Re: More on spark delay

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

Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>" <Kchdlh-at-aol-dot-com>

Malcolm (& all)-

Comments interspersed...


>
> Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <m.j.watts-at-massey.ac.nz>
>
> Hi Ken,
>         Despite my lack of responses to your posts I do read them and 
> take an interest in what you are doing.
>
> On 22 Apr 2002, at 17:11, Tesla list wrote:
>
> > Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>"
> <Kchdlh-at-aol-dot-com>
> > 
> > I've put forth my hypothesis more than once that the relatively high
> > rate-of-rise of spark-gap-coil voltage is the reason that spark-gap
> sparks are
> > longer than SSTC sparks.  I've proposed that that high rate allows the
> toroid
> > voltage to rise higher during the time it takes for the spark to propagate.
>
> > That time period exists due to the necessity to heat and displace the air
> along
> > the spark's path.
> > 
> > The notion has received scant attention.  I may be repeating myself but
> here's
> > an observation I just made today:   In my SSTC, the toroid voltage's rise &
> > fall is extremely stable from spark to spark.  So, I can sync the scope to
> it
> > and accurately gauge the rise and fall.  I find, at commencement of the
> spark,
> > that the toroid voltage falls abruptly, at first, to about 70% of the level
> > that it holds during the remaining 5 milliseconds of the spark's duration. 
>
> Just to be clear; is that 70% of the initial voltage or ?  The drop 
> is to be expected of course. A drop of that magnitude suggests that 
> 50% of the energy in the top capacitance is being dissipated.




See my other posting of today.


>
> > That fall seems to take place well within 1 cycle of the excitation, which
> is
> > at ~140 KHz , and it always occurs at a negative half-cycle (indicating to
> me
> > that the spark initiates when the toroid is 'crowded" with electrons and
> not
> > otherwise).  It then takes just about 100 microseconds longer for the
> voltage
> > to decline further to the steady level.
> > 
> > Clearly, at the instant of the 30% drop, the impedance of the initial
> spark has
> > appeared in parallel with the impedance of the capacitance between the
> toroid
> > and ground.  But at the end of that instant, the spark impedance is still
> > relatively high since its presence in the series circuit of
> > ground/secondary/toroid:ground capacitance/ground (across "toroid:ground
> > capacitance") has diminished the toroid voltage by only that 30%.  The
> spark
> > impedance then relatively-slowly decreases during the following 100 us,
> causing
> > the toroid voltage to correspondingly diminish.
>
> Which supports the observation of prolonged ringing for air streamers 
> and a still rather high loaded Q under those conditions.



I agree as to the Q; but I don't understand "prolonged ringing for air
streamers".



>
> > So my supposition remains:  It is the capability of spark-gap systems to
> > deliver higher power during the (at least first part of) 100 us or so that
> > allows for the longer sparks.  And it is the physical/thermal inertia of
> the
> > air in the path of the spark that causes the 100-us phenomenon to exist.
>
> I've heard the phrase about the abrupt rise of terminal voltage prior 
> to breakout as akin to "shocking the air" or something similar. So 
> what happens if the system is operated on the edge of breakout? Of 
> course, further powerful shocks in the form of subsequent ringups 
> would do the lengthening trick alright. Single shot would probably do 
> no better than a slow ringup to breakout voltage. Indeed, my largest 
> disruptive coil stretches five times further when repetitively fired 
> than it appears to do when sshotted. I say "appears" because there is 
> a distinct possibility that streamers which would be visible under 
> only the darkest conditions streatch out a good deal further. I have 
> observed this phenomenon more than once.



Very interesting about the single-shot length vs. repetitive; I don't know why
that would (seem to) be.  Your gap's behavior may have a bit to do with it
although it possibly is the case that just 1 shot , from a disruptive coil, may
not give much of a visible spark.  My ~5 ms duration yields quite a fat, bushy
one--albeit relatively short.

It seems to me that your "ringup to breakout", given identical gap and
primary-charging behavior, is going to be the same from spark to spark--unless
your repetition rate is fast enough so that the air's heating does not
substantially dissipate between sparks.  When that heating remains, I'd think
that your sparks would tend to congregate along the same path.  Perhaps you
observe this?

As to operating "on the edge of breakout", don't know quite what you mean... 
If no breakout, no spark except, perhaps, for wispy "leaders".  Those leaders
would not heat or displace air, appreciably, so likely they would not much
affect any subsequent large sparks.

Ken Herrick