The bases for Plasma based Technology
A plasma coil is a small disk/coin shaped object, the inside of which has metal tubing and/or wiring. When plasma is run through it, it generates an electromagnetic field. It is often used in the manipulation of plasma itself.
Plasma Coils come in varying sizes, from the hand size to the minute to fit into the barrels of blasters, a few are massive, but the most common are at best the size of a very small plate, or a large coin.