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Top Terminal Shape

Subject: Top Terminal Shape
  Date:  Thu, 8 May 97 14:27:47 EDT
  From:  pierson-at-gone.ENET.dec-dot-com
    To: 
       
mail11:;;;;;-at-us4rmc.pko.dec-dot-com-at-us4rmc.pko.dec-dot-com-at-unknown.domain.pupman-dot-com;;;;;;;;
(-at-teslatech)
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        pierson-at-gone.ENET.dec-dot-com


[note to the chop crew:
I'm manually limiting the line lenght to 70ish]

>dw pierson wrote:

>>         Assuming the rest of a given coil setup can provide some
>>         arbitrarily high voltage (eg: low enuf losses, high enuf
>>         power), the spark over distance is set, largely, by the
>>         terminal (toroid, sphere, whatever) diameter.  A 'zero radius'
>>         terminal (point) will spark out at a relatively low voltage.
>>         If LOTS of power is available, there may be long sparks.  A
>>         larger toroid will 'allow' higher voltages before sparking over.
>>         (Of course changing toroid size also changes tune, all the
>>         way back thru the system, so simply dropping on a new toroid
>>         to see the effect can be complicated to interpret...)


>There are several points to consider here:

>1)  Why then wouldn't a hemisphere on the top of the coil be the best?
        There's 'best' and there's 'good enough so it does not matter.'
        Van de Graaf's, with generally lower power inputs are more or
        less forced to a squished ball shape, to get max voltage.  With
        more power to work with 'coilers have a little more freedom.
        I believe this has been reported here by a number of people,
        that 'balls' are as good as toroids, theoretically a smidge
        better, but practically not enough to be worth doing.

        Also, its the smallest radius anywhere (exception below) that
        is expected to control, so a thin wire to a separated sphere
        is questionable, in fact designing the to terminal so as to
        be phyisically mountable, while not having sharp curvature
        is tricky.  I noted a few weeks back that someone reported (?)
        improvement by spacing the toroid up WITH A LARGISH CYLNDRICAL
        METAL SUPPORT, to there was minimal corona loss off the wire.
        (Was i hallucinating, did somoen so report?)  If so, it would
        want a slit in one side to avoid 'shorted turn', effects, i
        think.

        Also, i suspect (and here i emphasize i am speculating that
        the top terminal, at least before breakout, may or may not be
        the physical terminal present, but actually an ion cloud
        surrounding it, maintained by the corona.  If so, the
        terminal shape may be more spheroidal than the shap appears.
        Which (not coincidentally 8)>>) leads to:

>2)  Some coilers report getting longer arcs by putting breakout points
>    on their toroids, which actually compromise the hold-off voltage.
        A subject for investigation.  One _speculation_ might be that
        this establishes an ion cloud, present, but invisible, thru
        which restrike (as in natural lightning) is easier.

>3)  The streamer itself, once formed, acts as a sharp wire hanging off
>    the toroid, further compromising its holdoff voltage.
        ...and as in natural lightning, the streamer is actually
        multiple strokes, pumping successive discharges through the
        same, ore related channels, too fast for the eye to see.  As
        the streamer forms, each shot may extend.  Once it extended,
        say as far as the available power or voltage, it may stop
        growing, but waiver around, at quasi constant length.

        Also, i will wager any number of cookies, that the maximum
        voltage, by definition, ONLY exists 'pre breakout'.  Once
        breakout occurs, the secondary terminal voltage will fall
        radically, for as long as the spark/streamer exists.  In
        otherwords, once the terminal discharge starts the 'maximum
        voltage' conditions do not exist.

>It would seem that a larger radius of curvature isn't necessarily
>the reason for longer arcs.
        There are usually MANY reasons for an effect.  Its simpler, and
        easier to think of one thing causes one thing.  In a system as
        complex as a TC, MANY things will influence the reuslts.

        Part of the challenge is to optimize, each, in succession, and
        then go back and REOPTIMIZE.

>What else could it be?
        Input power
        Efficiency (all of them, of each component)
        Ground 'goodness'.
        And a few more....
        All at once.

=========varia====================
Tesla the man:
        There is another list where this is discussed, i fell off it,
        some one else may supply the current addresses.

======varia ii========================
'...lighting as dc...'
One rapidly gets into a swamp of semantics, unless exceeding care is
taken.  Arguabl;y, the only really DC _DC_ is from a battery.  For
electrical (or mechanical, or _any_ other impulses) There is an
alternating 'component' associated with the edge.  All working
enigneers use this if deailing with impulses in the frequency domain.

The mathermatical technique called the Fourier Transform 'decomposes'
a non sinusoidal wave, including a single shot impulse, into
component freqs, each of which is very real (eg: can be seen on a
spectrum analyzer, heard on a radio, etc.)  Its been a while since
i had to do this rogorously, but, if i recall, 1/rise_time (or fall
time) gives the main component of a single shot pulse.  In the
case of a '2 by 50' lightning pulse, this means one component at
500 Khz, another 20 KHz and a bunch of harmonics up above those.

regards
        dwp