Re: More on sparks--& AOL's goofs

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

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

Ken and all,

No problem Ken - I'll try to fill in some of the gaps. This stuff is
difficult to begin with, and trying to compress it into readable emails
unfortunately means that much of the meaning gets lost in the crunching.
Everything below refers to air breakdown for nonuniform gaps at Standard
Temperature and Pressure (STP). 

A nonuniform gap is simply a configuration of electrodes where one
electrode has a much smaller radius of curvature than the other. The
smaller (active) electrode is connected to the HV source, while the
other is often at ground potential. Because of the smaller radius, the
e-field is markedly stronger near the smaller (or active) electrode. You
can contrast this geometry with a sphere-sphere or point-point gap,
where there's no preferential field enhancement at one electrode versus
the other. A "rod-plane gap" is one of several standard nonuniform gap
configurations commonly used in high voltage testing. Other nonuniform
gaps include the sphere-plane and point-plane gaps. A toroid on a Tesla
Coil can be considered as another type of nonuniform gap, with the other
"electrode" being the earth and the toroid's other nearby surroundings. 

Avalanche Breakdown:
Here's a bit more of an explanation about avalanche breakdown and a
rewording of Raizer's explanation. Electrons do virtually all of the
work in gaseous breakdown, and one key mechanism is the electron
avalanche. An electron avalanche begins with free "seed" electrons in
the gap (originating from cosmic rays, background radiation, UV light -
they're everywhere!). Free electrons are accelerated by the electric
field around the HV electrode. Since the e-field is strongest near the
active electrode, electrons closest to the electrode are subjected to
greater force than those further away. Now suppose the e-field is strong
enough to accelerate some of these electrons to the point where they
collide with air molecules, creating MORE free electrons. If the rate
that new electrons are created becomes greater than the rate they are
are lost, then the number of free electrons will increase exponentially,
creating an electron "avalanche". Avalanches redistribute charges in the
gap, and individual avalanches wink into and out of existence in 10's of
nanoseconds. 

Electron avalanches form more easily near a positively polarized
electrode. This is because, as electrons move towards the positive
electrode, they move towards a region having an even stronger e-field,
enhancing the avalanche process. Once the electrons hit the HV terminal,
they are removed, leaving behind a positive space charge made up of
positive ions, effectively "extending" the field of the active
electrode. However, if the HV electrode is negative, electrons are
repelled into a region of weaker e-field, tending to inhibit further
avalanching. This is a rewording of Raizer's text - and it's the
fundamental reason why initial breakout (i.e. avalanches) occurs
preferentially when the active electrode is positively polarized. 

Breakdown of a long gap in air at STP occurs in stages. These begin with
electron avalanches, streamer formation, and ultimately leader formation
and extension. Groups of avalanches "feed" current towards the HV
electrode in a fan-like pattern of discharges called the streamer zone.
The overall degree of ionization in the streamers is still low, and the
temperature in the region is only slightly above room temperature. If
the HV electrode's potential is increased, the number of avalanches
increases, streamer current builds, and under the right conditions, a
leader forms - a projecting path of hot plasma that's attached to the
electrode on one end (the "root"), and connected to groups of streamers
at the other (the "head"). Individual avalanches in the streamer zone
"feed" current into the head of the leader, such that the leader current
is approximately the sum of the total avalanche currents. If the
electrode voltage continues to rise quickly enough, displacement
currents flowing through the leader help keep it hot (conductive). Under
appropriate conditions (i.e., further rising electrode voltage), the
head of the leader can move outwards, advancing out from the HV
electrode in a series of discontinuous jumps, as a sort of conductive
stick. Streamers continue to feed current into the head of the leader as
long as the head potential is sufficient to maintain the evalanches.  If
the extending leader makes it all the way over to the other
electrode/ground, we get a spark discharge ("power arcing" in a Tesla
Coil). The spark can also evolve into a true arc if the short-circuit
current can be sustained, as in a power line fault. What we usually
observe in a Tesla Coil are actually "arrested leaders" - leaders that
grow to a point, but usually failing to completely bridge the gap to
ground. 
  
Hope this clears up a bit of the mystery...

Best regards,

-- Bert --
-- 
Bert Hickman
Stoneridge Engineering
Coins Shrunk Electromagnetically!
http://www.teslamania-dot-com       


