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Re: Corona and Sphere - Puzzle

Hi Malcolm and Boris,

At 06:47 PM 10/22/1999 +1200, you wrote:
>Greetings all,
>                     Following some very interesting discussions offlist 
>with Boris Petkovic, I and he have a problem we would like to throw 
>into the arena for comment and possibly some answers.  I can 
>formulate the problem like this:
>
>       I have a sphere of a chosen diameter. It is connected to the top 
>of a TC. At some voltage, the sphere will issue a visible streamer.  
>We know that air streamers do not lower the Q of a system to a high 
>degree. The relatively high Q of system doing this has been 
>measured by myself and others and manifests itself as a continuing 
>series of beats which are visible on an e-field oscilloscope trace of 
>the secondary.  

Streamers do lower Q considerably but I guess that is sort of in the "eye
of the beholder" ;-)

>
>         The question:  Why does a single rooted streamer or just a 
>few streamers appear when the breakdown voltage of the sphere 
>has clearly been reached?  Why not all around?

IMHO - It depends on how much power is available.   A large top terminal
usually has one or few streamers while a small one has many more (all else
being equal).  A large terminal has one high power/voltage streamer while a
small terminal may have many but at lower voltage and power.  Once a
streamer forms, it will be the path of least resistance and will attract
the following energy along the established path.  If there is enough power
left after that, a second (third, forth...) streamer can form but they each
drain the system somewhat and take their toll on the available energy.

>
>     Something to consider: I have operated a coil such that the 
>voltage it reached was not sufficient to promote a visible streamer 
>but if the sphere was viewed in *very* dark conditions, the terminal 
>was indeed surrounded by a ball of wispy streamers issuing in all 
>directions.

I think you were seeing just the first hint of streamers but the energy was
not high enough for the current to repeat the path of the first little
spark.  Thus, every beat of the power cycle was creating a unique little
"one shot" streamer rather than the pulses taking the same path.

>
>     Does anyone have any comment on this? Is there any *good* 
>reason why, when terminal voltage has reached such a level as to 
>allow the issuing of a single bright streamer more are not issued?

A streamer is a hot path of air that has low resistance and will attract
the following pulses along that some path.  Thus, the streamer will grow
outward building on the path that has already been established by the
previous pulses.  The first streamer will leave little extra energy needed
to create a second streamer unless you have much more energy to spare after
the initial streamer.  There is probably a critical point where the air is
ionized enough so that the following pulses will follow an established path.

>
>      The problem appears simple on the surface but perhaps not so 
>simple if one delves deeply enough into what is happening.

These things are never simple ;-)

>
>      I've tried to formulate the problem as concisely as I can but 
>perhaps Boris might like to elaborate if he feels I have missed 
>something. This is written in some haste as I am about to go away. I 
>look forward to reading the answers when I return.

Another interesting twist is that one often sees a "little streamer" on the
exact opposite terminal side of the main streamer.  I think that is do to
electrostatic and sudden discharge effects in the space around the terminal
but that is only a "feeling" on my part without any data...

Cheers,

	Terry

>
>Regards,
>Malcolm
>