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Re: Why does top capacitance work?

This post has rekindled a thread from awhile back.....

> > Doesn't this apply only to the very first gap firing, when the power is
> > first applied?  After that, the first streamer would adversely compromise
> > the holdoff voltage of the toroid, acting electrically as a long needle
> > that projects from the toroid.  The subsequent gap firings appear to only
> > build upon the initial streamer, unless the gap rate is so low that the
> > ions start to re-combine between gap firings.  This results in a large
> > number of small streamers.
> > 
> > I am still mystified as to why a large toroid makes a difference.  My
> > favorite explanation is that the addition of a huge toroid forces the
> > operator of the TC to add capacitance to the primary, for tuning purposes.
> > More capacitance increases the thruput power, which is always better.
> > 
> > -GL
> 
> Greg,
> 
> You certainly make some compelling points. I agree that the "root" of a
> streamer, once formed, will act as a weakenned area during subsequent
> bangs, since it's the area of highest current density and should have
> the greatest degree of thermal ionization. The advantage a large toroid
> may provide on higher-impedance coils may come from providing a low
> impedance energy reserve (i.e., a capacitor to ground) during the time
> that the streamer is re-propagating along the weakenned path blazed by
> its predescessor. Adding larger capacitance will provide a
> proportionately larger reserve of energy once the voltage rises to the
> point where reignition occurs. The fact that there seems to be little
> benefit on your large coil may be due to it's relatively low impedance -
> it doesnt need any help, and adding the toroid merely reduces peak
> voltage. Ultimately, it IS power which governs ultimate streamer length.
> 
> Certainly your last point is true, although Wysock and Cox's coils tend
> to have BIG tank caps, small primary inductance and small top
> dischargers -- Adding tank capacitance alone seems to be sufficient for
> BIG streamers on larger coils. 
> 
> Looks like there are still lots of mysteries to be solved! BTW, what do
> you do for a living that lets you play with these nice toys?? :^)
>
> -- Bert --

    This is something that I am hoping to investigate. It is not true 
that all systems will benefit from extra capacitance in the primary 
system. One very experienced coiler reported that increasing his Cp 
from 0.1uF to 0.2uF resulted only in burning up the gap faster with 
no extra output length (power went from 25kW to 50kW). Others have 
also noted this effect. The question then is: why does doing this 
benefit some systems and not others? I earlier suggested that this 
might well be a function of the secondary characteristics and that 
there is an ideal primary configuration for a particular secondary. I 
will be looking for evidence of this in the coil design notes kindly 
sent to me by some list members.

Malcolm