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The cap engineers that I know tell me that the best material for a Tesla coil capacitor dielectric is polypropylene.  I do recall a roll of 1/2 mil teflon Richard Hull was commenting on in one of his tapes, he mentioned that it may be something to look at in capacitor construction.  I mentioned that material to Gary Greiser, a cap engineer at C.S.I. Technology in San Marcos, California.  Polypropylene, he said, is the best, you'd have to triple up the sheets of teflon because there would surely be a piece of conductor or some small flaw in the sheets and the ability of teflon to take heat is a non-issue as polypropylene doesn't heat up at Tesla frequencies either.  Obtaining a sufficiently pure teflon would be the biggest problem, although I've seen reference to caps made with this material, no specs or operating results though.

Best material for the plates?  Metallized polypropylene and aluminum foils are common while the best material is, of course, gold!  Being a bit on the expensive side, the next best would be copper foil, also a bit expensive at $500.00 for a 1/4  pound of 1/16 mil.  This cost discourages most, if not all, coilers.  One of these days...........  I have found a little info on caps using silver for the plates, the VTV ULTRA-TONE Silver Foil Paper in Oil, no technical data yet.

Impregnating a cap?  I have a number of custom caps and the first one CSI made for me was 0.0137uF at 15kvrms.  Its Q was measured dry at 14,000.  It then dropped to 12,500 after impregnating it with oil and Gary did say that the cap would be better if left dry but I was insistent.

Another example is the extended foil, twin, 0.1uF below.  It's dry and has a Q of over 24,000.  This was an exercise by Gary to see how high a Q he could obtain.  It can be configured for 0.05uF, 0.1uF or 0.2uF.  $500.00 and I made the acrylic case.

Above left:  CSI twin 0.1uF,  15" across the face, 11" deep, 4" tall.  Right:  Two CSI, extended foil, high Q 0.01768uF 15kv capacitors.  $380.00 for the pair and I made the acrylic cases.  I swapped out my 105 piece mmc (0.01768 uF) for one of these and the bi-polar TC performed as normal, no easily noticed differences.

June 20, 2004.  Tesla List.  Pulse capacitors are constructed with a number of equally sized sections connected in series or series-parallel. During the design phase, a sufficient number of sections is used such that each section will "see" a worst case voltage in the range of 3500-5000 volts/section. This reduces the peak voltage stress seen (both at the edges of the metallization and within any voids) below the point where corona normally forms - called the "corona inception voltage". Capacitors that are designed to withstand high voltage reversals are constructed with more individual sections than simpler DC or lower percent voltage reversal capacitors. A capacitor rated for 80% voltage reversal (Q~7 - highly oscillatory) is actually constructed with a dielectric system that can withstand 180% of the rated pre-discharge DC voltage. For example, compare the x-ray of the 15 kVac Plastic Capacitors BNZ series pulse cap (20 sections in series) with the DC-rated 80 kV LN series cap with only 8 sections in series in Mark Rzeszotarski's recent article, "X-Ray Pictures of High Voltage Capacitors".

Pulse capacitors designed to endure severe voltage reversals sometimes use a layer of oil-saturated kraft paper between the foil and the polypropylene (PP) film. The oil-soaked kraft paper acts as a higher dielectric constant (k ~ 4-6) buffer zone between the foil and the film, helping to even out the e-field stresses seen by the film. Because of the lower dielectric constant of the PP film layer (~2), the film actually sees most of the voltage stress in the cap. The oil-kraft paper layer is more resistant to corona damage (if it should occur during accidental overvoltage conditions) making for a more robust dielectric system during highly oscillatory discharges. The combination of a thicker dielectric system and more capacitor rolls/cap also mean that caps rated for high Q discharge duty will be significantly larger than their DC-rated or low % reversal counterparts.  Best regards, -- Bert --