Most people have a rough idea about what plasma is, usually thanks to the plasma TV craze that hit many years ago and has since been replaced by LCDs, but few will know that plasma is actually one of the 4 fundamental states of matter right along side solid, liquid and gas. The transition between a gas and a plasma is done through a process called ionization/deionization which converts the gas into an electrically conductive cloud which can be done by either inducing a large voltage difference or by subjecting the gas to extremely high tempreatures. The following video shows the latter and is a rather cool demonstration of the transition process.
The short run time for sustaining the plasma cloud is simple, given enough time that superheated cloud of carbon atoms would start to melt the pyrex container which would free the plasma to wreck all sorts of havok on the microwave itself. I’m not sure how long it’d last though as it looks like the atomised carbon atoms need to be cluster together for it to work, hence the spool up time require to set up the initial plasma reaction. Indeed if my experiments with bananas are anything to go by (it’s relatively safe but still, I’m not going to recommend you do it) you’d instead get little flashes rather than the sustained cloud.
What really interested me was the hum that was generated as it was pretty regular and I couldn’t really figure out what would be causing it. As it turns out there’s actually a couple things that could be responsible and, interestingly enough, the frequency could change depending on the input frequency of the power source going to the microwave. That link also suggests another, similar experiment with cut in half grapes that’s supposedly a lot safer (although this site argues otherwise) and the results look very similar to my results with bananas. It seems there’s all manner of things you can use to create plasma in the microwave, something I didn’t expect.
This is one of those experiments that I reckon would be really great for class demonstrations (this is probably also the reason why I shouldn’t be allow to teach science in schools but come on, fire and explosions are awesome!).