Re: Sparks to Mid-Air (fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Sun, 28 Oct 2007 12:02:23 -0500
From: Bert Hickman <bert.hickman@xxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: Sparks to Mid-Air (fwd)

Tesla list wrote:
> ---------- Forwarded message ----------
> Date: Fri, 26 Oct 2007 21:40:18 -0500
> From: Crispy <crispy@xxxxxxxxxxx>
> To: tesla@xxxxxxxxxx
> Subject: Sparks to Mid-Air
> 
> Hello,
> 
> This is something that's been bugging me for a while.  I like to
> understand things that I see and do, but I can't figure out how a Tesla
> coil can create sparks into mid-air.  You'd have to have some voltage
> generated in the air relative to the Tesla coil topload (I think), but
> how?  Is this dependent on the frequency at all?  Like, would a
> theoretical Tesla coil operating at a very low frequency still generate
> sparks into mid-air?
> 
> Thanks,
> Crispy
> 

Hi Chris,

All long sparks were, at some time during their development and growth, 
mid-air sparks (sometimes called "arrested streamers"). Long sparks 
develop by (very rapidly) evolving through a sequence of corona, burst 
corona/streamers, leader formation, and leader-streamer propagation. For 
many DC or low frequency HV devices operating at 100's of kV (Van de 
Graaff (VDG) generator, DC and AC power transmission lines), air 
breakdown often stops at the corona or burst corona stage, OR the spark 
very rapidly propagates across the entire gap to complete a high current 
spark to ground or a return electrode. As a result, you tend to see only 
  corona or complete sparks with these systems.

It takes time for a spark to completely propagate across a long gap. If 
you were able to suddenly remove the high voltage source before the 
spark completely bridged the gap, you'd see "arrested streamers" that 
only partially bridged the gap. Some early long spark research was 
performed using a HV impulse generator that created a short, high 
voltage pulse which initiated the spark propagation process across a 
long gap. However, before the propagating spark could completely bridge 
the gap, the HV was suddenly removed, arresting any further streamer 
growth. This allowed the faint propagating leader and streamers to be 
photographed and studied without being flooded with light from fully 
bridged spark discharge. Not surprisingly, photos of these look 
remarkably similar to TC air discharges.

Large DC or low frequency HV sources can exhibit burst corona from 
rounded terminals/conductors. Unlike higher frequency Trichel pulses or 
short glow discharges, these rooted branching discharges typically 
develop when the terminals are positively charged. The discharges are 
significantly longer than simple corona discharges, and they can make a 
very characteristic dull "popping" sound. The popping noise can 
sometimes be heard (and seen) around HV AC transmission lines as well. 
This is about as close as you will get to mimicking TC discharges from a 
DC or low frequency source. An example of 10" long burst corona 
discharges can be seen on California coiler Steve Cole's large VDG 
generator (8 second exposure). The straight stick-like section of the 
discharge is an emerging leader, while the blue haze is actually 
countless microscopic streamers discharges feeding current into (and 
heating) the leader:
http://capturedlightning/photos/HVStuff/VDG-Corona.jpg

Tesla coils have a complex, repetitive waveform that is ideal for 
creating arrested streamers. The rapidly rising secondary voltage 
envelope during ring-up, combined with a typically large terminal size, 
curvature, and capacitance, provide high initial breakdown voltage, 
driving voltage for streamer growth, and a reservoir of electrical 
charge necessary to favor stepped growth of leader-streamers. The RF 
components of the waveform further contribute to keeping the spark 
channel hot (via ohmic heating from RF displacement currents into/out of 
leader capacitance). Finally, the rapid repetitive application of these 
HV waveforms permits further growth from one bang to the next. Since 
most TC's are designed to maximize the distance from the topload to 
other objects, the majority of streamers effectively become 
"self-arrested" - the TC simply runs out of voltage and/or energy before 
  any sparks can fully bridge the gap.

It may be possible to design a coil that is so large that the 
combination of low operating frequency and relatively long ring-up times 
begin to adversely impact the "efficiency" of the spark growth process. 
Although such a system would still be capable of generating streamers, 
the maximum streamer length (versus input power) may begin to adversely 
diverge from John Freau's empirical spark length vs power formula. This 
may become a limiting factor in the design of very large Tesla Coils 
intended for long spark research.

Bert
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