Re: On sparks

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

Original poster: "Kennan C Herrick by way of Terry Fritz <twftesla-at-uswest-dot-net>" <kcha1-at-juno-dot-com>

Bert (& all)-
 
I pretty much agree except I'd put a bit different spin on the matter here &
there.  I'll intersperse some comments & leave everything else here for the
record:
 
Ken
 
On Thu, 29 Mar 2001 07:53:06 -0700 "Tesla list"
<<mailto:tesla-at-pupman-dot-com>tesla-at-pupman-dot-com> writes:
> Original poster: "Bert Hickman by way of Terry Fritz 
> <<mailto:twftesla-at-uswest-dot-net>twftesla-at-uswest-dot-net>"
<<mailto:bert.hickman-at-aquila-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. 
 
It would not be the ring-up time per se that limits the energy-replenishment
rate but rather the seeming lesser capability of my system to deliver power
into the secondary; that lesser capability is shown to exist by the need for
more cycles of ring-up before spark break-out.  But I believe we mean the same
thing.
 
> 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). 
 
That larger amount of energy is going into building up the secondary's magnetic
field to the level necessary to create the toroid voltage required for spark
break-out.  With half the break-out radius, I'll need half the current, etc.
 
> 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.
 
Right...clamped by virtue of the spark's extremely tiny effective break-out
radius.  The radius is no longer the 3" of the toroid but rather the end or
ends of the spark.

> 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. 
 
Yes, because of that 3" toroid radius.
 
> 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.
 
Agree.
 
As I've remarked, the ring-up time is lengthened by the use of the 6" x 24"
toroid, from which the spark is to emanate.  When I add a smooth 3" diameter
copper sphere on top of the toroid as the break-out point, my previous 10 cm
breakout amplitude on the scope diminishes to about half that and break-out
occurs in about half the time, which figures since the break-out radius is just
half that of the toroid.  But the amplitude during the remainder of the
sparking time remains the same, which also figures since a) the rest of the
system is the same and b) the nature of the spark is the same (with its tiny
radius).
 
> 
> 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...
 
See my other recent posting on this matter.

> 
> Excellent work, Ken!
> 
> -- Bert -- 
> -- 
> Bert Hickman
> Stoneridge Engineering
> Email:    <mailto:bert.hickman-at-aquila-dot-net>bert.hickman-at-aquila-dot-net
> Web Site: <http://www.teslamania-dot-com>http://www.teslamania-dot-com
 
Still from KCH:
 
I don't fully understand yet the seeming differences between the performance of
my system and that of a typical spark-gap system of comparable input VA, for
these reasons:
 
1.  I readily develop enough secondary voltage to break out from the smooth 6"
x 24" toroid alone.  In fact, judging from the slope of the voltage rise as
eyeballed on the scope, I could diminish the secondary turns by 20% or so and
still break out from that toroid.  That's going to be a follow-on project, to
see if I can reduce the turns ratio and thus increase the current into the
spark.
 
2.  I should think that my primary:secondary coupling coefficient is comparable
to that of conventional systems.  The center-line of the ~1"-dia. primary
bundle lies ~1" below the first turn of the secondary and that bundle is of the
same nominal major diameter as the secondary winding.  I think I can improve
that somewhat, however, by raising the bundle ~1/2" into a recess in the
secondary's end-cap and I will try that soon.
 
3.  My system is likely more efficient in that I lose no power in a gap.  The
biggest power loss is in the MOSFETs: 48 MOSFETs x (indirectly measured 3 A
current, each)^2 x (data-sheet RDSon of 0.28 ohms) = ~121 W.  And that is when
drawing ~1500 VA from the power line.  (But can't do that for very long out
here in poor old power-starved CA.)
 
4.  My secondary is constructed conventionally--at least, in terms of size,
turn spacing and quantity of turns, wire size, etc.
 
Possibly there is something about my system that produces rather "bushy" and
markedly branched sparks rather than long, skinny ones.  The energy would be
going into making them fat rather than making them long.  But what would cause
that?  Perhaps it's the environment:  There are many conducting surfaces in my
shop around the coil, 6-12 ft. away from the toroid and only ~4 ft. away
directly overhead--although the sparks emanate horizontally for the most part. 
But I should expect those surfaces to induce longer sparks rather than fatter
ones, if anything.  Perhaps when I get it out into the open I will see
different performance.
 
KCH
> 
> Tesla list wrote:
> > 
> > Original poster: "Kennan C Herrick by way of Terry Fritz
> <<mailto:twftesla-at-uswest-dot-net>twftesla-at-uswest-dot-net>"
<<mailto:kcha1-at-juno-dot-com>kcha1-at-juno-dot-com>
> > 
> > John (& all)-
> > 
> > Comments from Ken Herrick interspersed:
> > 
> > On Mon, 26 Mar 2001 12:03:37 -0700 "Tesla list" 
> <<mailto:tesla-at-pupman-dot-com>tesla-at-pupman-dot-com>
> > writes:
> > > Original poster: "by way of Terry Fritz
<<mailto:twftesla-at-uswest-dot-net>twftesla-at-uswest-dot-net>"
> > > <<mailto:FutureT-at-aol-dot-com>FutureT-at-aol-dot-com>
> > >
> > > In a message dated 3/26/01 5:07:04 AM Eastern Standard Time,
> > > <mailto:tesla-at-pupman-dot-com>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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