I though since people throw out microwaves far more than Neon signs an easy solution is to use a MOT. A large number of people say it is not possible to build one with only one MOT, or at least some form of voltage multiplier circuit.
a MOT puts out only 2k and over 500mA (A cap on the low voltage side will limit current)
Well it is possible to build a tesla using a single magnetron microwave (It is disappointingly weak though, an ignition transformer from a car puts out more juice, and is my preferred tesla coil transformer) I got about a 1cm spark, but it throws RF like crazy. It is a semi decent beginner coil being made mostly from scrap, and needing no power transistor drive circuit.
Step 1: The Circuit
A MOT will jump the spark gap, but it will not jump it in a coil circuit. To let it build the voltage I used a diode. The diode does not let the cap discharge. So the voltage may build and jump the gap. (Still appears to fire each AC cycle) It is not a voltage multiplier just a rectifier. A multiplier would improve the output considerably but cannot be made with one Microwave.
WARNING USING THE DIODE ALOWS THE CAP TO KEEP ITS CHARGE FOR SEVERAL MINUTES AFTER USE
There is a built in resistor in most microwave caps but please short the terminals anyway before servicing
(should go without saying) the same with the ballast cap.