Beams have bunches in wrong bucket; abort gap??

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mugwort
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Beams have bunches in wrong bucket; abort gap??

Post by mugwort » Tue Dec 08, 2009 4:31 am

As of 8-Dec 0329 (CERN time), the operator reports that the bunches in both beams are about 128 buckets behind where they should be; cause unknown.

They're expressing concern because this puts the bunches only 83 slots ahead of the "abort gap". Does anybody here know what role the abort gap plays, and why it's a problem if there is beam in that gap?

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jmayes
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Re: Beams have bunches in wrong bucket; abort gap??

Post by jmayes » Tue Dec 08, 2009 5:04 am

I think this is directly related to my Beam Crossover question in this thread
http://lhcportal.com/Forum/viewtopic.php?f=4&t=142

I basically was wondering if the 2 beams (beam paths) are separated until collision is wanted, but if not and the 2 beam paths are permanently combined while in the experiments then control of the bunch timing is critical to getting the collisions to happen at the right spot in the experiments. I think what is happing now is the collisions are 83 periods off (which they call buckets) and now collisions are hitting in a an area they call the "abort zone". I would guess if the bunch timing moves any more they will have to dump or a collision may occur before or after the detectors intended area which would cause damage.

Am I right?

Looking for some experts views here :)

Thankx!
J

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Xymox
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Re: Beams have bunches in wrong bucket; abort gap??

Post by Xymox » Tue Dec 08, 2009 5:16 am

Not sure... im no expert.. Sounds reasonable to me...

mugwort
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Re: Beams have bunches in wrong bucket; abort gap??

Post by mugwort » Tue Dec 08, 2009 5:54 am

The two beams run through separate pipes except at the crossover points, and I've seen diagrams that show the beams being focused and entering the crossovers at a significant angle -- so the site of collisions shouldn't be very timing-dependent.

It's also my understanding that the RF system keeps bunches tightly within their bucket slots; compression in all 3 dimensions is necessary in order to get a good collision rate for a given beam current. From the operator's comments, it appears that bunches somehow landed in the wrong bucket, rather than drifting across a number of buckets. Maybe they were mis-located at load time, due to a handshake problem from SPS or farther upstream?

As I recall, something like 80% of the bucket slots are occupied when the LHC is running at design luminosity, so the other 20% are reserved somehow; the abort gap must be one of those. However, I don't think I've ever spotted a diagram of beam-bunch layout...

It's interesting to witness the issues that arise when shaking-down a project of this magnitude.
jmayes wrote:I think this is directly related to my Beam Crossover question in this thread
http://lhcportal.com/Forum/viewtopic.php?f=4&t=142

I basically was wondering if the 2 beams (beam paths) are separated until collision is wanted, but if not and the 2 beam paths are permanently combined while in the experiments then control of the bunch timing is critical to getting the collisions to happen at the right spot in the experiments. I think what is happing now is the collisions are 83 periods off (which they call buckets) and now collisions are hitting in a an area they call the "abort zone". I would guess if the bunch timing moves any more they will have to dump or a collision may occur before or after the detectors intended area which would cause damage.

J

daavery
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Re: Beams have bunches in wrong bucket; abort gap??

Post by daavery » Tue Dec 08, 2009 6:10 am

think of the buckets as cars in a railroad train. each beam has 2808 buckets in it. there is at least 1 gap between the end and the beginning of the train. that gap is the "abort gap". the gap is needed to allow the dump steering magnet to switch from off to on during a time when there are no particles passing through the dump steering magnet just like you never would want to throw a train track switch when a train is passing over the switch.

the beam rotates around the ring 11245 times a second or 88.9 uSec / rev. the abort gap is 3uSec or about 3.3% of the beam length

mugwort
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Re: Beams have bunches in wrong bucket; abort gap??

Post by mugwort » Tue Dec 08, 2009 6:36 am

Oops, I see I responded hastily to jmayes; at the four crossover points, both beams do share the same pipe, and protons collide head-on (low angle). As somebody stated in the crossover thread, collisions are turned on by shifting beam path rather than bunch timing.

Daavery, thanks for the info! They need a 3uSec gap so beam doesn't go spraying at other components while the dump steering magnet engages. Given the energy at "only" 450GeV and 1/500 max current, there's good reason not to want the beam going astray...

(Imagine: a machine that *starts* at 450GeV.)

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chriwi
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Re: Beams have bunches in wrong bucket; abort gap??

Post by chriwi » Tue Dec 08, 2009 6:39 am

As to my understanding (read somewhre here or in one of the cern documents linked from here) you have to imagine the bunches of protons as a big wheel rotating in the LHC (ofcorse the material is only flying protons) the buckets are just virtual positions on this wheel positioned around the circumference of the wheel. Each bucket can hold a bunch of protons or can be emty.
The abort gap is a number of buckets in a row which are supposed to be never loaded with protons and form a gap in the wheel. The purpose of this gap is that in the dump process it takes a short, but signifficant time to switch on the magnets which divert the beam from the LHC into the dump. Supposedly the dump magnets should be onlyswiched on during the abort gap is passing them in orer to give the beam only 2 fixed paths (circulating or dump) but nothing inbetween which could damage equipment surrounding the dump magnets or the dump path. Nevertheless its also possible in emergency to switch to the dump any time but then ther is alway a chance of damage done by the beam, but ther have already been tests of switching aoutside the abort gap at leas while the energys are as low as they are until now.
Therfore it is safer if the beam commissioninggroup can manage to maintain a well defined abort gap to ensure a clean dump, but at least at current energies it is also not really dangerous if they cant (I think also for maximum energy and no abort gap only minor damagesto the system are expected).
bye

chriwi

daavery
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Re: Beams have bunches in wrong bucket; abort gap??

Post by daavery » Tue Dec 08, 2009 7:45 am

heres the latest description of the LHC filling pattern I have found:

http://cdsweb.cern.ch/record/691782/fil ... te-323.pdf

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Xymox
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Re: Beams have bunches in wrong bucket; abort gap??

Post by Xymox » Tue Dec 08, 2009 7:54 am

Interesting reading..

You gotta love all this great info on the CDS

mugwort
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Re: Beams have bunches in wrong bucket; abort gap??

Post by mugwort » Tue Dec 08, 2009 8:32 am

They now have refilled for one bunch into bucket #1. It does seem to have been a Super Proton Synchrotron vs LHC timing issue, per operator's log:

B1: CIRCULATE AND DUMP to check that now we inject bunch 1 in bucket 1. Philippe confirms that this is the case but that there is a phase problem between SPS and LHC and he is tuning this.

Fudge
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Re: Beams have bunches in wrong bucket; abort gap??

Post by Fudge » Tue Aug 30, 2011 1:10 pm

I like reading this good stuff. really interesting and useful for me:)

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