James Wilkinson Sword wrote:


Perhaps the power going back into the klystron causes heating? Perhaps
the power goes into other parts and causes damage?

Microwave ovens use magnetrons, not klystrons. When power is applied to the
magnetron, it is pretty independent of the RF load, you apply a couple
thousand Volts minus to the cathode, and the anode is grounded. But, due to
the magnetic field, the electrons spiral outward instead of just heading
straight outward radially to the anode. Passing the resonant cavities
repeatedly during that spiral path builds the RF resonance.

The oven chamber develops standing waves. If there is nothing to absorb the
RF, it is reflected back into the magnetron, and the anode runs hotter. I
think the thermal switch on the anode is more to cover the condition where
the fan motor has seized up than no food in the oven, but may handle both to
some extent.

Jon