When you start the microwave and you warm up a dish, a bowl or another container, some electric arcs appear, joining the top of the microwave and the plate on the right side.
Whereas there is no metal in your dish (spoon or other cutlery).
In fact, it comes from the food projections on the side plate (mica plate) that deteriorated it with the time.
This mica plate allows you to clean the microwave without touching the magntron that sends the waves on the dish. It must be in a proper state, otherwise the magnetron will send the wave on itself and will be damaged.
The electric arcs are by consequent the result of a food surplus (carbon surpolus) on the mica plate.
You have to change the mica plate thats costs between 5 and 30 dollars.
If the plate is not too ruined, you can withdraw it. Warning, it is fragile as a plate in cardboard recovered with a senitive plastic film. Lay it flat on the ground, clean it with a wet spontex sponge, slightly by doing circles. Don’t eradicate the notches. Remove a maximum of soot and carbon. Finally, dry it.
When you will put it back, try to insert the most clean face and side in front of the magnetron, inside the microwave cavity.
To prevent premature aging of the mica, think about a regular clean of the microwave room and to recover the food container with a plate.