Here are the tank circuit capacitors used for the big 12" Tesla Coil. Each individual 
capacitor is .25uF @ 6kVAC. So, wired together in the picture (5x2) this yields .1uF @ 
30kVAC. This configuration is used when running 14.4kV in. With an input voltage of 
7.5kV, the coil performs much better with a 3 x 3 arrangement which gives .25uF @ 18kVAC. 
Unfortunately the internal construction of these cap's is unknown. It is probably not 
polypropylene, P-styrene, or other low-loss material since they heat up after a little while. 
But: they have survived a LOT of abuse with this system. We love these caps.