Capacitors
What can I say.... Capacitors... the bain of Tesla coiling activities. Of all the components of the Tesla Coil, this part takes the ultimate in abuse. For a high quality capacitor... you WILL spend a fortune on it, no way around it. To date, the best bet is to do the MMC ( multi-mini-capacitor) assembly. Yes you can try to build your own capacitor, but given time it WILL die. The alternate ways around the high cost of the capacitor is the Saltwater Cap, the homemade plate cap, and the home made rolled cap. But, these are power hungry, weak, and messy to make and eventually you will spend more on these than if you had originally done a MMC.
The capacitor is the "heart" of the system, it stores the charge supplied by the transformer. Once the charge/voltage reaches the value required to jump the spark gap, it jumps across the gap ( which acts like a switch ) and allows the charge to rush through the primary coil. As the voltage dies down the gap quits conducting and the pulse that was sent through the primary makes a electromagnetic field that is induced into the secondary. This switching effect of the gap occurs VERY fast.... up to 400 times a second on average. So what we see here is a complete voltage reversal inside the capacitor 400 or so times per second. And considering the voltages that are pulsing through the cap at this rate, its amazing that they do hold up. These voltages, by the way, range from 6000 to 20,000.
If you plan to make a homemade capacitor, here are some things to consider ....
Plate caps are easy to build and rebuild and rebuild and rebuild... get the picture ;)
Rolled caps are hard to build ... they never roll right the first 7 times, the sheeting slips and gets out of position.
Some type of dielectric oil is needed and its messy.... and it needs to be almost perfectly clean.
Mylar is great for its insulative properties BUT it can't take the heat... and the frequency of the voltages pulsing through the cap put alot of strain on the plastic, thus melting it and allowing the electricity to punch through the insulation. Once this happens, its all over... the Cap is dead.
Polyethelene is a very good choice for homemade caps but, you will need several sheets of it between the aluminum plates to be effective. The problems encountered here are air bubbles between the sheets... they allow corona to form and corona gets HOT... once again melted plastic, blown cap. If you could find a way to remove ALL of the air, then you will have a decient cap. Most sources of inexpensive Polyethelene ( tarpulines, visqueen, etc) have impurities in them. These impurities act as voltage magnets and create weak spots in the insulation ( dielectric ).
Salt water caps are the cheapest way for a homemade capacitor. But they
are bulky, messy, very power hungry and the glass will break if abused.