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On 12/6/13 2:27 AM, Colin Tregenza Dancer wrote:
The toroid nicely protects the top of the secondary/tertiary from breakout by extending the field down and out from the former. You're much more likely to get breakout from the coil top with a sphere (I've tried both).
For a VDG, the sphere provides working room inside for the roller, pickup, etc. and for a given size with have a significantly greater radius of curvature which gives you a higher breakout voltage.
A lot of VDGs use a sort of oblate hemisphere on top of a half toroid.
The toroid part on the bottom provides good field control where the
column and belts come up into the top electrode. The flattened top
reduces the field where the VDG is closest to the ceiling.
VDGs that run in a tank often have hemispherical tops, because that
matches the tank top shape, so you have a very even spacing between
grounded tank and charged top electrode.
It's all about avoiding field concentrations and breakdowns in a VDG.
VDGs tend to operate a much higher voltages than Tesla coils, and they
try to avoid small discharges as well, because the steady DC is what is
desirable for running an accelerator. You might look at info from
National Electrostatic Corp, makers of the Pelletron.
Tesla coils, on the other hand, want to control the field along the
secondary. A toroid is a good choice: no sharp edges on the bottom,
large extent in a horizontal direction at right angles to the field.
And, for most TC applications, "display of sparks" is the objective, and
sparks to the side is most desirable, so a "flat top" or even a concave
top is wanted.