Re: On sparks

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

Original poster: "Bert Hickman by way of Terry Fritz <twftesla-at-uswest-dot-net>" <bert.hickman-at-aquila-dot-net>

Ken and John,

Very interesting experiment, Ken! The "ring up" time is significantly
longer for this system than for comparable disruptive systems of the same
average power. The long ringup time (or lower primary circuit peak power)
appears to also be limiting the rate that energy can be replenished to the
secondary once breakout has occurred. The initial "collapse" of secondary
output voltage during initial leader propagation is very significant for
this system - it appears to be much higher than for a disruptive system.
Afterwards, the system appears to be unable to "overpower" the additional
energy loss from the leaders, preventing the output voltage envelope from
increasing much beyond about 20% of it's pre-breakout level. 

There are some interesting energy insights here... 
The E-field and base current measurements imply that it takes a
comparatively large amount of energy to initially form the leaders versus
the amount required to maintain them (at least for near CW operation). The
measured pre-breakout E-field was 10 cm pk-pk, declining to a "steady
state" level of 2 cm on the scope. So, once the initial leaders were
formed, the system reached a new energy balance at a point where the output
voltage was effectively "clamped" to about 20% of the pre-breakout level.
This implies that the creation of the initial leader channel required that
the resonator ring up (over about 32 cycles or 200 uSec) to a peak energy
that was about _24 times greater_ than the energy level required to sustain
the fully formed leaders. And, while displacement currents flowing through
the leaders were sufficient to maintain the leaders, the primary couldn't
supply sufficient additional energy to the secondary to overcome the
heavier loading. The resulting inability to further increase the output
voltage apparently prevented any further leader growth past the initial
length. 

I suspect that John's insights about peak power are correct - additional
power must be available for transfer to the secondary at a rate _faster_
than its loss from streamer/leader loading in order for additional leader
growth to occur. Higher peak currents or tighter coupling would be
necessary in this system...

Excellent work, Ken!

-- Bert -- 
-- 
Bert Hickman
Stoneridge Engineering
Email:    bert.hickman-at-aquila-dot-net
Web Site: http://www.teslamania-dot-com

Tesla list wrote:
> 
> Original poster: "Kennan C Herrick by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <kcha1-at-juno-dot-com>
> 
> John (& all)-
> 
> Comments from Ken Herrick interspersed:
> 
> On Mon, 26 Mar 2001 12:03:37 -0700 "Tesla list" <tesla-at-pupman-dot-com>
> writes:
> > Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>"
> > <FutureT-at-aol-dot-com>
> >
> > In a message dated 3/26/01 5:07:04 AM Eastern Standard Time,
> > tesla-at-pupman-dot-com
> > writes:
> >
> > >
> > >  Just to see what would happen, I thought to extend the function
> > of my
> > >  128-cycle counter so as to interrupt the primary's excitation in
> > bursts
> > >  of 128 cycles over the entire 7 ms duration.  400 us on, 400 us
> > off, etc.  [Not 128 but rather, 64--kch]
> > >
> > >  The sparks appear identical in form--jagged & branched--but they
> > are, of
> > >  course, less "fat" when interrupted.  But that they appear
> > otherwise
> > >  identical tells me that each burst of sparks travels essentially
> > the same
> > >  path during the entire series of 400 us bursts.  I suspect that
> > it's the
> > >  heated air, along the path, that induces all the repeating sparks
> > to
> > >  follow along.
> >
> > Ken,
> >
> > That's an interesting test.  You seem to be implying that the
> > sparks
> > are still the same length as before. This would suggest to me that
> > the duration of the application of current to the arcs does not
> > seem
> > to make them shorter, but just dimmer or thinner.
> 
> Well, I'll try to be a little more accurate, as I might have been in the
> first place:  I've just taken a more careful look, in the dark, at the
> sparks when in the 3 modes of operation--~400 us duration, ~7 ms but
> periodically interrupted, and ~7 ms steadily on.  The character of the
> sparks is the same: branched, often if not usually with 2 or 3 branches.
> But the general length does seem to increase, perhaps 30%, overall, from
> the shortest- to longest-duration modes.  And, of course, the apparent
> thickness increases along with that.
> 
> > By reversing
> > this thought, it seems to suggest that applying current for a
> > longer
> > duration would not make the spark longer either.
> 
> It makes it longer, but not a whole lot longer--nowhere near
> proportionally to the duration.
> 
> > I'm assuming
> > that since the coil is off for half the time during a burst now,
> > that
> > the total average current supplied to the arc is much less overall.
> > Are you thinking the same way?  Am I missing some point?
> 
> It is definitely less; I haven't measured it exactly but it is surely, in
> my system, going to be directly proportional to the overall duty cycle of
> sparking.  When I make no sparks, I have close to zero line-input current
> since everything drawing static current in my system, with the exception
> of a few pull-up resistors, is CMOS or MOSFETs, & their static current is
> negligible.  Essentially all the line power goes into charging the
> storage capacitors, and all of their output currents go into the primary.
>  The main losses are in the switching supplies' MOSFETs and the
> primary-loop MOSFETs, and those are modest.
> >
> > Maybe it's the peak currents, rather than the average currents
> > that are important in determining spark length?  Any thoughts
> > about this in light of your experiment?
> 
> That's worth some thinking about--or experimenting.  I haven't looked
> into it; my primary current is only slowly variable, dependent upon the
> partial discharge during the pulse-burst of the electrolytic storage
> capacitors.  I don't have any handy way of varying it abruptly.  I am
> reminded that I do notice a definite reduction in length when I increase
> the rep. rate so much that the capacitor voltages sag pretty far down.
> So there's a clue.
> 
> >
> > For instance if you can channel the power you "saved" by
> > having the coil off during half the burst time, into higher peak
> > powers instead, that may make the sparks longer.  Since
> > the MOSFETS are only "on" half of the time now, they should
> > be able to handle higher peak currents without burning up.
> 
> I tend to think not for this reason:  That 400 us spark is definitely 30%
> or so shorter than the 7 ms one and yet it emanates from the toroid at
> the instant when the toroid voltage is upwards of 8x the voltage there
> while the 7 ms spark is occurring.  Surely (correct me if I'm wrong) the
> peak power at that initial instant is much greater than the power being
> expended at comparable instants during the 7 ms.  So if the conjecture
> were correct, one might expect that initial spark to be, although
> thinner, longer and not shorter.  But it isn't.
> 
> What I'm going to need is more ampere-turns in the primary.  I'm not
> analytical enough, unfortunately, so far, to be able to dope out ahead of
> time what is the optimum mix of switches/capacitor-banks vs. quantity of
> primary turns in my current-loop primary system.  Right now I have 4
> switches, 4 capacitor banks and 3 turns.  I happen to have 1 more set of
> circuit boards from which I could make up a 5th set of
> switches/capacitors.  If I can dope out how best to wire that 5th section
> into a 3-turn primary configuration, I may take a shot at that; I think
> the MOSFETs would stand the current.  That will raise the ampere-turns by
> ~5/4 and surely make the spark longer--but to what extent, who knows?
> 
> >
> > John Freau
> >
> 
> Ken Herrick
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