Original poster: Paul Nicholson <paul@xxxxxxxxxxxxxxxxxxx>
I think the primary coil design must be the most tricky bit of
the whole project. Not least of which because that nice symmetry
of the voltages, with zero in the middle, is only an ideal case.
In practice the secondary voltage will find its own balance - at
a point dependent on how well matched the end capacitances are,
and on how evenly the streamers are formed at each end. The zero,
therefore, may be drawn away from the center of the coil by
any asymmetry present - such as drawing an arc from one end to
ground.
This is not good news for the primary, and to cope with this
inevitability, it would be a good idea to build in a lot more
voltage breakdown capability than normal. If the inner diameter
of the primary was raised to say 12-14", the inductance increases,
the coupling reduces, and the voltage breakdown tolerance is
raised. Putting in the 3" gap is a good move - it reduces the
otherwise excessive coupling and increases breakdown tolerance.