Critical TC Size

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

Original poster: "Greg Leyh by way of Terry Fritz <twftesla-at-qwest-dot-net>" <lod-at-pacbell-dot-net>

[Note -- Re-Re-post...  The former attempts seems to have vanished?]

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Original poster: <Michael.Day-at-USPTO.GOV>

Greetings all,

The Lightning-On-Demand web page at www.lod-dot-org states the following:

"The physics of air under electrical breakdown limits the maximum practical
size of a Tesla Coil to about 12 stories in height. Greg Leyh started
Lightning On Demand (LOD) in 1996 as a focus for designing and constructing
a Tesla coil system that would operate at this limit. In addition to posing
an interesting engineering challenge, this facility will provide a unique
scientific research platform for studying natural lightning phenomena -- and
operate as an world-class educational attraction for the general public."

Does anyone know why the physics of air under electrical breakdown limits
the maximum practical size of a Tesla Coil to about 12 stories in height?

-mike day
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Here, I am considering the special case of a Tesla Coil...
one designed to produce maximum arc length.
This is quite a different optimization than a system such as
Tesla's Wardenclyffe, where an arc breakout of any kind was
to be avoided.

As one begins to increase the scale of an arc-producing coil,
the power consumption increases roughly as the square of the
scale, as John Freau has documented.  This is not necessarily
a disadvantage however, in that a coil that maximizes arc length
is effectively an 'air incinerator', and more throughput power
is desirable.

A low duty-cycle pulsed mode of operation is more efficient at
maintaining the ion channel than a CW mode, as the ions have a
persistence of several mS that can be exploited between pulses.
If the duty-cycle is increased, then the peak power starts to
approach the CW average power, and this pulsed advantage is
eventually lost. Note that CW coils suffer far lower arc length
to input power ratios than pulsed coils, even though their arcs
are decidedly 'bushier'.

If a formed ion channel has a finite lifetime, then it follows
that there will be a *minimum* firing rate that can properly
exploit this ion persistence.

Now as the coil continues to grow in size, far past the human
scale, it's natural resonant frequency continues to decrease.
Consequently, the fill time required to transfer energy to the
secondary increases.

At some point this fill time must adversely impact either the
maximum possible break rate, or the operating duty-cycle. In
either case the coil's performance will begin to suffer, and at
larger coil scales a rather unfortunate knee feature appears
in the 'arc length vs. grid power' curve.

To compensate, one can employ a variety of tricks to prop up the
resonant freq, such as space-winding the secondary, but then at
some point the secondary impedance will begin to drop too far,
interfering with the coupling of the power to the arc.

Eventually at some critical scale one runs out of design options
for maintaining the coil's arc production efficiency as the resonant
period of the coil collides with the fixed ion lifetimes in the arc
channel... which moves the operating mode from pulsed to CW. Increasing
the coils' scale beyond this critical point results in considerably
larger increases in the prime power draw to obtain longer sparks, and
very quickly one runs out of options for connecting such a beast to
any practical point on the US power grid. (Now France, perhaps...)

Ultimately the coil's maximum size will be defined by the finite
persistence of the ions in the arc channel, which I shall assume not
to be adjustable.  My first-order attempts at designing a coil to
this critical scale result in a secondary somewhere around 110ft in
height, and 22ft in diameter.  This is the present design target for
the Advanced Lightning Facility.  These dimensions are likely to
change to some degree, as more detailed information on arc creation
and maintenance at these scales is collected, and/or if I can coerce
the TSSP at some point to simulate such an impractical example.  ^_^


-GL
www.lod-dot-org