Hi Greg,
There's no need to probe your primary circuit in order to measure
quenching performance. You can tell exactly when your coil is
quenching by simply connecting your scope to a small wire antenna
that's placed well outside of the sparking range of your coil (see
John Freau's earlier post). You'll need to carefully set up your
scope's trigger and timebase so that you can view a complete bang on
your scope every time the gap fires. When set up properly, you'll see
one or more sine-like RF envelope "humps" that progressively become
lower in amplitude. By counting the number of humps on the secondary
waveform, you can directly determine when your primary gap is quenching.
If your gap fails to quench, bang energy will continue to cycle back
and forth between primary and secondary LC circuits, continually
losing energy all the while. Once the gap quenches, the energy
interchange process stops, and all the remaining energy becomes
stranded within the secondary circuit. The remaining energy then
decays within the secondary circuit (ideally mostly through
leaders/streamers). If you could achieve a single-notch ("perfect")
quench, you'd simply see a single secondary-side ring-up followed by a
much slower decay (from Richie Burnett's excellent site):
http://www.richieburnett.co.uk/quench1.gif
If your gap quenched on the 2nd notch you'll see two secondary peaks,
with the secondary decay occurring just after the 2nd peak. And so
on... Here's an example of 3rd notch quenching from Richie's site:
http://www.richieburnett.co.uk/quench2.gif
I also agree with John - "perfect" single-notch quenching is not
essential to obtain excellent coil performance. A given spark gap coil
will show considerable quenching variation based upon the operating
power level, degree of streamer loading, coupling coefficient, etc.
You'll also see significant quenching variation from bang to bang even
at a fixed power level, especially when using a static or asynchronous
rotary spark gap. A very well written in-depth discussion about
quenching can be seen on Richie's site:
http://www.richieburnett.co.uk/operatn2.html
The bottom line:
There is simply no need to risk your scope (or potentially your life)
trying to measure your primary circuit when a simple secondary antenna
will provide you with similar quenching information.
Bert