TSG instability

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

Original poster: "Lau, Gary by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Gary.Lau-at-compaq-dot-com>

I have been making slow progress with my triggered gap experiments.  I use
a modified Jon Tebbs zero-crossing detector, driving a capacitor-discharge
circuit to power an EG&G trigger transformer.  The circuit works well,
delivering a consistent 3/8" spark regardless of the phase control setting.
 All of my experiments so far have been done using an array of halogen
lamps to serve as a dummy-load in place of my primary coil.  This works out
well as the lamp brightness directly indicates the phase "sweet spot" and
is far less subjective than streamer length.

While I'll ultimately be interested in finding the least lossy electrode
configuration, I've been stumped by instability when approaching the timing
sweet spot.  Just as with a mechanical synchronous RSG, the lamp brightness
increases as the trigger delay increases, approaching the sweet spot.  The
sound of the gap is a strictly even drone.  Scoping the gap waveform, the
gap is firing as the gap/cap voltage is still increasing.  Increasing the
delay, the lamp brightness continues to increase and the gap fires as the
waveform crests.  But now, every second or so, the gap misfires.
Continuing to increase the delay, the brightness increases considerably and
the gap fires as the waveform is on the downswing, but the frequency of
misfires increases correspondingly with the delay setting.  Clearly, from a
performance perspective, the gap should be firing at this point, but the
misfires cause dangerously high voltages across the cap.  If I was using my
primary and secondary rather!
 than a dummy load, racing sparks would surely result.

I wanted to ask other TSG users what their experience has been with regard
to this instability.  I don't think my trigger circuit is misfiring when
the NST is drawing maximum current and causing the mains to droop.  The
300V capacitor discharge power supply dips only a few volts when the
instability is at its worst.  I have verified that the timing is stable
across a wide latitude of AC input voltages.  That the timing vs.
power-delivered and stability is so similar to a mechanical SRSG, it makes
me wonder if there is some mechanism inherent in sync gaps that can't be
overcome.  But I don't want to believe this.

I'm using a 15/60 NST with a .03uF cap, for about a 3X LTR factor.  Since
the instability seems most evident when the timing is beyond the crest of
the cap voltage (optimal for LTR operation), it seems that LTR operation
may be a factor.

Regards, Gary Lau
MA, USA