Microwave oven modification

I bought this broken microwave oven cheap at a flea market (high voltage capacitor was shorted). After replacing the capacitor I put a burning toothpick into the oven to produce those cool ball lightning-like plasmoids. Didn't work too well - I rarely managed to produce any plasmoids. I think it's because of the low oven power (500W).
But putting evacuated vessels (light bulbs, fluorescent tubes, old HeNe lasers) inside the oven cavity worked nicely - except that now 500W was too much power and the vessels melted very fast. So I decided to make the oven power adjustable. The microwave oven's own power selector doesn't work as it just turns the magnetron on and off for variable amounts of time (some seconds). The food with its high thermal capacity doesn't mind, but this way of power reduction is not suitable for plasmas. The easiest thing would have been to simply connect the microwave oven transformer (MOT) to a variac. But this way I would not only have adjusted the cathode voltage but the heating voltage, thus the filament temperature, as well. So I kept the original MOT to heat the filament and used my 4 MOT power supply for the cathode voltage. This solution has another advantage - instead of the original half wave doubler the magnetron is now powered by the full wave rectified output of the four MOTs - which makes the plasma more stable. Of course when powering the magnetron with 4kV now, power is much higher than before, but because of the robust construction of magnetrons this shouldn't do much harm for short periods of time - as long as it doesn't melt.

I mounted the oven on stilts so I can insert a vacuum hose into a hole in the bottom of the cooking cavity (and to bring the oven to eye level).

Schematic of the oven. I removed the noise filter unit (already included in my control box), turn table motor, timer, power selector, termal switch, capacitor and diode.

Side view. Heating transformer, cavity light and magnetron cooling fan can be turned on individually. The latter is necessary to cool the vastly overpowered magnetron during experiments without leaving the MOT turned on.
The control box I use to control the transformers, light and fan incorporates an interlock circuit which cuts power when interrupted. It is connected to the door- and start switch of the oven to turn it off when the door is opened and make the oven more foolproof (I can be pretty thoughtless...).

Closeup of the connectors.
From left to right: Heating transformer, cooling fan, cavity light, interlock, auxilary ground, high voltage connectors. You can also see the two rectifier strings.

Door switch (background) and start switch (foreground). The start switch is closed when the start button on the front of the oven is pressed and opened when the door is opened.

Top view. Not much to see here except for the waveguide and the cavity light.

Closup of the magnetron. Cooling is improved with the four cooling fins. Under and over the fins the ferrite magnets are visible. The metal case under the magnetron contains an RF filter to reduce microwave leakage.

For photos of the microwave oven in action take a look at the microwave plasma page.