What's the maximum bunch intensity they can go for?

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jmc2000
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What's the maximum bunch intensity they can go for?

Post by jmc2000 » Thu Sep 15, 2011 2:44 pm

On fill 2109 they're going for a bunch intenstiy of 1.4x10^11 protons per bunch which seems way higher than 1.2x10^11 last week. So how high can they go?

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DCWhitworth
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Re: What's the maximum bunch intensity they can go for?

Post by DCWhitworth » Thu Sep 15, 2011 8:49 pm

I guess they probably can't go much higher, but no one knows exactly how high they can go I think because they are already beyond the originally envisaged maximum intensity of 1.15x10^11. As far as I know they've already pushed it up to 1.35x10^11. This is new territory beyond what was originally expected of the machine.

I suspect the limit might be dictated by the injector chain rather than the LHC itself.
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tomey36
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Re: What's the maximum bunch intensity they can go for?

Post by tomey36 » Thu Sep 15, 2011 10:58 pm

I read that they were going for 1.6x10^11 per bunch. I think it was in one of the progress reports.

adam_jeff
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Re: What's the maximum bunch intensity they can go for?

Post by adam_jeff » Fri Sep 16, 2011 10:05 am

The 'nominal' LHC bunch was 1.15x10^11, as DCWhitworth said, so we're already well beyond that. But there is an 'Ultimate' LHC bunch which I think is 1.7x10^11. This is what they hope to achieve after a few years of running, and with a few minor equipment upgrades. The main issues in getting to this intensity are various instabilities, mostly space charge in the PS/SPS and electron cloud in the LHC.
The 'High-Luminosity LHC' upgrade, (which is just a design with no funding as yet, and at least 10 years away) would see bunches of 4x10^11, but this would need a major upgrade of the injectors.

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PhilG
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Re: What's the maximum bunch intensity they can go for?

Post by PhilG » Fri Sep 16, 2011 5:07 pm

For now the main limiting factor is actually the cryogenics. With 50ns spacing they can go to about 1.7E11 before the cooling rate required from the cryogenics is too much. The exact amount may depend on the e-cloud build-up and such. They injected bunches with over 2E11 during MD so the injectors are probably not the limit, and stability was not a problem either, at least with small numbers of bunches.

When they go to 25ns spacing (next year?) they will only be able to reach nominal bunch intensity 1.15E11 before cryogenics become an issue. This means that 25ns will not directly help improve luminosity, but it could reduce event pileup allowing them to go to higher luminosity by using more squeeze.

For the HL-LHC they will need an upgrade to the cryogenics as well as the injector chain.

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Re: What's the maximum bunch intensity they can go for?

Post by Harbles » Wed Sep 21, 2011 7:28 pm

It appears the higher intensities have been taking their toll on the Vacuum system, from the daily news;

Fill # 2130, 2131, 2133, 2134 dumped by vacuum spikes left of IP2 (Fill # 2132 scrubbing at 450GeV).

10:20 Stable beams fill #2135, 2.5E33 cm-2s-1. Lower bunch intensity (1.2E11/b).

So a sensible choice to sacrifice some Instantaneous Luminosity to enable longer dump free runs which will increase the Integrated Luminosity over the long term.

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