Its the damnedest thing you've ever seen. Please realize that although we haven't actually damaged any of the ovens we've done this with, the potential exists to damage or destroy the microwave that this is done in, and the possibility also exists that it could harm a human being if the proper precautions are not made. Please see to it that you are willing to pay $200 for a new microwave before you try this, and that you have a fire extinguisher nearby. If you are under age 18, please seek the supervision of a parent or guardian. I accept no responsibility should this experiment cause damage or injury. How to make a glowing ball of plasma in your microwave with a grape:
What's going on? Well, I only have an idea for some parts. The first part is speculation -- the sparks are resulting from an electrical discharge between the sides of the grape, as is evident by watching and as supported by the 120Hz buzz that coincides with the grape flaming. It could be that the particular size of the grape relative to the wavelengths of the microwaves in the oven causes the discharge, but I don't know if I believe this yet. What is the mechanism for charge separation?
The discharges vaporize the sugars in the grape and then cause them
to combust. The combustion products, if they get hot enough due to the
continued electrical arcing, form a plasma (gas where the nuclei and electrons
have been ripped apart from one another). This plasma is electricially
conductive, and so absorbs microwaves keeping itself warm, and causing
the cloud to glow. Since the cloud is hot, it rises to the top of the oven.
However, the cloud seems to stay remarkably coherent. Why doesn't the could
dissipate over timescales of several seconds?
Cut your grape in half, then cut the half into halves (quarter-grapes) leaving a small bridge between the halves. Dry the cut surfaces.
Place the cut crape with the cut surfaces up onto a platter.
Put them in the microwave's hot spot.
Within 3-4 seconds of turning on the microwave electrical arcing between the two grape quarters emits light, a buzzing noise, and periodic flames as the sugars in the grape burn.
If you've been lucky and good, you may get treated to a show of a ball of glowing plasma rising through your microwave oven.