NST will require a dielectric 62.5 mil thick. To regain the lost capacitance you must increase the size of the plates. Any high voltage conductor can generate corona. It appears as a hazy purple glow on the conductors surface. Placing the conductors in oil will reduce corona formation considerably but there is a limit to the oils ability.
Corona can form in air bubbles between plates and cause localised heating.
Corona can also eat away at the dielectric, weakening it. This has the effect of sharing the voltage equally between the caps, but only if all the series wired caps are identical in construction. The most important component of any capacitor is the dielectric layer. For Tesla Coil use there are three specific requirements to be aware of. The second, and less obvious, property to be aware of is the materials dissipation factor .