That is the antenna of the magnetron. Most likely some contaminant got on the surface which caused a hot spot by absorbing the the intense field energy at that location.
Contrary to common belief, metal objects can be placed in a microwave oven. The oven walls are metal. You just must lot allow metal objects to touch other metal objects or there will be sparks, nor do you want foil or thin metal plating which will heat-up and melt due to the eddy currents and low mass.
Thanks for the enlightenment. I was aware that it was the antenna, but a little puzzled about the shape, particularly inside the burned part. I guess that there is a 45degree plate/reflector in the waveguide too, but will have to wait to look ... it is someone else's unit. Is it a resonant cavity ... something similar to a LASER ?
The closed end of the waveguide is discoloured. That is the only damage that I noticed beyond the antenna. The burned side of the antenna pointed that way. I am now concerned that there may be damage to the waveguide, and that a new magnetron will quickly follow the original.
