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If you go to the Atlas experiment and click the status button, there's an AFS reading at the bottom with a current value


The 50ns seems to refer to the bunch spacing, the 228b the number of bunches, but what do the other numbers refer to?

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You'd have to ask someone involved in that experiment. These kinds of things tend to report on the status of all kinds of things, and the unlabled numbers give no hint. Beam focus, polarization, halo sizes, currents, ... the list of candidates is endless. – dmckee Apr 15 '11 at 17:23
What you quote is not displayed at the moment. It is the LHC status page 1, . – anna v Apr 15 '11 at 19:00
@anna: click on "Latest LHC Run Status" and you get the screen the OP is talking about. pretty cool that they are live-feeding the experimental data to the web like that.. – BjornW Apr 16 '11 at 9:58
@Bjorn right. It was not running at the time I posted the previous. – anna v Apr 16 '11 at 11:06
up vote 2 down vote accepted

The Link Bjorn posted contains most of the information,

currenty the AFS (active filling scheme) reads:


50ns: bunch spacing

480b: bunches per beam

424: colliding bunches in the interaction Points 1 (ATLAS) and 5 (CMS)

12: colliding bunches in the IP 2 (ALICE)

468: colliding bunches in IP8 (LHCb)

36bpi15inj: 36 bunches per injection and 15 injections per beam

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36x15 = 540 bunches per beam, whereas 480 is stated? – John McVirgo Apr 22 '11 at 12:51
the first injection usually is the injection of a probe beam with just one bunch, and the second injection is with a lower bunch number (12). All following injections are with 36 bunches for much more detailed information, there is an java app you can download here and run via java web start abwww/ap/dist/lhc/lhc-app-injectionschemeviewer/PRO/… – luksen Apr 22 '11 at 13:40
They are currently running with the same AFS value you've quoted, yet have achieved a higher instantaneous luminosity. How have they managed that? – John McVirgo Apr 23 '11 at 10:43
I am by no means an accelerator physics expert, but I can imagine that a better squeezing and adjusting of the beams would lead to increased luminosity even if the injection scheme is the same. Also if you have higher beam intensities (that is more protons per bunch) this would increase luminosity as well – luksen Apr 23 '11 at 11:19

The LHC filling-scheme coding is described here. Found after some digging :)

Very cool live displays, I must say! It's almost like having your own backyard accelerator...

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Just need to get some popcorn and then sit back and enjoy the data! This is awesome indeed. Thanks @Bjorn – user346 Apr 16 '11 at 10:51
@Deepak is a great way for keeping up to date with what's going on at the LHC. It was hoped to at least run the LHC through 2011 with 75ns spacing, 200 bunches to achieve 1.5/fb of data. If they can get it to run with 50ns, 1400 bunches, they could collect 14/fb of data. Right now they are running with 50ns spacing 336 bunches so it's interesting to see how they are steadily making spectacular progress over what was initially hoped. – John McVirgo Apr 16 '11 at 11:27
@John thanks for that link. It seems useful. Question: I'm guessing a "bunch" is some constant number of protons. How much data is generated for every proton-proton collision? How much data/bunch? – user346 Apr 16 '11 at 15:27
@Deepak, I haven't a clue :) – John McVirgo Apr 17 '11 at 21:45
a bit OT but: coming from a computer background, I find it fascinating this whole stuff with the amazing amount of data generated every second from the detectors. there must be a huge amount of automatic processing to mark and flag interesting events.. and a LOT of work to verify any candidates.. – BjornW Apr 17 '11 at 21:50

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