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[TCML] Re: Re: Spark models, revisited

Kurt Schraner wrote:


This thread is really interesting. Would be nice, to have a formula for
spark capacitance, relevant for TC's. Antonio's appoach seems very
convincing, because it only relies on basic physics law's and the model of
a transmission line. May be, the transmission-line model can be
questioned for this case. On the other hand, the formulas given in
Wikipedia

http://en.wikipedia.org/wiki/Capacitance seem well established. I see kind

of a dilemma.


I think Antonio's transmission line approach is correct. Along a straight
wire, capacitance and inductance conspire in such a way to make the
velocity equal to c.
I do have a problem with his assumption of the inductance of 1uH/m
being mostly independent of the radii we consider here.

The spark model I've described needs a capacitance of about 15pF/m.
This could be due to branching sparks. The model doesn't take these into
account.

I've been wondering about the non branching sparks, which can be generated
by DRSSTCs with a slow voltage ramp up. If the conducting spark channel is
very thin, I'd expect sideways breakout from it. My assumption is, that
the arc pushes out charge carriers during the rise of voltage of each RF
cycle. This charge cloud would reduce field strength near the arc,
preventing  breakout.
The charge cloud would also increase arc capacitance.

Another problem with the spark model is, that the ratio of
capacitive and resistive load seen by the coil is too large compared to
experiment. If the model is modified by adding a resistor in series with each capacitor, that ratio can be brought into agreement with observation. 
I believe a charge cloud would have a similar effect, adding a dissipative
component to the capacitors. I'm currently trying to model this, based
on drift velocities of charge carriers but don't have any results yet.

Udo


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