Magnetron Working Principle
I have just read the Wikipedia article as I also know very little about cavity magnetrons. It is much better. What I read from the wiki page the following is wrong/missing from the text above:


The electrons do NOT travel in a circle but in a spiral finally hitting the anode. (Yes @DKNguyen you where right)
This causes local charge which has to be equalized an thus a current starts flowing in the anode. (The local charge also makes that the next electrons want to land elsewhere)
Due to the cavities that current has to go "around" the holes.
This causes microwave radiofrequency energy.


Please do not upvote this. I know nothing about magnetrons, I only repeated the wike data.