Hi radicalsniper, welcome to ETO!
I have seen Tesla Coils built using ZVS drivers and bucket capacitors, though as you say it's not very efficient and there is a chance of it causing the Tesla coil to operate very poorly. An alternate option which I would prefer over a bucket capacitor is an array of beer bottle capacitors. Their construction is fairly consistent (glass thickness, etc) and they are able to withstand around 40kV (though I would not suggest going above 15kV for Tesla coils). Bucket capacitors may work fine, though the size of the bucket, type of plastic it's made out of, impurities in the plastic, etc can greatly affect the operation.
I used beer bottle caps in one of my Tesla coils and found that, if built properly, one bottle can give you between 0.5 and 0.7 nanofarads of capacitance, with a ~40kV rating.
You can calculate dielectric breakdown voltage using the following formula:
where "Vbd" is the breakdown voltage, "Eds" is the dielectric strength of the material (you can find charts online, like at
this link), and "d" is the distance between the plates (which, assuming the plates are directly against the dielectric, is simply the thickness of the dielectric). Note that the voltage is in kV, Eds is in kV/mm, and d is in mm. So, for example, let's say you make a capacitor with a sheet of 2mm thick polypropylene:
According to the chart, polypropylene has a dielectric strength of 23.6 kV/mm.
Thickness of the polypropylene is 2mm.
Therefore, the breakdown voltage of the dielectric is:
It is important to know, however, that you should NEVER drive a capacitor at, or even remotely near, its breakdown voltage. Most capacitors are derated significantly to prevent the user from applying voltage anywhere near its breakdown voltage. For example, you may have a capacitor rated for 10V but it may not break down until you apply three or four times that. When creating your own capacitors, you don't have the luxury of having a preset voltage rating that takes this deration into account. You only have the breakdown voltage. Personally, I calculate the required breakdown voltage as follows:
Take the voltage you plan to apply to the capacitor (let's say, 10kV). Multiply that by 2 to get the normal operating voltage rating of the capacitor (20kV). Then, multiply that by 2 again to get the breakdown voltage you need to shoot for (40kV). In order to drive a Tesla coil at 10kV, your homemade capacitor should be designed to withstand at least 40kV. This is just for safety reasons. Obviously you have a little bit of play, so don't stress too much if you can only get a 35kV rating, but keep it as close to or above 40kV as possible if driving at 10kV.
One thing to note about any homemade capacitor, make sure it's well-vented. Beer bottles have been known to explode if they are tightly sealed. Internal arcing can cause the air inside the bottle to heat up and expand, causing significant pressure buildup. Make sure to leave a vent hole in the cap. The same goes for any other type of homemade capacitor.
I hope this helps! Feel free to let me know if you have any further questions. Good luck!
Regards,
Matt