Re: Limit on coil size

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Greg Leyh <lod@xxxxxxxxxxx>

Hi Dave,

Alberta Chu has copies of Electrum available at http://www.asklabs.com/electrum

The physical size limit I mentioned in the talk applies specifically 
to coils designed for generating long arcs.  If your goal is to 
simply generate higher voltages without breakout, the coil should 
just continue to scale.

This practical limit revolves around two key features of impulse-driven coils:

A)  Impulse coils take advantage of ion lifetimes to maintain the arc 
channel integrity, until they can recharge and restrike the arc.

B)  Impulse coils multiply the input power dramatically, often by 
factors of 100's to 1000's, greatly increasing their ability to 
strike long arc channels over CW machines.  The impulse coil is 
analogous to a hammer, compared to a press.

When increasing the size of a coil, the problem arises in trying to 
maintain both conditions A and B, listed above.  If we set a minimum 
acceptable value for the power gain, then our duty cycle has a 
maximum limit.  And if the coupling is already fixed at an optimal 
value, then the envelope represents a fixed number of RF cycles.

Here is where ion lifetimes come into play.  Effective ion lifetimes 
for coil arc channels seem to range from about 3 to 12 msec.  Many 
coilers observe that break rates around 80-150Hz (7 to 12msec) seem 
to yield best arc-length efficiency.  Now as the resonant freq (Fo) 
of the coil decreases, the envelope length gets longer in time, 
therefore the break period must also increase to maintain a low duty 
cycle.  At some Fo the break period must increase to the point 
(>12msec) where the dead-time between firings allows the ion channel 
to begin dissipating.

The result is that at some critical size, either feature A or B has 
to be compromised, which would diminish the arc production efficiency 
of the machine.  Beyond this point the coil will operate in a 
quasi-CW, then finally a CW mode of operation.

-GL


> Original poster: Dave Leddon <dave@xxxxxxxxxx>
>
> I attended a most inspiring talk given by Greg Leyh last night in 
> San Francisco in which he showed a documentary video detailing the 
> construction of his Electrum coil (I sure would like to have a copy 
> of that, Greg) followed by a discussion of his plans to construct a 
> lightning laboratory in Nevada.  He mentioned that there is a 
> physical limit to the size of a Tesla coil beyond which it cannot 
> produce an arc due to the frequency being too low to sustain a 
> continuous stream of ions.  I believe he said that an ion would 
> extinguish before the next one could be generated.  Does anyone have 
> an idea how to determine this minimum frequency?
>
> Dave
> Pleasanton, Ca