Original poster: "D.C. Cox" <resonance@xxxxxxxxxx>Solenoidal primaries will produce overcoupling if used in any coil dia. above the 4 inch size. Flat spirals work best for efficient energy transfer without "racing sparks".
Racing sparks are always caused by overcoupling or too small toroid size (causes coil to produce standing waves because certain harmonic frequencies and complex frequency interactions are not being surpressed enough). Use a toroid that is at least 2 x sec coil dia (3 or 4 x for max performance systems) and you will not have racing sparks assuming your sec coil is properly elevated above the pri. We have used motor driven adjusters to dynamically tune 4", 6", 10", 12", 18" 24", and 36" dia. sec coils while they are running to determine optimal sec coil heights above the horizontal plane of the primary to produce a maximum length secondary spark. 4 inch systems respond well approx 1.125 to 1.25 inches above the pri top turn (flat spiral) or even with base if using a 15-20 degree cone. This will produce the optimum spark length when driven with a 60 mA xmfr.
Someone advocated adjustment until racing sparks occur and then backing off. This is horrible engineering. Once a spark punctures a solid insulation it does not "cure" or repair itself. Once the damage is done it is permanent unless the coil is rewound. You can back off the coupling to reduce sparkover, but then you always have a weakest link in your coil system. Properly sealing the sec coil and then not puncturing it will produce a long lasting coil.
Dr. Resonance
If you place the primary off center in a half wave coil you will find that the performance drops quickly. ( and If you tilt the primary the performance drops like a rock) Solenoidal primaries work just fine. I like them better then pancake primaries they are easier to make and tune. You can wrap tube around a drum and get a nice copper spring. Compress the spring and the inductance goes up. Stretch the spring and inductance goes down. You can make spacers and spiral them into place or weave nylon rope between the winds and epoxy saturate the rope for a rigid structure. Sincerely: Jared Dwarshuis