensity (different bunches take
different ways and so a larger horizontal plane is occupied in the pipe and the chance
a falling piece is hit increases)
2. Some electrostatic effect: A piece of dust sits at the upper wall.
When a bunch passes it may be shaken a bit, if there are more bunches passing,
the chances are increasing that it gets shaken loose or picks up charge that will
move it around.

Re: Current Events Discussion

Posted: Mon Oct 04, 2010 7:01 am
by Kasuha
josch222 wrote:I don't understand why they didn't
cleaned the beam shields before they assembled them (or burred them before packing),
it seems to be a known problem in accelerators.
They did. It would be much more frequent if they didn't.
josch222 wrote:So why is the rate of this events intensity dependent (and so well correlated)?
At lower intensities the shower generated is less intense as well and does not trigger beam dump. They actually scanned log data from past successful runs and found multiple such events which just weren't strong enough to cause a problem.

Re: Current Events Discussion

Posted: Mon Oct 04, 2010 12:06 pm
by josch222
Kasuha wrote: At lower intensities the shower generated is less intense as well and does not trigger beam dump. They actually scanned log data from past successful runs and found multiple such events which just weren't strong enough to cause a problem.
So lets assume that the total rate of beam-dust events is constant,
regardless of intensity. If there is a fixed radiation energy threshold involved the
event rate above this threshold rises with intensity.
That would explain the strong correlation of the 7 events (that triggered a dump)
pretty well.
For the other events given in the "Sub-threshold UFOs" diagram the same applies,
but with a detection threshold given by sensitivity and noise-floor of the BLMs and
shielding effects. If one would add this events the correlation would be weaker.
So if this (somewhat macroscopic) model were right, the correlation would disappear
if one would be able to detect all beam-dust interactions.
But if one looks at the nuclei-level this model is still wrong because more protons around
increase the possibility of interactions with the dust and in the end there will be always a correlation with intensity. Even if the dust-falling-rate would be constant.
Hope I got it right this time :rolleyes:
I wonder if there are some non-linear effects too, depending on what amount
of dust gets evaporated, atomized, ionized or quarkized (? :-) per time.
Or can the cross section be viewed as constant (for a given shape and mass)?

Another question:
I'm not sure because I did not understand what Myers were talking about,
but I believe Myers talked about possibilities to cure the dust-events.
I understand something like "clean it out with high beam intensity" and/or
something about degassing (in the LEP). Did he meant to use the heaters
in the pipes (or screens) to fix any plastic stuff around by melting it?
It is around 00:17:43 at
http://indico.cern.ch/materialDisplay.p ... fId=105780

Re: Current Events Discussion

Posted: Mon Oct 04, 2010 3:18 pm
by DCWhitworth
200 Bunch fill next up !! More records to fall !

Re: Current Events Discussion

Posted: Mon Oct 04, 2010 5:56 pm
by Kasuha
josch222 wrote:I'm not sure because I did not understand what Myers were talking about,
but I believe Myers talked about possibilities to cure the dust-events.
I understand something like "clean it out with high beam intensity" and/or
something about degassing (in the LEP). Did he meant to use the heaters
in the pipes (or screens) to fix any plastic stuff around by melting it?
It is around 00:17:43 at
http://indico.cern.ch/materialDisplay.p ... fId=105780
There's deep vacuum in the pipe, but during construction the pipe is kept under normal atmospheric pressure and during that molecules of atmospheric gas may get deposited within thin layer of the internal pipe surface. When there's vacuum in the pipe, they can escape again and disturb the beam. Degassing means controlled release of these molecules e.g. by heating up the pipe so that as many as possible of them escape during that period instead of during production runs.
They mentioned LHC does not have such problem because its beam is positively charged so I assume these particles have a positive charge too. They are also individual atoms/molecules rather than dust particles.
UFOs are apparently caused by particles which are much bigger.

Re: Current Events Discussion

Posted: Fri Oct 08, 2010 2:25 am
by ferar
Hi, why is Atlas BGND2 so big (it reads 1454 now) compared to CMS (1.710)? In wich unit is it meassured? And the difference between BGND #1, 2 and 3?
Thanks very much!
Regards;
Fernando

Re: Current Events Discussion

Posted: Fri Oct 08, 2010 2:31 am
by jmayes
The Christmas Tree is getting Brighter!

2e52+13, 150ns_248b_233_16_233_3x8bpi15inj :dance: :clap: :dance: :clap: :yawinkle: :goodman: :D :D :D

Re: Current Events Discussion

Posted: Fri Oct 08, 2010 2:50 am
by CharmQuark
Good Job guys :thumbup: :clap: :dance:

Nice pic Jmayes hehehe :whistle:

Atlantis Christmas tree needed

Posted: Fri Oct 08, 2010 6:30 am
by 7holography
Fantastic new bunch train :clap:

but where is the Atlantis live collision display ? :angry-screaming:

Don't stop climbing the barn mountain !!! :happy-sunny:

Re: Current Events Discussion

Posted: Sat Oct 09, 2010 11:17 pm
by tomey36
atlas has recorded over 1 trillion collisons! this has to be some kind of milestone, and hopefully enough for some exciting new physics ;)