"Jon Elson" wrote in message
...
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.

Reflected waves can augment or cancel - or anywhere in between.

AFAIK: the extremes are flashover in the magnetron or excessive current
draw. I doesn't take much excessive current draw to cause shorted turns in
the mains transformer.