MAGPIE is currently undergoing maintenance as the college install new lighting to replace the ageing fluorescent tubes we currently have. We’ve swaddled all the delicate bits in white plastic sheet, and so the machine waits patiently to burst afresh from its cocoon.
Still, we haven’t been idle. One of the four Marx banks didn’t discharge properly on the last shot on Wednesday. This usually indicates a problem with the gas switches inside the bank. We pumped all of the oil out overnight, and on Thursday we went in to remove the switches.
The inside of the newly drained Marx bank is dripping with transformer oil, and the smell of rotten eggs pervades the tight space – when the sulphur hexafluoride we use in the switches burns with the plastic hydrocarbon casings, then hydrogen sulphide, with its characteristic smell, is produced.
There are twenty four capacitors inside each Marx bank, weighing about 100 kg each, in four columns of six. The capacitors hang from above so they don’t touch the ground and short circuit. Twelve gas switches connect pairs of capacitors.
The switches contain two electrodes, and a metal disk between them. One electrode will be charged to a positive voltage, and one to a negative voltage, with the gas in between preventing any flow of electrical current – the switch is open. When the metal disk in between is rapidly charged, a spark will fly between the electrodes, and the switch will close, conducting electricity.
This spark knocks off some of the brass from the electrode, and so a sort of brass dust builds up inside the switch. When the dust gets too much, the switch will close even when the metal disk isn’t charged, and the Marx bank won’t charge up properly. At this point, we have to remove all the switches, open them up, wash them in a big tub of soapy water, clean the electrodes in a lathe, and then reassemble them.
I enjoy these bits of my physics career – it gets me away from the computer, and gives me time to think. Mundane tasks can be very satisfying when you can quickly get measurable results, and the space to think can be very useful for overcoming a mental block.