Let's go hump hunting!

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Tau
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More hump info

Post by Tau » Fri Apr 16, 2010 7:14 am

Thanks Chris for the hint.
Here some more enlightening remarks from that shift and the next one:
  • central hump frequency seems to move much faster inside the spectrum since we increased the bunch intensity...
  • (an hour later:) B1 lifetime with 9e10 is about 30h. Lots of hump in the tune spectrum
  • (a few hours later:) we could inject nice and tiny beams with regulating all parameters and thanks to the absence of the hump
I did not put in any pictures from the log, since they do not give much information; if you have an external account, look at the elog from 15 april night shift (which actually is 16 april).

Anyway: we just learnt that the hump can "just" vanish, and that it appears to depend on the intensity of the beam.
Intruiging, isn't it?
- Tau

Kasuha
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Re: Let's go hump hunting!

Post by Kasuha » Fri Apr 16, 2010 7:59 am

Hmm so beam energy does not affect the hump (only its amplitude) but beam intensity does?
Couldn't that be some kind of self-inducing effect? A resonance between beam and pipe wall in some place where the beam travels off-center?

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Re: Let's go hump hunting!

Post by chriwi » Fri Apr 16, 2010 8:25 am

Looks like it also depends how firm they are alreadywith the setups, before they had also the hump with the low intensity beams (what was all they had there) now when they go to higher intensities the hump is onlythere and often gone for the routine setups of low intensity.
bye

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Tau
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New hump measurement

Post by Tau » Tue Apr 20, 2010 6:08 am

I found a graph showing the lifetime of the beam, measured one night last week:
Lifetime.png
Lifetime influenced by hump
Lifetime.png (37.97 KiB) Viewed 5022 times
Here you can see that if the hump is there, it indeed has a clear cycle that resembles some machine that regulates something.
Since this graph shows beam lifetime instead of hump frequency, we cannot deduce a lot on the hump frequency, but we can clearly see it is something that goes up and down every 7 minutes or so.
- Tau

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mrgumby
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Re: Let's go hump hunting!

Post by mrgumby » Tue Apr 20, 2010 7:06 am

I don't know anything about this stuff. but......could it be something external to the LHC? Like a train passing....? :think:

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Re: Let's go hump hunting!

Post by Kasuha » Tue Apr 20, 2010 7:31 am

mrgumby wrote:I don't know anything about this stuff. but......could it be something external to the LHC? Like a train passing....? :think:
That'd probably affect both beams about equally which is not the case.


I wonder... is the tunnel completely "quiet" when the beam is in? As far as I know, the tunnel consists of curved sections and straight sections. If something heavy was moving through the tunnel passing a sector every 7 minutes it might produce similar effect (e.g. frequency rising when going through curved part and stationary when going through straight part). It would require speed of about 8 m/s which is rather fast but not too fast. How fast does TIM move anyway?
Last edited by Kasuha on Tue Apr 20, 2010 7:38 am, edited 1 time in total.

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Re: Let's go hump hunting!

Post by mrgumby » Tue Apr 20, 2010 7:35 am

perhaps.....wouldn't the 2 beams have opposite magnetic fields?

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Re: Let's go hump hunting!

Post by Kasuha » Tue Apr 20, 2010 7:43 am

mrgumby wrote:perhaps.....wouldn't the 2 beams have opposite magnetic fields?
Yes but these are oriented to bend the beam horizontally. Hump is mainly in vertical direction in which both beams are getting about the same field strength most of the time.

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mrgumby
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Re: Let's go hump hunting!

Post by mrgumby » Tue Apr 20, 2010 7:56 am

Fair enough....Looking at the wave shape.....Typically a slow rise then an abrupt fall. Like a motor starting up then turning off. A vacuum pump?

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Tau
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Shape of the hump frequency

Post by Tau » Thu Apr 22, 2010 6:50 am

mrgumby wrote:Fair enough....Looking at the wave shape.....Typically a slow rise then an abrupt fall. Like a motor starting up then turning off. A vacuum pump?
Well, that graph is beam lifetime, instead of "hump intensity" (whatever that may mean). So it is a bit hard to determine the shape of the hump from this graph.
The best picture of the shape of the hump is made a while ago by the operators when they set the FFT tune tracker to follow the hump.
When reading the picture below, ignore the vertical stripes: these are an artefact of the tracker that misses the hump sometimes. Try to look "through" these lines to see the actual shape: it is more or less triangular.
20100326125410.png
Hump frequency followed by tune tracker
20100326125410.png (41.08 KiB) Viewed 5084 times
- Tau

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Re: Let's go hump hunting!

Post by mrgumby » Thu Apr 22, 2010 7:20 am

yup, I can see the triangle you refer to, it still has a more gentle slope "up" than "down" (mostly) it still looks like a motorized something starting up then turning off.

I don't do physics, but I am a "fix it" person.

If I was diagnosing this, I would try to find out if the hump correlated to a specific area of the tunnel, if it doesn't then its probably outside the tunnel, (its a 7 minute (ish) cycle and most of the stuff in the tunnel operates at thousands of cycles per second) if it does, look at the current (as in V over I) records of the devices in the tunnel and look for a match. If its outside the tunnel, look at... I dunno....trains, nearby industrial sites, Overhead flights...all of these can cause electromagnetic disturbances.
Its "interesting"

:think:

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Tau
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Hump reading material for diehards

Post by Tau » Fri Apr 23, 2010 7:59 am

By googling around, I found these documents on the hump. You probably need a CERN account, but I am not sure:
https://lhc-commissioning.web.cern.ch/l ... RECTED.pdf
https://lhc-commissioning.web.cern.ch/l ... bility.pdf
- Tau

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mrgumby
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Re: Let's go hump hunting!

Post by mrgumby » Fri Apr 23, 2010 8:15 am

Tau....thanks for that.....I think I withdraw my idea of outside interference....

still interesting tho....I'm still curious if there is any data on spatial location, I mean does the hump happen more in any particular part of the ring?

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Tau
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Location in the tunnel?

Post by Tau » Sat Apr 24, 2010 7:47 am

Yes, it would be great to have an idea where it is in the tunnel. I couldn't find anything on that; I am not sure if that means they don't know, or that they just didn't write it down.
- Tau

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Tau
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Log entry about the hump

Post by Tau » Sat Apr 24, 2010 1:32 pm

Today's (24 of april) log have more hump info. The most interesting is this, in the shift summary:
Hump active from 7:30 - 9:50 then quite calm.
In other words: the hump varies much more wildly than I thought!
Looking further in the log, there was another "hump attack" from 17:40 to 18:10.
So whatever this hump is, it switches off and on whenever it wants.
- Tau

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