Waveguide Covers - Mica
Most microwaves have either a plastic or mica cover called a waveguide cover. It sometimes resembles a piece of grey cardboard and is usually located either in the top of the cavity or on one of the side walls.
The waveguide covers the hole through which the microwaves are fed into the oven cavity from the waveguide.
It acts as a barrier to stop grease and food particles from entering the opening of the waveguide.
If the part begins to arc or spark during a cook cycle, it needs to be replaced. Continued use will cause burning of the paint and cavity and will eventually damage the magnetron tube.
The covers can be replaced with the factory part made for the unit or they can be cut from a sheet of mica material. (This generally only works if you can use the old cover as a template to cut out the new one).