Watching this YouTube video about the LHC at around 3 minutes and 50 seconds into the video the narrator says...

For machine protection reasons we are not allowed to inject a high intensity beam into an empty machine, therefore we start injecting a very low intensity probe beam.

However the video doesn't provide any further explanation as to why a high intensity beam can't be injected into an empty machine. It goes on to say that when the high intensity beam is injected, the probe beam is kicked out.

My question is why can't (or possibly why shouldn't) a high intensity beam be injected into the LHC when the machine is empty?

  • 1
    $\begingroup$ total guess, but would the beam not have to be aligned first, to make sure it is not going to hit off something delicate. Most of the magnets are there to keep it aligned, not so much to speed it up, afaik $\endgroup$
    – user74893
    Apr 5, 2015 at 18:48

1 Answer 1


As irish guessed in the comment, this is about beam alignment and tuning.

A high intensity beam can damage the beam pipe (as in cut right through to the helium jacket) or cause a unusually hard superconducting magnet quench. The former is disastrous and the latter has some potential outrun the quench protection with similarly unhappy consequences. Having a low intensity beam in the accelerator gives you the opportunity to see problems developing before they reach that level.


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