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Scopes Away!

Hello everyone,
                I have just spent the weekend investigating the 
vagaries of disruptive 2 coil systems (in my living room!). Among the 
things I investigated were:

- the effects of "current-limiting" chokes
- loading of the secondary
- claims about esoteric phenomena associated with "coherence times"
- optimum dwell/quench times
- CW operation in a disruptive system (and how difficult it is to 
  achieve. This is where the VSWR scenario makes itself known.
- a host of other things too numerous to list

     The setup consisted of a two coil system with the spark gap 
replaced by 4 paralleled MOSFETS (MTP3055E). The secondary coil with 
sphere resonated at 153kHz and measured a Q of 184 unloaded, 123 
decrement. I should explain that the coil was wound with 1900 turns 
of rather fine wire (0.2mm). I wanted a compact resonator that 
operated at a moderately low frequency. I think it will perform 
rather well under real conditions.
     The primary was 5.3 turns of 1/4 inch copper tubing about
2.8" high. The bottom turn of the secondary was about 1/2" above the 
primary top turn for k = 0.1 exactly. The primary cap consisted of two
(polyester?) 560nF caps in series. Primary Q without the MOSFETs 
measured around 24.
     With the FETs driven with a gate drive of 8V from a pulse 
generator terminated with a 50 Ohm resistor at the gate/source 
connection. Primary Q measured 9 ( from waveform decrement). 
This is rather close to what you would get with a standard spark gap 
incidentally.
     The DC power supply went up to 30V -at- 1.5A or so. At low
rep rates, I fed the circuit with a 10k resistor to characterize 
basic operation, then switched to a 9mH inductor with and without a 
diode in series. Secondary signal pickup was by a short aerial wire
hung about 4 feet away.
    The pulse generator was a Philips PM5712 and allowed me to set
dwell and rep times over a very wide range.

     First point - you can easily see the waveforms I've been using 
the storage scope to capture with an ordinary scope with this setup.
It is now within the power of anybody to do investigations of this 
type without risking equipment and life and limb. Under some 
conditions I was able to draw power arcs off the terminal of 1/2" or
so. It is worth noting that these conditions correspond with rather
less efficiency than one would expect of a TV flyback transformer due 
to the low value of k I was using. I took nearly two rolls of film off 
the scope - I hope they come out.

     Brief summary of results:
- the spikes generated when the gap is "quenched" at the wrong time
(zero-crossing in the primary - mag field maximum) are horrendous.
Jim's measurements are well substantiated. The ring associated with 
these is easily related to wiring inductance in the primary and stray 
circuit capacitances.
- there was no evidence whatsoever of any sudden jump in the secondary 
amplitude associated with "coherence time in the secondary" as stated 
in the 1990 ITS Symposium Notes. The energy was dissipated either 
both coils, the relative proportions depending on the dwell time.
- some resonant charging was apparent when a power feed choke was 
used. You can easily see the large humps generated by scoping across
the drain and source of the "gap". It really took effect with the 
inclusion of a diode at the kind of rep rates we use. The diode 
allowed the cap to charge without subsequently discharging. I was
swinging a 6 Volt supply up to the Vds of the MOSFETs (60V or 
thereabouts) no trouble at all with moderate rep rates.
- you can easily verify the optimum quench time vs the calculated 
value (single primary ringdown to zero). With a looong dwell time,
I obtained as many as 6 energy trades, clearly visible as the well 
known DSB beat envelope. 
- by increasing the rep rate to the kHz region and phasing the "gap"
on/off times carefully, I was able to achieve CW operation. I should 
stress this is with the secondary unloaded. Resonance does not last 
long at all with loading. In fact you can leave a long dwell, move a 
hand near the terminal/ connect a resistor across the sec/ touch the 
terminal etc. and watch the beats disappear and the primary revert to 
a single ringdown. It was very difficult indeed to adjust to the pulse
gen to do this. You can imagine the effects of loading and consequent 
detuning of the secondary. Easier to use a CW supply straight off. I
was effectively operating in class C.
- many many more......

A few conclusions: At moderately high rep rates, your 
limiting/boosting chokes have to be a compromise between allowing a
quick rate of recharge (low inductance) and protecting the supply
from the shorted gap (high inductance). The "synergism" appears in 
graphic detail in both primary and secondary waveforms and reflects 
the experiences of Ed, Richard and others. If your gap speed does not 
match circuit reactances well, you can end up with very little power 
going in. At the other extreme, you can easily blow fuses. The 
inductor and primary cap are the main culprits although in a real 
system, the transformer, variac filter chokes etc. all band together 
to play a part. Resonances abound and become more and more prolific 
with high break rates. Watch what your primary cap is being subjected 
to. Nuff said.

    I took a couple of photos of the equipment and test setup which
I hope to mail to Jim at some stage.

Regards all,
Malcolm