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Re: [TCML] Odd VTTC Streamer Behavior

Hi Bert,

It would be interesting to build a Tesla coil in which
various parameters could be easily varied, such as envelope shape,
envelope duration, tuning during the growth cycle, etc.  This might
help to tease out the important parameters for sword-spark formation,
and make it easier to discover the mechanisms which lead to
sword-spark formation.  Such work might lead to an ability to
better control the spark formation of spark gap TC's and
solid state TC's too.  I remember someone on this list was
talking about building such a coil.  The coil would be basically
like a modulated/pulsed radio transmitter powering the coil.

I suspect the 8mS RF envelope duration of a VTTC is
a little too long for best results.  When I tried using a
mechanical interupter for a shorter envelope duration,
I think the sword sparks were smoother (less tendency
to have branching or fuzziness).  But my memory of those
experiments is a little fuzzy too at the moment   :^)


-----Original Message-----
From: Bert Hickman <bert.hickman@xxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Sent: Wed, 22 Oct 2008 7:34 pm
Subject: Re: [TCML] Odd VTTC Streamer Behavior

Those are some very interesting observations! The phenomenon of streamer branching is not very well understood. It is thought to be a consequence of chaotic instabilities at the streamer tip where the tip becomes broadened and then splits. As with many types of chaotic phenomena, branching is also known to be quite sensitive to initial conditions, including HV electrode shape, local E-field, source impedance, etc. 
In VTTC's or SSTC's operating in pulsed (staccato) mode, the combination of comparatively slow envelope growth and longer ringing times appears to be uniquely suited to forming smoothly growing, fat, sword-like discharges with little branching. Branching, when it does occur, is considerably more limited than with spark gap systems or DRSTTC's, where the Primary-secondary energy transfer rate (and resulting dV/dt of the TC secondary voltage envelope) is considerably faster. 
In addition, the RF arc-like discharges form VTTC's and SSTC's also develop a curious spiraling appearance around the main channels. The cause of this phenomenon is also not understood (and to my knowledge, has NOT even been reported in the mainstream literature). 
Some academic studies of pulsed streamers and streamer propagation simulations suggest that branching increases as you add resistance in the HV path, and decreases when you add inductance. Branching also increases with higher ambient E-fields. Minor differences in coupling, impedance of the incoming power source, electrode tip geometry, (and seemingly just about anything else!) may tip the balance one way or another for coils "on the edge". I suspect its a delicate balancing act involving ringup rate, where the spark can grow during each RF cycle without the E-field becoming so high that splitting results. Perhaps slight changes in coupling may throw a given system one way or another. 
One thing for sure, significant mysteries still lurk within these discharges... :^) 
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futuret@xxxxxxx wrote: 
Seems it's not a tube problem. If the cap is failing, it can 
change the tuning, which can effect the forking. 
But the forking effect could 
be something else. I had a lot of trouble with spark 
splitting on my small 4-250A coil which gave 20" sparks, 
and on my (2) 833A coil which gave 24" sparks. I found 
it to be sensitive to the type of power supply even. 
For example if I used a 4450 volt plate transformer, 
then the splitting occured. But if I used an MOT with 
level shift, and the correct number of level shift caps, 
then the splitting didn't occur. But it split occasionally 
anyway. It can be difficult to get longer and longer 
sparks and continue to avoid the splitting. It may 
help to lower the breakout point on top of the toroid 
or something like that. The toroid affects the electrostatic 
conditions in that area and may help to funnel the spark 
straight up. I think the splitting has a lot to do with 
how the spark originates at the breakout point as 
it begins to grow at each burst. If it splits early 
as it's forming, then it will stay split. The trick is 
to get the spark to form without splitting. This 
makes it sensitive to tuning and various adjustments also. 
Did you try varying the main tank tuning some? 
It may even be beneficial to use an MOT which 
saturates, because the waveform distortion which 
results can help to stop the splitting perhaps. 
In any case, varying the number of level shift 
caps may help. 
-----Original Message----- 
From: Phillip Slawinski <pslawinski@xxxxxxxxx> 
To: Tesla Coil Mailing List <tesla@xxxxxxxxxx> 
Sent: Tue, 21 Oct 2008 7:15 pm 
Subject: Re: [TCML] Odd VTTC Streamer Behavior 
I just tried the 833A I have laying around. It was the same situation
with the Penta 833C. I also examined the inside of the C and
looked perfect. 
Would the cap really cause the streamers to fork like that, and not
gain any 
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