desirability of high coupling...Re: primary INSIDE of secondary?

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Jim Lux by way of Terry Fritz <teslalist-at-qwest-dot-net>" <jimlux-at-earthlink-dot-net>

At 12:03 PM 4/3/2003 -0700, you wrote:
>Original poster: "boris petkovic by way of Terry Fritz <teslalist-at-qwest-dot-net>"
> >
> > Spark-gap type coils will work with the primary inside the secondary.
> > How well they will work, depends on the coupling and how much
> > insulation is between the two coils. Too much coupling is generally a
> > bad thing: it can cause "racing arcs" and poor efficiency/short
> > sparks.
> >
>---
>Higher coupling is desireable in all TC systems for
>better efficiency.The only question is how geometry
>and insulation can withstand E-field stress without
>letting the discharge where one doesn't want to (ie.
>flashover primary-secondary,racx sparks etc.)


I don't know that high coupling is always desirable... Here's some ideas 
why not (without any real analysis behind them, so all you analysts.. flame 
on!)

1) With respect to "efficiency", defined as the amount of energy lost to 
resistive losses. Is the loss of the primary or secondary higher? The 
primary cap has more loss (in a percentage way) than the secondary (which 
has air dielectric, and is very low loss).  I'm not so sure about the 
primary L vs secondary L, but off hand, I'd just think the primary L is 
lower loss than the secondary L.  Considering the entire primary circuit, 
the spark gap is probably the big loss, so high coupling gets the energy 
out of the primary quickly, reducing the spark gap losses...

2) There is a huge effect on spark growth and shape from the RF envelope, 
and the coupling (and relative tuning) of pri and secondary.  It's possible 
that you don't want too fast a rise of the RF energy in the secondary, to 
give the sparks time to develop and lengthen?  Consider that a Marx bank 
with a single impulse is sort of a similar thing to a very highly coupled 
TC (i.e. all the power is there at the first RF cycle)... Marx banks 
generate sparks that look very different from a TC.  (Has anyone built and 
tested a 120 pps reprate Marx with a couple hundred kV output?  At first 
glance, one might expect the output to look like a TC)

3) Part of the coupling thing is the relative resonant frequencies of the 
primary and secondary (leading to "beats" (aka "notches"))...again, this 
affects the RF envelope, which probably affects the spark "look".  There's 
also a change in the resonant frequency of the secondary as the spark 
develops...