The Hump

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Danny252
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The Hump

Post by Danny252 » Mon Mar 01, 2010 11:09 pm

CERN has some fun names for problems apparently...

It seems that the "Hump" is an unwanted oscilliation that's been observed since last year - and it sounded like some people were rather pissed off that it hadn't gone away after Christmas! It seems they're having a fun time trying to find the cause of it and eliminate it.

The last few pages of this briefing give some info about it:
https://lhc-commissioning.web.cern.ch/l ... 1_1700.pdf (They changed the link! Now works)

Does anyone have any better understanding of the Hump than myself?

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Tau
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Re: The Hump

Post by Tau » Tue Mar 02, 2010 7:10 am

I recommend to read this from the forum: viewtopic.php?f=4&t=219
There PCatom explains what tuning is.
You can see a tuning graph with hump on page 10 of http://indico.cern.ch/getFile.py/access ... nfId=85751
The tuning graph (sometimes shown in the OP log) ideally shows two sharp peaks, but there is a vague "hump" on it that moves around. The fact that it moves around signifies that the oscillation comes from the outside.
What the hump is, is not clear yet, but indeed it is an unwanted oscillation of some kind. Or, there's a camel in the tunnel :-)

(Later addition: There is now a hump hunt thread: viewtopic.php?f=5&t=498&p=4632)
Last edited by Tau on Wed Apr 14, 2010 9:30 am, edited 1 time in total.
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Danny252
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Re: The Hump

Post by Danny252 » Tue Mar 02, 2010 1:53 pm

Tau wrote:The fact that it moves around signifies that the oscillation comes from the outside.
Which would explain the table on Page 8 of the link in the first post, with the turning off of various possible influences and comparing the hump (or lack of) when these happen.

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Re: The Hump

Post by Danny252 » Tue Mar 02, 2010 11:05 pm

Insert 1 complicated diagram of the hump(s) here.
Attachments
Hump.gif
Hump.gif (78.35 KiB) Viewed 6170 times

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Tau
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Re: The Hump

Post by Tau » Wed Mar 03, 2010 11:43 am

The "right" hump is actually supposed to be there, it is the "tune", that you can recognise from the diamonds. The left one is the real hump.
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Re: The Hump

Post by Danny252 » Wed Mar 03, 2010 4:20 pm

Actually, the log I nabbed the image from noted both as humps - and if you look closely, it is 3 humps with 1 between the diamonds.
LHCOP wrote:B2 hump reference
one hump in V at 0.21 and another 0.305 .below the vertical tune.
no hump visible in H
EDIT: Actually, looking at today's hump pics, I'm wondering if the red diamond actually signifies the hump.

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Tau
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Hump measurements, march 4: some computations

Post by Tau » Thu Mar 04, 2010 8:33 am

The sharp peaks with diamonds are the "tunes": they are supposed to be there.
It is the "vague" hump that are the problem.
The needles with a spacing of about .01 are actually result of the quadrupoles: you can ignore these. (For physicisists: there are about 100 FODO cells in the LHC).
The hump resides around 0.2; I've seen it move from around .18 to about .21: that's about 2 kHz.
I wonder if someone took the sound from inside the tunnel and checked it for a 2 kHz whistle tone...
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Re: The Hump

Post by Angrond » Sun Mar 07, 2010 5:23 pm

From what I have gathered from various posts and presentations:
These 'hump' oscillations appear to be tiny and more than ten thousand times smaller (<< 100 nano-metre) than the average beam size (~ 1 mm) at injection. They seem to get some attention only because one of their frequencies is by-chance close to the nominal vertical machine tune (Qv=0.31), amplifying these excitations by a factor of 100-1000, and in turn causing a slow continuous growth of the beam size. Either way, mitigating whatever is causing these 'humps' (the clean solution) or simply shifting the vertical tune working point to another suitable value (the practical solution) seems to solve this.
This may turn out to be one of those effects like the French TGV train or lunar/solar tides on LEP in the 1990s.

To the best of my recollection,
[http://accelconf.web.cern.ch/accelconf/ ... PAP054.PDF],
RHIC had, at some point, millimetre level oscillations due to flow of cryogenic cooling liquids inside their triplet magnets. They seem to have mitigated this by changing the helium pressure.

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Tau
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Nice paper found on hump

Post by Tau » Thu May 13, 2010 1:36 pm

I just found a paper from end of 2009 that has very detailed information on chromaticity and the hump. They also talk about an 8 kHz oscillation that has been fixed in the mean time:
https://espace.cern.ch/acc-tec-sector/E ... _02_RS.pdf
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Re: The Hump

Post by LarryS » Thu Oct 21, 2010 12:58 pm

Tucked away in the OP Log during the current Technical Stop is the following small comment"

"21-10-10 12:55 end of activity with collimator for the hump hunting"

While it has not been mentioned in quite awhile, "hunting" is still going on.

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LarryS
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Re: The Hump

Post by LarryS » Thu Oct 21, 2010 1:05 pm

With respect to all grammar majors, please allow me to change the last two words of my previous post from "going on" to "on going" under the rule:

A preposition is a bad word to end a sentence with.

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Re: The Hump

Post by josch222 » Fri Oct 22, 2010 9:15 pm

Hi LarryS,
is it possible to get access to the OP Log as a normal person?

I saw the hump mentioned in the "commissioning latest news" rarely in the past weeks.
At least once the hump was mentioned and a second time was about installing additional
magnetic probes (or moving the previously installed ones) to some location in the ring.
I haven't seen any results from that measurements yet.

I think they had some more serious problems to address lately:
The UFOs, electron clouds, injection problems at LHCb and at a septum (MSIA/MSIB)
and others that seemed to affect stable beams much more than the hump.

Has anybody seen a report about the bended RF-Fingers at MSIA/MSIB?
I was searching for some information (and pics) but haven't found anything so far
(besides the x-ray pics in the meeting slides from "latest news").

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LarryS
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Re: The Hump

Post by LarryS » Fri Oct 22, 2010 9:44 pm

I believe the only access to the OP LOG (and many additional logs) is through a CERN ID.

If anyone knows of another way, please post that information.

Thanks ...

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Re: The Hump

Post by Kasuha » Sat Oct 23, 2010 9:10 am

You need Cern External Account to see OP logs, but there is very little info about the Hump. They are not the ones investigating it, so it only gets mentioned when it's exceptionally annoying or when they do some special operations aimed at Hump hunting.

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Re: The Hump

Post by PhilG » Fri Dec 10, 2010 10:05 am

Here is the latest on the Hump from Evian
http://indico.cern.ch/getFile.py/access ... fId=107310

The UPS is now the prime suspect.

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