Weekends and nights are a quiet time these days in the control room, but the lack of noise isn’t from a lack of collisions. Last weekend, in a single run, ATLAS collected almost as much data as in the previous months, thanks to a new peak luminosity record of 5 x 1029 cm-2 s-1. Every crossing of the beams brought, on average, more than one pair of protons colliding!
During the day, the LHC performs tests and calibrations that allow a gradual increase in the number of collisions per second and continue the fantastic trend of doubling datasets. In the ATLAS control room, experts spend the day also testing, calibrating, improving, breaking, fixing, and tweaking small corners of our humming giant. At night, when the experts sleep and the tweaking pauses, the LHC and ATLAS run smoothly, collecting data, under the watch of dedicated and caffeinated night shifters.
Testing and tuning aren’t the only reasons for collisions to stop, however. Last week, a lightning bolt caused a power fluctuation which caused the beam to dump. A problem with a file server system, not developed by the LHC, also caused a delay.
These problems, however, are small and unimaginative compared to others in the past. The predecessor of the LHC, LEP, which was in the same tunnel, untangled some rather unexpected issues. The water in Lake Geneva compresses the ground in the region. When the water level changes due to seasonal variations, the nearby topology changes. The change is very slight, but the accelerator was so sensitive that even this tiny shift in the position of the magnets had to be accounted for.
The position of the moon and its gravitational field also had an effect on the energy of the LEP beams. Not all mysteries had natural origins, however. For months, LEP observed noise in their system that they couldn’t trace. Only when the French railway workers went on strike and the noise disappeared did they realize that the passing TGV was inducing unwanted current in a vacuum chamber. And finally, more locally, some difficulty in getting the beams to circulate more than 15 times was traced to a single quadrupole magnet. When the magnet was examined closely, two empty beer bottles were found actually inside the accelerator.
The LHC, at least so far, has had only one (at least, of which I am aware) bizarre externally-induced problem: the notorious day it was brought down for a short while by a bird with a baguette.
These sorts of problems might make the accelerator seem delicate. It is, after all, more than 26 km of some of the world’s most precise technology. Pushing the forefront of engineering and technology — the fastest humans have ever made something go, the smallest scales we have ever seen — requires building something crazily large and crazily sensitive. But its fragility is strengthened by adaptability, and by a team of experts working round the clock to solve problems bizarre and routine alike. As problems arise, solutions are found. It was not designed to be started one day with a big button and left running unobserved for years; unexpected problems are expected. As long as the LHC is kept safe from its own stored energy — and that is what many of the current tests are ensuring — baguettes and lightning bolts may cause a few minutes of excitement in the ATLAS control room and a period of hard work in the LHC one, but, after a pause, the collisions will continue.
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