Re: More on sparks- Bert's 4/29

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

Original poster: "K. C. Herrick by way of Terry Fritz <twftesla-at-qwest-dot-net>" <kchdlh-at-juno-dot-com>

Bert (& all)-
 
An admirable run-down.  Thanks!  I'm sure it will benefit others as well as me.
 
One more thing occurs to me to ask about & then, perhaps, I'll shut up:  With a
positive active electrode, electrons that hit it are "removed"--but I wonder if
it makes a difference as to the nature of the path by which they are to be
removed.  In many instances, that path might be a relatively low-impedance one,
back to a power supply.  But in the case of a Tesla coil, it would seem to be
via the secondary coil to ground.  Is the ease of "removal" different in the
two cases and if so, would that make a difference in the characteristics of the
spark?
 
Ken Herrick
 
 
On Mon, 29 Apr 2002 00:15:57 -0600 "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-qwest-dot-net>twftesla-at-qwest-dot-net>"
<<mailto:bert.hickman-at-aquila-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
> ctive 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>http://www.teslamania-dot-com       
> 
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