Re: Gap Arc Voltage Was: Natural streamer

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

> A plane-plane gap would spark at 30 kV/cm, or a bit less, from DC to
> at least 300 MHz, so the frequency of a Tesla coil doesn't make
> difference (for single sparks).

I think this is true only for small gaps (on order of 1 cm), where the
leader propagation time is much less than a cycle (and clearly, where the
gap is a small fraction of a wavelength, so you can consider it as a
quasistatic electric field).


> Power, heating the electrodes and the air between then, makes a big
> difference. Breakdown voltages are inversely proportional to the
> absolute temperature (Kelvin) and directly proportional to the
> air pressure. Consider that the temperature of a primary circuit
> gap in a Tesla coil can reach two or three times the ambient
> absolute temperature, maybe more, and the breakdown voltage would
> get divided by the same factor.

Lower "air temperature in the gap" is probably why blown gaps, in any form,
no matter how feebly blown (or sucked) work more consistently.  An even
bigger effect would be hot spots on the electrodes which provide copious
electrons from thermionic emission (like the cathode in a vacuum tube) to
get the breakdown process rolling.


> But a plane-plane gap must be short relative to the widths of the
> terminals, and the edges of the electrodes shall be rounded adequately
> to avoid electric field concentration there (there is an optimal profile
> for this).
Several profiles, actually, depending on how sophisticated you want to get
(Bruce, Rogowski, etc.)

 With sharp corners, the gap may operate as a needle gap, that
> starts to emit corona as soon as the voltage exceeds ~300 V, and
> eventually start to operate as a ball gap, with the "balls" being
> formed by ionized air around the needles. A rule of 10 kV/cm can
> be applied to needle gaps in the usual conditions.

Given that the standard needles for low voltage gaps are 00 sewing needles
(or maybe 000, I can't remember), which come to a very sharp point, I wonder
if anyone has done a theoretical analysis of the fields, or if it is
entirely empiricism.  With such a small radius of curvature, the field would
be very, very high at the tip, and things like field emission would probably
be important.
>
> Antonio Carlos M. de Queiroz
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