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Ingrid Gregor | DESY | Germany

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Power Rangers

PowerFollowing the press coverage of the LHC startup is quite entertaining. There seem to be journalists sitting in front of the computer watching any tweet or blog where they might catch some information about something going wrong at the LHC. Or they assume something is wrong with LHC when the web pages are not online or anything out of the ordinary happens. A power cut at LHC makes it very easily into breaking news these days.
Therefore I decided to write about power cuts in the daily life of an accelerator and the experiments. A power cut is a nuisance, but it is part of life when working in particle physics. One has to understand that most of the power cuts are due to safety measures in the whole system. If the current in the power system fluctuates – maybe due to a small problem in the power plant, or it’s the half time of a soccer game and many fridges are opened at the same time to get out the beer – it might happen that our power supplies trip. A power supply is the device between the power outlet in the hall and our detector and in particle physics and it regulates the voltages and currents for our detectors and magnets. These devices were usually specifically designed for exactly the system they serve. Tripping means that the voltage is quickly ramped down because some over-current was detected. With these safety measures, we can make sure that detectors (or accelerator sub-systems) are not damaged when something unusual goes on. And this system is very fast. For example if at home the light flutters due to a glitch or fluctuation in the power system, the TV does not even show that there was a problem. A particle accelerator or a huge experiment would already be shut off just for safety reasons in the moment the light at home just flutters.

Power cuts are normal. Everybody who worked at PS, SPS, LEP or HERA (I guess it is the same at Tevatron) experienced more than enough power cuts.
And what happens then? Within a fraction of the second a lot of power supplies around the accelerator and/or at the experiments are turned off. Then we cannot just turn one switch and everything is back on. It is just normal that it takes some hours, as we first have to see why things were turned off, which parts of the things were turned off etc. When this is established, people have to start switching everything back on, usually in a certain order. And it case of the power cut there can be many devices in a funny state. Mostly they can be reset remotely using software, but sometimes more than a soft reset is needed. It can be that some of the supplies might be on the other side of the accelerator, in case of LHC some km to drive – someone has to get up, (get dressed if the power cut is in the night) and drive there. For example when I was in charge of the ZEUS calorimeter, almost every time when there was a thunderstorm in Hamburg (a usual weather condition in summer)the lights went out in HERA and the experiments. I don’t have an exact statistics, but on average it took something like 6 hours to recover from a power cut.
In case LHC is down due to a power cut for some hours, it does not mean that it is damaged or a bird from the future was harming it. It is normal daily business and we have to live with it….

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