Re: A photographic tutorial of Pancake Coil winding...with movies...(fwd)

["Tesla list" <tesla@xxxxxxxxxx>]

---------- Forwarded message ----------
Date: Sun, 24 Jun 2007 02:07:05 -0500
From: Bert Hickman <bert.hickman@xxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: A photographic tutorial of Pancake Coil winding...with
    movies...(fwd)

Tesla list wrote:
> ---------- Forwarded message ----------
> Date: Sat, 23 Jun 2007 19:29:02 -0500
> From: David Thomson <dwt@xxxxxxxxxxxx>
> To: 'Tesla list' <tesla@xxxxxxxxxx>
> Subject: RE: A photographic tutorial of Pancake Coil winding...with
>     movies...(fwd)
> 
> Hi Bert,
> 
>> For a variety of reasons, your secondary would 
>> make a very poor waveguide at GHz frequencies. 
> 
> What made you think my flat spiral coil was acting like a waveguide at
> microwave frequencies?

You previously stated:
"Bart just pointed out that the metal pipe you are using acts like a 
waveguide, or third coil."

My point was that waveguide behavior will only meaningful at frequencies 
in the GHz range, not at 428 kHz or 1300 kHz. I fail to see how the 
operation of your 3rd coil is anything like an air cavity Helmholtz 
resonator. The only similarity is that both systems are exhibiting forms 
of resonance, one acoustic, the other electromagnetic.

> 
>> Given the 
>> above, I don;t believe that Helmholtz resonators (audio 
>> frequency sound resonating chambers) apply to the physics of 
>> your system (or for magnifiers in general).
> 
> Your logic is at odds with my experimental results.  It is clear from the
> stationary coronal discharge on the topload that longitudinal standing waves
> with no current was achieved (i.e. no wasteful streamers were produced).

Not exactly - my conclusions are significantly different than yours, and 
they are consistent with observed behavior, with known Tesla Coil 
theory/practice, and with the known physics of air breakdown.

Let's get a bit more specific - I assume you are referring to the 
experimental setup shown on your web page:
http://www.tesla-coil-builder.com/FlatSpiralSolenoidCombo.htm

First, a brief digression about the physics and appearance of 
progressive stages of air breakdown to bring everyone to the same 
starting point. Breakdown (within a nonuniform field - such as around 
your topload) occurs in several progressive stages as you increase 
maximum topload voltage. Breakdown initially begins as relatively short 
corona and glow discharges, and (because of the RF output voltage) both 
positive and negative corona is observed.

Corona initially appears as short bluish-violet tufts at points where 
the electric field gradient is sufficiently high. Corona initially 
begins when the E-field at the terminal surface exceeds about ~26 kV/cm 
(for negative terminal polarity) or ~30 kV/cm (for positive terminal 
polarity). Under RF excitation, once corona begins, it may not be 
extinguished until the voltage is reduced significantly (due to the 
effects of injected space charge into the region surrounding the the 
terminal).

With increasing terminal voltage, these discharges evolve into longer, 
disruptive plume-like discharges called "burst corona" or "streamer 
corona". Each burst corona discharge has a single root leading to a 
diverging, conical discharge region that gets progressively dimmer as it 
spreads out. You are actually seeing a myriad of visible avalanches as 
the air becomes partially conductive. On systems with larger terminal 
capacitance, burst corona makes a distinctive "popping" sound with no 
corresponding streamer. Burst corona plumes are typically 2 - 6" long, 
but they can be longer on higher voltage systems at the threshold of 
breakout.

Most experienced coilers have heard and seen burst corona (sometimes 
referred to as "gas jet" discharges), but they may not have recognized 
them as such. Burst corona most typically appears in inefficient or 
underpowered systems, especially if they also have small toploads. 
Multitudes of small, conical discharges will appear all around the top 
terminal, looking very similar to the blue flames of a gas stove burner.

Since these are low energy discharges, they aren't hot enough to create 
strong thermal currents in the air, so each discharge root tends to stay 
pretty much rooted in place. Burst corona discharges are often seen in 
systems operating at low power levels, particularly at higher (> 1 MHz) 
frequencies. [In fact, you may be seeing a bit of "shock excitation" of 
your third coil, which would induce low energy ringing at its natural 
frequency (~1.3 MHz)]. Burst corona also appears on systems with poor 
tuning, systems with very lossy primary circuits (lossy tank caps, 
poorly quenching gaps, etc), or on systems with too small a topload. 
Coils with small toploads lack sufficient capacitance to provide the 
high current pulses needed to support initial streamer formation and 
leader growth. Your system has many of these attributes...

In order to develop longer streamers and leaders, it is necessary to 
develop significantly higher terminal voltages, combined with 
significant topload capacitance. Although well designed, well tuned 
systems easily progress to develop streamers and leaders, poorly tuned 
or underpowered coils may not. In analyzing your system, I conclude that 
you are getting inefficient energy coupling to the tertiary coil. This 
results in low output voltage, with a small topload C, perhaps combined 
with a dash of low energy 1.3 MHz shock excited ringing - the system 
cannot progress beyond the burst corona stage.

The above discussion reflects the hard-won knowledge and measurements of 
countless coilers, independent researchers in gas discharge physics, and 
my own direct observations of many systems. I know that you believe you 
are seeing something else, but at least I've presented the "logic" I 
employed to arrive at my conclusions.

How did you conclude that a lack of streamers means a lack of current? 
Was this confirmed by any measurement? You must know that the fact that 
you are seeing visible discharges means that current IS being exchanged 
between your topload and the surrounding air. This is fundamental to, 
and easily demonstrated for, any kind of gas breakdown. If your model 
says otherwise, it simply doesn't conform to the realities of electrical 
discharges in gases.

> 
> Dave
> 
> 
> 
> 

Bert
-- 
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