Re: More on spark delay

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

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Ken,

On 23 Apr 2002, at 13:52, Tesla list wrote:

> 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".

The difference between an air streamer which still allows the 
secondary to retain much of its energy as seen in ongoing ringing and 
energy trades, and a spark to grounded object which stops ringing 
more-or-less dead in its tracks.  
 
> 
> >
> > > 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 is a well worn observation that repetitive hammering of the air 
lengthens streamers well beyond the length obtained in single shot 
mode. An example: Electrum gives a bushy single-rooted air streamer 
about 7' in length when breakout voltage is just exceeded in single 
shot mode, but can send a connecting spark out around 40' or more in 
repetitive running. Each succesive shot makes use of the hot air path 
of its predecessor.  

> 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?

Absolutely. 

> 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.

Right. By "on the edge of breakout" I mean an attained terminal 
voltage which just allows breakout given the ROC of the terminal. 
    I hope this is clearer.

Regards,
malcolm