The reason why your waveguide is sparking is because it has become damaged and the metal inside the waveguide cover is exposed and leads to the magnetron causing sparks when it comes into contact with the metal.
Waveguides contain some metal which can be exposed when the waveguide becomes damaged.
Microwaves that spark or arc without any other metal being inside the microwave is often caused by a damaged or a dirty waveguide cover although microwaves sparking can also be caused by metal objects inside or a faulty magnetron.
A waveguide cover can become damaged due to normal wear and tear or even food buildup and they are easy to replace and cheap to purchase new as well.
The waveguide cover in the microwave is a small panel inside your microwave, which directs the microwaves into the cooking chamber of the microwave.
Food splatters or grease buildup on the waveguide cover can become overheated and also cause sparking or a damaged or crack in the waveguide cover can also lead to sparking and arcing.
A common culprit of a sparking microwave is a damaged waveguide cover that is located inside the microwave area where you cook the food and it coves the hole where the magnetron sends the actual microwaves.
When that becomes torn or damaged it can result in sparking of the microwave.
You can use your microwave after it sparks as long as the sparking stops and is not a result of a problem with the microwave itself.
If the microwave sparked due to some metal that was on a dish or something in the food caused the microwave to spark or a bug or something else caused the microwave to spark it's generally safe to continue using the microwave.
In some cases the sparking of the microwave can be caused by a damaged waveguide cover inside of the microwave which is fitted over the over the magnetron on the inside of the microwave on the side.
It's a little cover that is similar to a piece of cardboard and sometimes that can result in sparking if it's damaged.