Tesla list wrote:
> 
> Original poster: "K. C. Herrick by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <kchdlh-at-juno-dot-com>
> 
> I'm back with Juno for this one.  I found that I never received, via AOL,
Bert
> Hickman's posting of 26 April or John Freau's of 27 April.  I remember also
> that back in March I hadn't received a couple of List postings regarding my
> goofy spark-booster idea.
> 
> I did receive Bert's 4/25 posting but forgot that I''d wanted to comment as
> follows on that:
> 
> This is not to put Bert on the spot or even to ask him for more
> explication...but the quote he gave from Raizer somewhat baffled me.  Perhaps
> it's because I learned my college EE and physics, such as it was, half a
> century ago; or perhaps it's because maybe English is not Raizer's first
> language...?
> 
> "The avalanches at the _rod anode_ travel to it from the outside; as
> they come nearer, they enter the region of progressively stronger field.
> This factor _facilitates_ the multiplication of electrons and
> _stimulates the avalanche-streamer transition_. In the case of the _rod
> cathode_, multiplying avalanches move further from the electrode into
> the region of progressively weaker field. The multiplication process is
> therefore _slowed down_ and the _avalanche-streamer transition is
> inhibited_. Moreover, in the case of the positive rod, electrons sink
> into the metal, leaving behind a noncompensated positive space charge,
> which enhances the field at the electrode. In the case of the negative
> rod, however, the field of the corresponding negative space charge is
> somewhat compensated for by the field of positive ions, all of which
> stay in the gas."
> 
> 1.  Firstly, I don't appreciate what the physical configuration of the
> electrodes is (but perhaps Raizer disclosed that elsewhere in his paper).
> Also, how that configuration would relate to a typical Tesla coil
> configuration.
> 
> 2.  1st sentence, "...travel to it..." and "outside": unclear to me exactly
> what is meant.  And in fact, "avalanch" itself: is that a process, a
phenomenon
> or a physical entity?  (Right off, I'm sure many of you can now assess the
> extent of my knowledge!)
> 
> 3.  2nd sentence, "...multiplication of electrons":  where do the electrons
> come from?  Does "multiplication" perhaps mean "incorporation of more"?
> 
> 4.  Next sentence, "...multiplying avalanches move further...": what
means it,
> "multiplying": "becoming bigger"?..."becoming fatter"?..."becoming more
> numerous"?  And what means it, "move further": moving to a different
> location?...extending in length?...?
> 
> 5.  If I understand the "Moreover..." sentence correctly, that one would seem
> to be the most pertinent:  He seems to say that regardless of the electrode
> polarity, there's always in existence, at that electrode, a positive
> space-charge.  When the electrode is positive, that charge adds to the field
> and when it's negative, it subtracts.  Showing my ignorance again, is
that the
> case?
> 
> Bert, as to your 26 April posting (beginning, "The breakdown sequence..."),
> allow me to say this about that:
> 
> 1.  Mea culpa again:  I don't appreciate exactly what "avalanch", "streamer"
> and "leader" mean.  If you should be inclined to say to me, "Go read a
book", I
> will fully & completely accept that--& maybe I will.
> 
> 2.  Other than that, your posting is admirably clear to me.  It confirms my
> general understanding that the smallest effective radius of curvature, of a
> conducting surface that is a part of a closed high-voltage system, is the
> location from which a spark is going to originate.  But there seem to be
> qualifications associated with that general rule--as to polarity, size and
> shape of other such surfaces in the system, the nature of intervening
gases if
> any, etc., that remain unclear.
> 
> As to John Freau's 4/27 posting (beginning, "I was thinking some more..."),
> 
> 1.  I can, without too much trouble, and will, modify my s.s.t.c. system for
> cycle-counting:  I will put in a switch-selection for pulse-burst lengths of
> 64, 128, 256, 512 and 1024 excitation-cycles.  That's from the beginning
of the
> ring-up time.  Ring-up to break-out will still take about 30 cycles, at 140
> KHz.
> 
> 2.  When I had written previously that I'd had an interrupted-burst mode
of 64
> cycles on, 64 cycles off, etc., I should have said 128 cycles (& I made that
> mistake once before); my gate signal was taken from the Q7 output of a
CD4020,
> and 2^7 is 128, not 64.  I'm going to replace that IC with a CD4040.
> 
> 3.  Due to circuit considerations, I'll no longer be able to have that
> interrupted mode.
> 
> 4.  I'll still be able to operate at any sub-multiple of the max. 120 Hz
> spark-rate.  Later, perhaps I'll beef up my low-voltage supply (for the
MOSFET
> gates) so that I can fire the coil asynchronously & so at a higher rate than
> 120/second.  Once I do that, I'll really be able to tweak the duty cycle
of the
> sparking & see what happens.
> 
> 5.  I'll repeat my observation that, when I did have that interrupted mode
> operative (128 cycles on, 128 off, etc.), I noted that the probe-sensed
> field-voltage a) dropped (close) to zero during each off-time and b)
resumed at
> the same level at the end of the off-time.  In other words, it did not
> appreciably rise, toward what it was before the initial spark break-out,
during
> that off-time.
> 
> 6.  For now, I think it most likely that John's phrase, "permit higher
voltages
> or currents to be utilized" is the key to longer sparks.  Get all that
power up
> front & cram as many electrons into the electrode as you can, as fast as you
> can.  Ring-up in 2 cycles rather than 30.
> 
> Ken Herrick