Re: Enhancing Corona

["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>

Scott and all,

I think you are asking how one can generate cold streamers instead of
leaders. Streamers are relatively long, "cold" discharges that rapidly wink
in/out of existence, collectively forming a diffuse bright bluish glow that
extends outward from the HV terminal before leader breakout, or outward
from the tips of leaders once they've formed. This type of discharge is
often (but mistakenly) called corona. It's cold streamers that are
primarily responsible for generating the large quantities of ozone
associated with running small coils. Leaders are the "hot", arc-like
discharges which most coilers desire, which are (but mistakenly) called
streamers. It gets kind of confusing sometimes... :^) 

Anyway... it turns out that it's the rate of rise of the terminal voltage
combined with the availability of "instant" charge at the topload and how
fast it can be replenished from the resonator that governs the form and
character of the discharges you get. Smaller, higher frequency systems
simply don't have as much topload charge available to support leader
formation and growth. So, the resulting leaders tend to be shorter and
(because each "step" is shorter), more branched. And, because the peak
currents are lower, their appearance is also less arc-like (more purple
instead of white colored). As you go outward, all leaders grade into
streamers, eventually dissipating into the surrounding air. 

Skin effect is a function of the electrical conductivity and magnetic
permeability of the material and the applied frequency. All other things
being the same, a poor conductor will always have a larger "skin depth"
than a good conductor. The relatively low current levels and channel
temperatures in Tesla Coil streamers and leaders simply does not result in
very thorough ionization of the air in the path. So while a leader may be a
much better conductor than un-ionized air, it's a MUCH poorer conductor
than even the poorest conducting metal. The conducting diameter of a leader
channel is much smaller that its "skin depth" even at very high
frequencies. So, skin effect is not likely a significant factor in the
physics of Tesla Coil streamers and leaders.   

There are several things that you can try for creating just streamers:
1. Reduce the resonator self capacitance and topload capacitance by using a
physically smaller system that operates at a higher resonant frequency. 

2. Use a smooth toroidal having a relatively large radius of curvature but
a small major diameter to inhibit initial breakout and leader formation.
This will add minimal topload capacitance. Mount the toroid close to the
top of the resonator. The smaller reserve of topload charge means that
terminal voltage rapidly drops once we begin to draw any appreciable
displacement current via discharges. The collapsing terminal voltage stops
any further leader propagation in its tracks until the terminal charge (and
voltage) can be replenished.

3. Reduce the main gap spacing until the system is just below the point of
leader "break out". You may need to use a DC charging system of a SRSG on
an AC system in order to achieve sufficient uniformity of bang size.

4. Place a grounded object near the top terminal to increase the local
field. If placed properly, only streamers will traverse the space between.
To observe this properly, operate the system in a dark room with your eyes
dark adapted, shielding the light from the spark gaps. 

BTW, for a gorgeous timed exposure of streamers in the air space between a
toroid and a grounded plate suspended above, look at the images in the
"coolpics" section of Mike Hammer's site:
http://home.gallatinriver-dot-net/mhammer/coolpics.htm

Hope this helped!

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


Tesla list wrote:
> 
> Original poster: "Scott Fulks by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<darkthing-at-earthlink-dot-net>
> 
> I am interested in doing some experiments on normal-pressure plasmas, and
> was thinking of custom-designing a TC to create copious amounts of brush
> discharge as opposed to sparks.  Virtually all of the discussion on this
> forum seems to equate spark length with efficiency, but I am sure that many
> coilers have seen configurations which produce more corona than spark
> length.  I'd appreciate any feedback from coilers who have (accidentally or
> intentionally) produced a coil that puts out lots of corona.
> 
> The way I see it, the secondary frequency is the key parameter for brush
> discharges.  If the frequency is high, inductive effects similar to the
> well-known "skin effect" come into play more prominently.  The impedance of
> a single thick spark is so great at high frequencies that the discharge is
> more prone to seek multiple pathways, just as litz wire offers a lower
> impedance to RF currents than a solid copper wire.  Multiple small discharge
> pathways are seen as a brush discharge, since the human eye cannot resolve
> the individual lines of current and just sees a uniform glow.
> 
> If this is true, then I would expect the length of sparks from a coil would
> be in inverse relation to the frequency of the secondary.  A lower frequency
> would give longer sparks.  From the discussions of the importance of large
> toploads for spark length on this list, this seems likely.  Large topload =
> larger secondary capacitance = lower frequency = longer sparks.
> 
> I recall someone on this list awhile back say that small coils seem to
> produce more ozone than larger coils.  Is this because smaller coils operate
> at higher frequencies (low inductance secondary), or is there some other
> reason?
> 
> Regards,
> Scott Fulks (darkthing-at-earthlink-dot-net)