How do I calculate the high voltage transient?

[DandDMan]

I want to power a miniature tesla coil using the short duration, high voltage transient created when the current powering an inductor is shut off. I'm hoping to get the a voltage high enough to jump a spark gap or perhaps a high voltage diode; to make the pulse unidirectional.

Can anyone tell me how to calculate the high voltage transient created when power is turned off to a coil?

How can I generate the highest possible potential spike?
A short duty cycle and very rapid shut off. Device suggestions? Transistor part#?

Iron or air core? Higher or lower inductance? I would think a very long air inductor would be best. Is this correct? Is a single layer or multi-layer coil best? Is a flat coil better for higher voltages spikes? I have noticed that a bifiler wound coil wired in series has a lower resonant frequency than a normal coil, but the same voltage transient created when its shut off.

I would love to hear any suggestions you can offer.