If you have been following our LHC news features for a little while you will know that the Large Hadron Collider has been dogged by a particularly annoying form of interference that the beam engineers have dubbed “The Hump“. Yesterday they ran some tests to see if the source could be the GSM phone network that they run throughout CERN. By switching off the GSM transmitters while a 450 GeV beam circulated in the collider ring they were able to show that the interference did not go away. Something else must be to blame.
The Hump takes the form of a hump in the spectrum of beam oscillations that drifts up and down the frequency range. The beam is normally given a tune frequency away from any interference to keep it stable, but because the hump frequency keeps changing it is difficult to avoid whatever tune frequency is chosen.
When the hump drifts over the beam frequency it destabilised the beam causing it to spread out vertically. The first effect of this is to decrease luminosity because as the protons spread out they are less likely to collide, then as the beam spreads out further the protons hit the collimator causing losses that reduce the lifetime of the beams. In the worst case the losses can trigger an unwanted dump of the beams.
The LHC has a number of inbuilt dampers and other features that help to stabilise the beams by steering wayward protons back into the centre of the beam, but at high intensities these don’t seem to be quite enough to avoid the effects of The Hump. It has been around since the LHC was restarted at the end of the year and has thwarted all attempts to track down its cause.
The history of collider building provides several stories of interference that was hard to track down in the past. When LEP was running at CERN another nasty problem was finally diagnosed when the French rail workers went on strike and the interference disappeared. It had been caused by a nearby TGV line. For the last few months the LHC beam teams have been looking for causes of The Hump and yesterday’s GSM test is the latest in a series that have eliminated many possible causes such as vacuum pumps, and Cryogenic coolers, bur whatever they do the hump remains.
Meanwhile we can look forward to the LHC engineers testing for even less likely sources for The Hump and hope it does not have too bad an effect on the continuing build-up of luminosity.