The microwave oven shown here was modified to allow it to operate with the door open. Running a microwave oven with the door open can damage the magnetron, since the tuned cavity is no longer present and causes the magnetron to slowly overheat. Escaping microwave radiation can damage nearby electronic equipment, so keep things like household appliances and cameras well away.
Microwave radiation can also do a lot of damage to humans.
The amount of microwave energy being put into the valves will not be as great with the door open. With the door closed the grill would have interfered with the photos I took, so I ran the oven with the door open. If the glass breaks on the valve's there may be some toxic chemicals that may escape. If doing the experiment yourself use an old microwave that will not be used for cooking food. Don't modify the microwave so that the microwave can be run with the door open.