Re: [TCML] Odd VTTC Streamer Behavior

[Bert Hickman <bert.hickman@xxxxxxxxxx>]

John,

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...  :^)

Bert
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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.
>
> John
>
>
> -----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 as
> with the Penta 833C.  I also examined the inside of the C and everything
> looked perfect.
>
> Would the cap really cause the streamers to fork like that, and not 
gain any
> length?
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