Re: Size DOES matter?

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Malcolm Watts" <m.j.watts@xxxxxxxxxxxx>

Hi Steve,

On 2 Jun 2005, at 11:52, Tesla list wrote:

> Original poster: Steve Conner <steve@xxxxxxxxxxxx>
>
> Hi all,
>
> I was pondering the relative lack of spark length from my DRSSTC.
> There seemed to be no shortage of energy going into the thing. No
> parts were heating excessively, so I assumed most of it was going into
> the discharge. But the sparks were not so impressive. I was getting
> maybe 33" at any breakrate above 100hz, and John Freau's formula
> predicts 40" for 5.5 joule bangs at 100Hz.
>
> What's more, they didn't grow that much with breakrate. I well
> remember Richie Burnett writing about a classic coil that made maybe
> 14" sparks in single shot mode but grew dramatically to 66" at 200Hz
> breakrate. (Richie was not born yesterday so I assume he checked that
> the tank capacitor charging voltage was the same in both cases.)

That is exactly how every disruptive coil behaves. I have measured
primary voltages in both repetitive and single shot operation and the
phenomenon is real. If any doubt remains as to the way streamers grow
in repetitive operation a slowed-down video of the action will expose
what is happening clearly. One of my coils mirrors the one Richie is
speaking of very closely. I can guess from the figures given that his
primary energy is around the 5J mark. Running a coil singleshot for
the first time was an eye-opening experience for me and exposed a lot
of popular mantras spouted at the time about coil operation for what
they were ;)

> Mine did 22" with a single bang and still only about 33" at 200Hz with
> 1kW power input. (by way of comparison my classic coil did 40" with
> 1kW input at 400Hz, and 17" with single bangs of very roughly the same
> energy.)
>
> So I got to thinking, maybe size does matter? Maybe the ultimate limit
> to spark growth is not how much power/energy your oscillator can
> deliver, but the size of the resonator? It's often been said that
> three times your secondary length is the maximum spark length you
> should attempt. Well maybe there is a law of physics that says you
> just can't get more than that no matter how much power you supply!

Size matters when it comes to both the output voltage the coil can
produce without flashing over before letting fly and efficiency in
general. There _is_ a weak dependence on output voltage. There is a
stronger dependence on output charge availability.

> So how could that be? Well for a start it's not too hard to see how
> toroid size could influence spark length. Streamers need a certain
> field strength to grow. And, the maximum field strength at the surface
> of any toroid is limited to the breakdown strength of air. So it seems
> reasonable that a smaller toroid might not "throw" the field (and
> hence possibly the streamers) out to such a long distance: simply
> because it has less surface area to accommodate field lines.
>
> The resonator length may also have an effect. If we admit the argument
> above, then the toroid should be brought almost to its breakdown
> voltage for most efficient streamer throwing. If you try to drive a
> large toroid with a small resonator then the resonator will flash over
> end-to-end before the toroid reaches breakout. Hence (even with a
> breakout point) the spark length will be limited by flashover rather
> than power availability.
>
> To prevent flashover of an undersized resonator you need to reduce the
> output voltage by fitting a breakout point and/or turning the peak
> power down (this is achieved by loosening the coupling on a classic
> coil, or turning down the DC link voltage- and increasing burst length
> to restore bang energy- on a DRSSTC) The reduced output voltage will
> probably lead to smaller sparks.
>
>
> Does anyone have any comments as to whether any of this makes sense ;)

I agree with all your conclusions based on what I've observed. It is no
accident that I've found operation to be most efficient and rewarding for
largish coils. I'd resort to such coils every time when trying to break
output vs power records. And operating at the edge of breakout is the way
to go in my experience. At any given power level and bang size there
appears to be a balance to be struck between bare Cself and Ctop.

Malcolm