Can we turn on the High Voltage?

If you want to turn on a light, or start your car, you rarely pause to think about possible damage that might result. But when beam is coursing through the CMS muon end caps, we think about it very carefully. In fact, we discuss in all seriousness when to turn on the high voltage, when there is beam in the machine.

Images of the CMS muon end cap detector (EMU for short) have been shown hundreds of times, including on the cover of Newsweek. Lots of copper surfaces, thick steel disks – what could be delicate about that?

One of the CMS muon end cap disks being lowered into the experimental hall

The chambers inside are very well made by experts in Russia, China and the United States. They are relatively robust in a mechanical sense, though we would not want any of them to fall from their mounting on those large steel disks.

The danger is in the gas and the wires. The detecting layers consist of thin gas layers sandwiched between cathode strips, with anode wires stretched in planes between the cathode planes. A large voltage (several thousand volts) are applied between the cathodes and the anode; this is part of the gas amplification mechanism which allows us to detector the wispy muon tracks as they pass through the chamber. In order to detect the muons, we need to have this high voltage turned on.

Diagram of the gas gap in a cathode strip chamber

This is a delicate instrument, and quite sensitive to minute amounts of ionization (on the order of 100 electron-ion pairs or fewer). You can’t spray it with intense concentrated charged particle fluxes or you risk damaging the instrument. Putting the beam through one of these chambers would be like igniting an old-fashioned camera flash in front of night vision goggles.

You might object that we splashed millions of muons through the detector in the famous beam splash events, which I wrote about a couple of days ago. It is true that this is an immense amount of ionization compared to a single muon or a muon pair. But it was spread around a hundred squared-meters of area, not concentrated in a narrow area close to the beam. (Nonetheless, we put the high voltage at low values just to be very safe, as mentioned in my posting.)

So, when the LHC operators are circulating beam, looking for the rough spots and making systematic adjustments to machine controls and monitors, we have to protect our delicate detector. At this point in time, when an eager physicist asks:

Can we turn on the high voltage?

the answer will often be:

be patient, not quite yet…