Herr Zapp wrote ...

Der Strom -

A safety factor of at least 2X the peak supply voltage is recommended as kind of a minimum margin. Of course this requirement is affected by the voltage safety margin that the capacitor manufacturer has built into his capacitor design; something that is unknown to the end user.

To start, multiply the RMS output voltage of your HV transformer (9KV in your case) by 1.414 to find the peak AC voltage that the capacitor must withstand. Then, multiply by 2 to get a target voltage rating for the MMC string.

If you have access to military or industrial surplus, or ham radio swapmeets, you may find other types of capacitors that are suitable for Tesla coil use, but rarely will they be the exact value you are looking for. High-voltage pulse-rated capacitors are found in industrial and medical laser systems, military radar and sonar systems, indistrial induction-heating systems, and high-power military and commercial radio transmitters. Sometimes interesting capacitors show up on eBay, but you never know their history, and purchase is always a gamble.

Building a conservatively-rated MMC for your system using the C-D 942 capacitors should cost you less than $100, and will likely give the best performance and reliability in the long run.

If you're going to take the salt-water capacitor route, I'd strongly recommend getting an inexpensive multimeter with a capacitance scale to measure the actual capacitance of your bottles. Knowing the actual capacitance value can help eliminate a long and frustrating exercise in "blind tuning".

Herr Zapp