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Re: [TCML] spark models
On 5/26/11 8:45 AM, Steve Ward wrote:
Jim,
I have great interest in this topic, but so far all I've managed to do is
come up with more questions. Some current questions are:
1) What impact does resonant frequency play? I think this must relate to
some basic principles about spark growth and basically the resonant
frequency plays a role in how often the leader gets a new "kick" which i
would suspect plays a big role (well, because my testing says so). Id guess
that there's probably an issue with displacement current through the
streamer's capacitance, my guess would be that higher frequency could be
more beneficial in that it would keep more current circulating through the
spark channel and keep it hot. I'd be curious to hear your thoughts.
i think resonant frequency does play some part in the dynamics of the
spark growth, although not a huge one. For instance, how is a TC spark
at 100kHz different from a 60 Hz spark? Or a pulse from an impulse
generator lasting tens of microseconds. I think that there *is*
something that optimizes for spark length vs energy.
It would be interesting to have a very controllable Marx generator where
you could independently control the rise and fall times fairly easily.
There were some experimenters in Australia (as I recall) who built a
very controllable impulse generator to experiment with lightning rods.
It had a bunch of independent arbitrary waveform generators, each
driving a transformer, all stacked up to get several hundred kV.
Hmm.. these days, that might not be too tough to do. A battery powered
circuit, auto ignition coils, DAC driving a suitable power stage driven
of a 400V DC/DC converter. Peak current in the auto coil is a few amps.
If you could stamp out stages for, say, $100 in parts each that would
put out, say, 30-40kV, for a couple thousand bucks you could do it.
(of course, a suitably controllable SSTC might work just as well. ARB
driving a good linear RF amplifier power stage that has 1 MHz BW (hah!)
into the TC...
2) What is the difference between spark growth behavior on a "transient"
coil vs something like my quasi-CW system (which in my opinion is much
easier to do studies on)? Pics can be found at:
http://www.flickr.com/photos/kickermagnet/
I think there is a huge influence between the RF envelope and the spark
growth. In the idealized coupled resonator, the secondary voltage rises
on each half cycle, but I'm theorizing that this doesn't actually occur
when there is a growing spark. That is, instead of the peak voltage
rising on each half cycle, the energy goes into charging a bigger and
bigger spark (and being dissipated in the channel too.. although I'm not
sure how much energy it really takes to heat a tube of air 1cm in
diameter and a meter long to 7000K.
Just ballparking..
Air is nitrogen so 28g/22.4 liters =
1cm x 1 meter is about 100 cc so around 0.125g total mass.
Specific heat is probably something around 1kJ/kgK..
so, 0.125E-3*7E3 = about 1 kJ
Make the channel a few mm in diameter, and we're down in the 100s of
Joules range.
Let's say a 1 meter spark takes a 500W tesla coil, minimum (Freau
formula) which is going to store about 4 Joules/bang.
This doesn't quite reconcile...
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