According to the information I gathered from CERN wiki pages, an LHC fill is a collection of several good runs. Am I right? Also, is an LHC run defined as a discontinuous period of data collection which contains several million events? Please clarify.


2 Answers 2


A single LHC fill has roughly the following steps:

  • A dose of protons enters the accelerator complex
  • Protons are accelerated through the chain (Linac, PSB, PS, SPS, LHC) to the maximum energy
  • Protons are kept in the LHC ring to collide at the interaction points
  • After some time (1-35 hours) the proton beam is dumped: it exits the ring

The duration of a fill can vary wildly. Some fills can end prematurely due to technical issues. Information about various fills can be found here. When the machine is ready for the next fill, the process is repeated. Sometimes there are other activities happening in LHC between the fills (short technical breaks). You can check the actual status in real time here.

A fill consists typically of $10^{14}$ protons, grouped into bunches which form the proton beam. During a fill there is roughly 600 million collisions per second, that is $10^{12}$ collisions per hour. Below you can see a time graph of a typical fill: the injection happened around 2 am, beam was dumped shortly after 3 pm. Energy per proton was 6.5 GeV, the beam intensity was gradually dropping due to losses in collisions.

enter image description here

LHC Run consists of many fills. A Run is either a single year span of LHC work, or sometimes it is understood as a period of several years or operation without major stops: right now we are in the middle of Run 2 which started in 2015.

  • $\begingroup$ If you go to the following link: imgur.com/a/KswZH you can see that in the Fill report there are several Runs. I am wondering what each of those individual runs are? Btw, your answer is helpful. $\endgroup$
    – CMSnoob
    Commented Jun 21, 2017 at 11:23
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    $\begingroup$ @CMSnoob CMS will have periods of stable operation that are (sort of) independent of the LHC's. The reasons for changing a run might be to change settings, if something goes offline and is fixed, etc... These runs are difference to the "LHC Runs" $\endgroup$
    – Gremlin
    Commented Jun 21, 2017 at 12:50

A "fill" is surprisingly difficult to define. The best definition I've found is that it's just a number that the LHC operators can increment by 1. You can think of it as a bookmark to refer to a period of activity.

The term is most commonly used to refer to a discrete interval of time containing a period where the beam is "full" (i.e. the beams have successfully been injected according to the desired filling scheme).

A typical "physics fill", in terms of machine modes, goes "injection" → "ramp" → "flat top" → "squeeze" → "adjust" → "stable beams" → "dump" → "ramp down". The fill number is then changed before the next injection.

However, there are lots of exceptions to this. A fill need not contain a ramp: it can remain at injection energy. There may be any number of beam dumps during a fill, particularly at injection and during "inject and dump" fills. There are even fills without any beam at all!

The word "run" is used very differently for the LHC and the experiments. For the LHC, a "run" is a period between "long shutdowns". Run 1 was 2010 to early 2013, Run 2 is 2015 to 2018, etc. There are thousands of LHC fills in a LHC run.

For the experiments, a "run" is a discrete interval of data-taking. Usually run changes occur when something about the detector itself changes, such as trigger configuration or a subdetector is enabled/disabled. There may even be a timer that changes run after a prescribed period (certainly this is the case at LHCb). There can be many runs per fill, or runs can occur independently of fills, particularly if there's no beam or the LHC is in "machine development".

  • $\begingroup$ A fill is not difficult to define: when the LHC is empty (no proton beams in it), and one fills it with protons (from the SPS), this is a new fill, like filling a bottle. In contrast to a bottle however, the LHC is never partially emptied. When the beams are dumped, all protons in the beams are extracted to the dump within one single orbit (roughly 90 microseconds, the time needed for the proton beams to go around the LHC once). $\endgroup$ Commented Jun 22, 2017 at 13:41
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    $\begingroup$ My answer addresses the deviations from the naïve definition. It is difficult to give a definition that covers all periods between increments of the fill counter. For example, there were 15 fills in 2014, when the LHC was in no condition to circulate beams. $\endgroup$
    – dukwon
    Commented Jun 22, 2017 at 17:08
  • $\begingroup$ ok.. good point about empty fills :-) $\endgroup$ Commented Jun 23, 2017 at 11:59

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