Re: [TCML] gaps

[Bert Hickman <bert.hickman@xxxxxxxxxx>]

Jim Mora wrote:
> Hi Bert,
>
> Please define big and power. My 15/120 used two 1" brass dowels 2" long each
> spaced about .5" if I recall (end to end). They were threaded and therefore
> easily adjustable. They would run 5-10 minutes and not get hot and cleaning
> off the Zink oxide we have talked about was occasionally needed for best
> performance. The faces were polished. And the air blast was very focused on
> the gap and velocity enhanced by the fire hose like tapered nozzle which had
> 2.5" input to .5" out tapered over 12" and set very close to the gap. 

<snip>

Hi Jim,

It depends... and obviously YMMV.  :^)

Getting good performance from a single static gap becomes increasingly 
difficult as you begin running at power levels above a kilowatt or two. 
It's certainly not impossible to scale up to higher power levels, but it 
may take significant design and cooling efforts. Let's look at some 
numbers...

It can be shown that a well-designed spark gap coil that quenches on the 
2nd notch will dissipate over 40% of the initial bang energy in the gap. 
And, if quenching is delayed until the 4th notch, this jumps to over 
70%! If you happen to drive your 15/120 system using variac-boost (140 
volts), and also run with a mains resonance or LTR tank cap, you could 
easily be pulling 1500-3500 watts of wall-plug power. This translates 
into pumping a minimum of 600 watts to almost 2500 watts of heat into 
your spark gap.

For a system that can transfer 85% of primary energy to the secondary 
(typical for an efficient spark gap coil), the gap losses (as a % of 
bang energy) climb as follows as a function of quenching notch. You can 
also see that, if you aren't getting topload breakout, you can burn up 
your gap:

Notch      Loss (% of Bang)
===== 	   ==============
1st		15.0%
2nd		47.8%
3rd		62.3%
4th		72.8%
5th		80.3%
6th     	85.8%

It's possible (as in Bart's example) to initially obtain good quenching 
with a cold gap. However, if there is insufficient cooling to "balance" 
heat flowing in and out of the gap, it will overheat. As the overheated 
gap begins quenching at a later notches, this further increases heating 
and the gap can undergo thermal runaway and complete quenching failure. 
Using massive electrodes is essential, but this must also be combined 
with cooling that is sufficient to handle worst case (no breakout) 
dissipation without inducing thermal runaway.

At higher power, this typically means using higher static gas pressure 
or higher air velocity air flow. At higher levels, it can also mean 
converting electrode material from brass or copper to machinable 
tungsten-copper, using water cooling, and even using gases with higher 
cooling effectiveness (hydrogen), or gas having higher dielectric 
strength (SF6). High power static gaps have been successfully used in 
repetitive, oscillatory discharge circuits and pulsed power applications 
at 10's of KW. Because of the significant design complexities and 
minimal performance advantages (for Tesla Coils at least), multiple gaps 
and rotary gaps provide more practical alternatives with greater 
flexibility.

Do you happen to know which notch your single-gap system is quenching 
at, and does it change during extended operation?

> I have often wondered how scaling this up would work. I have a 240v air
> curtain dual blower...I wonder what would happen at 5KVA with BIG dowels.
>
> Jim Mora
>
>

Well, there's one way to find out... :^)

Bert
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