LHC portal

We have a opportunity to ask questions about the displays and operation of the LHC and get the questions answered by people who run the machine.

Post some direct and single line type questions that we need to ask to better understand the displays or operation.. I will use these answers to help make our Info pages for the LHC.

Hi Xymox,

Can I start with a big "Thanks" to you personally, its a great job you are doing & much appreciated.

I can think of a number of bits of information that we could use:

1. A run down on the different page 1 displays as the LHC shifts from mode to mode and what information is contained therein.

2. Details as for page 1 on the status pages or equivalents for the various detectors. It would be good to find an overall status page for the CMS incidentally and also to get back the one for Atlas, but though there is a lot of information in the Alice status its mostly indecipherable.

3. A description of 'where to go and what to look for' in order to get some sort of overview of what is going when collisions are happening. I'm guessing that once physics starts in earnest page 1 & status ops will become very repetitive.

Thanks in advance to the CERN people (and others) who are coming up with these explanations.

Tim Bergel wrote: 3. A description of 'where to go and what to look for' in order to get some sort of overview of what is going when collisions are happening. I'm guessing that once physics starts in earnest page 1 & status ops will become very repetitive.

Try: http://lhcportal.com/Forum/viewtopic.php?f=3&t=84

Try: viewtopic.php?f=3&t=84

That's great for the status of the machine but won't show any collision info, I think. What I was hoping for was an overview showing how the various detectors were doing at logging collisions.

Obviously only the barest overview is possible ...

Please can someone explain the following message from the Page 1: AFS: Single_2b_1_1_1 (this one (fill 1438) is just a test ramp for checking some LHCb polarization change, but please explain what the 1_1_1 would mean otherwise)

the 1-1-1 stands for number of colliding bunches. when they run later in the week for physics you should see numbers near 300. also they corespond to the different experaments but i am not sure in what order. for example 150-5-300 could mean 150 colliding bunches in atlas 5 for alice abd 300 for cms. sorrey if i am not making much sense i am not used to giveing these kind of explanations. i hope this helps

Yes, the number of bunches for each different detector. This makes sense. Thanks

What would "AFS:" stand for?

I think AFS stands for Active Filling Scheme

Based on what I understand, the following describes the collision notation:

Sample: 150ns_312b_295_16_295_3x8bpi19inj

150ns - 150 nano seconds bunch separation (bunch is a group of individual buckets)

312b - total number of buckets injected

295 - number of buckets colliding at IP1(Atlas) and IP5(CMS) located 180 degrees apart

16 - number of buckets colliding at IP2(Alice)

295 - number of buckets colliding at IP8(LHCb) and can be different than IP1/IP5

3x8bpi - number of buckets per injection (may not always be the same number of buckets per injection thus the difference from 3 X 8 X 19 does not equal 295

19inj - number of injections or bunches


Note: IP1/IP5 are 180 degrees apart and thus the same bucket number always collides in each while IP2 and IP8 are displaced from this 180 degree line and require a difference of 8910 between bucket numbers for IP2 collisions and a difference of 8940 between bucket numbers (in the opposite direction) for IP8 collisions.

As I indicated, this is my understanding based on what I have been able to find while sitting here "across the big pond".

LarryS wrote:150ns - 150 nano seconds bunch separation (bunch is a group of individual buckets)

As far as I understand it...

bucket = imaginary box in which protons can circle around the ring. There are over 30,000 buckets in the LHC ring but during a fill most of them are empty and only some contain protons.

bunch = group of protons circling together in one of buckets

During injection from SPS, multiple bunches are injected to the LHC ring and the 150 ns is the time between these bunches within the injection. LHC is designed for as low as 50 ns spacing but getting there will take some time.

Kasuha wrote: LHC is designed for as low as 50 ns spacing but getting there will take some time.

The nominal spacing is 25ns, and 50ns spacing may start this week.

I'd love to see a detailed explanation of the CMS DAQ displays from CMSTV http://cmsdoc.cern.ch/cmscc/cmstv/cmstv ... frames=yes

Kasuha wrote:

LarryS wrote:150ns - 150 nano seconds bunch separation (bunch is a group of individual buckets)

As far as I understand it...

bucket = imaginary box in which protons can circle around the ring. There are over 30,000 buckets in the LHC ring but during a fill most of them are empty and only some contain protons.

bunch = group of protons circling together in one of buckets

During injection from SPS, multiple bunches are injected to the LHC ring and the 150 ns is the time between these bunches within the injection. LHC is designed for as low as 50 ns spacing but getting there will take some time.

See if I understand correctly:

1 - The electromagnetic field in the RF cavity is about 500 MHz, so this is 2 ns (nanosecond) between successive accelerating pulses.

2 - At 150 ns spacing, they were limited to about 400 bunches, corresponding to 1200 bunches at 50 ns. In fact I have read somewhere that the ultimate capacity is about 3000 bunches at a 25 ns spacing.

3 - Since the RF period is about 2 ns then it means that there are about 10 RF cycles between successive bunches (3000/300 or 25ns/2ns).

Now, assuming I understand the problem correctly, my question is: Do all the protons of a bunch cross the RF cavity during the same RF cycle?

I guess another way of asking the same question is: Do all the protons of one bunch reside in the same bucket?

LarryS wrote:Based on what I understand, the following describes the collision notation:

Sample: 150ns_312b_295_16_295_3x8bpi19inj

150ns - 150 nano seconds bunch separation (bunch is a group of individual buckets)

312b - total number of buckets injected

295 - number of buckets colliding at IP1(Atlas) and IP5(CMS) located 180 degrees apart

16 - number of buckets colliding at IP2(Alice)

295 - number of buckets colliding at IP8(LHCb) and can be different than IP1/IP5

3x8bpi - number of buckets per injection (may not always be the same number of buckets per injection thus the difference from 3 X 8 X 19 does not equal 295

19inj - number of injections or bunches


Note: IP1/IP5 are 180 degrees apart and thus the same bucket number always collides in each while IP2 and IP8 are displaced from this 180 degree line and require a difference of 8910 between bucket numbers for IP2 collisions and a difference of 8940 between bucket numbers (in the opposite direction) for IP8 collisions.

As I indicated, this is my understanding based on what I have been able to find while sitting here "across the big pond".

Thank you very much. this is the info that I was looking for. IP2 and IP8 are displaced from 180 degree. I did not know that! Thanks again.

P.S. This is a question for the site admin: Can I acknowledge each reply, or is this seen as a bad practice?