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Re: [TCML] spark models
Hi Steve,
Doesn't look like Jim responded as yet. I'm also interested in his
thoughts.
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.
The resonant frequency only plays an indirect role by helping to
maintain an already existing channel's conductance via displacement
current _at the fundamental frequency_ and reducing voltage ring-up time
for a given coupling coefficient. However, actual spark growth occurs
when the terminal voltage reaches a higher peak voltage than previous
peaks, so spark growth is more a function of the RF envelope. Sparks
grow only during a rising envelope. And, as Jim mentioned, growth occurs
in a series of quick jumps, each of the order of nanoseconds or tens of
nanoseconds. The detailed mechanism is significantly different depending
on the polarity of the terminal voltage.
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/
You are directly controlling the key parameter - the shape of the RF
envelope. And, tighter coupling causes the secondary's output to more
closely track the primary drive waveform, unlike a classical coil where
you're sloshing a decreasing amount of energy between P and S.
It seems that any good streamer model would also factor in things that
effect the conductivity of the air, that is, how long ago was it recently
ionized or heated up, and how long does it take for the ions to recombine or
for the temperature to come down.
Good ionic and thermodynamic models do exist, but they are quite
complex. A leader is an arc, and like any free arc in air, it decays
with a time constant measured in hundreds of milliseconds. An RF arc has
a virtually constant dynamic resistance (which, in turn, is an inverse
function of peak channel current), even though the channel is
continually being reignited after each zero current crossing. The
thermal lag of the arc channel is comparatively long, and the higher the
current (larger the channel) the larger the channel diameter, the hotter
the core temperature, and longer the thermal lag. For typical TC
leaders, the previous channel has fully decayed after 100 msec or so,
and the path taken by the NEXT leader is pretty much independent of the
previous one.
Im considering how i might be able to rig up a cheap(er) USB oscilloscope
(or some other data acquisition) to the topload of the coil so that i can
measure current between toroid and secondary, and current out of the
breakout point. Looked into USB-fiber optic converters, they are pricey...
hoping i might be able to make my own if i can get fast enough fiber-optic
parts.
Sounds like a great approach. A small, low bandwidth digital scope to
capture and store a single discharge event may also work.
Any other suggestions on how you could measure these currents? I seem to
recall terry fritz had some fiber optic probes, but i was really not sure
how that worked, and they also looked to be really expensive and probably
finicky if i had to guess. The direct approach would be nice, of course.
Ive already considered crude options like a LED bar graph that would read
back the peak current value, and of course tell me nothing about the
wave-shape. Could also make my own low quality data acq with some
micro-controller or something, which honestly might be just as useful unless
i have some other solution with really high bandwidth.
High bandwidth is only needed if you're trying to capture the nanosecond
growth spurt currents. At this stage, even being able to accurately
capture the secondary-to-toroid and toroid-to-spark current envelopes
would be very useful in developing a macroscopic model lumped model.
Bert
Steve
On Wed, May 18, 2011 at 7:08 PM, Jim Lux<jimlux@xxxxxxxxxxxxx> wrote:
I've written a short description and drawn a few diagrams of some
directions for future spark modeling. Comments welcome.
http://home.earthlink.net/~jimlux/hv/tcsparkmodel.htm
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