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Re: [TCML] Spark gap



Hi Vinnie,

Excellent question. In days past, coilers would build a series of pipes spaced apart as based on advise from others or their own experience. Then use trial and error taking the gap and transformer to the bench. Start by using 2 of the pipes, then 3, then 4, etc.. until the transformer would finally not fire across the gap. Then go back 1 or 2 pipes and check for consistent firing. This sets the gap for the transformer. This is still a recommended method to ensure you don't open the gap up too wide (for NST and Cap health). BTW, good to let the gap cool down between tests.

However on the front end of design, it is nice to have a good ball park for building (via advise or calculations). I use Javatc's Static Gap procedure. Input asks for the number of pipes, pipe diameter, and the total gap width. Javatc runs a somewhat extensive number of equations. It will determine your cap voltage, field strength, breakdown voltage, distance each electrode should be spaced, etc.. All these equations could be performed on paper, spreadsheet, or whatever as well. The breakdown voltage should be set just below the charging voltage (which is the peak voltage at the cap). Change the total gap input until arc voltage is just below the charging voltage. This gives a good ballpark to build from with some design data at the front end. But also you need to look at bps and percent cap is charged. These are also important and based on cap size and transformer power.

It is still important to take the static gap and transformer to the bench and test it once built.

An example: My RQ static gap is 6 electrodes of 1.25" diameter, total gap spacing is 0.25".
Here's an output of Javatc for this particular case:
----------------------------------------------------
Static Spark Gap Outputs:
----------------------------------------------------
0.05 [inch] = Gap Spacing Between Each Electrode
19799 [peak volts] = Charging Voltage
19414 [peak volts] = Arc Voltage
34822 [volts] = Voltage Gradient at Electrode
77654 [volts/inch] = Arc Voltage per unit
98.1 [%] = Percent Cp Charged When Gap Fires
9.265 [ms] = Time To Arc Voltage
108 [BPS] = Breaks Per Second
3.54 [joules] = Effective Cap Energy
382 [power] = Energy Across Gap
47.8 [inch] = Static Gap Spark Length
----------------------------------------------

For reference:
**
**The arc voltage is the voltage potential at which the air gap between electrodes becomes conductive.

The Arc Voltage is equated by first identifying the field strength at breakdown, then incorporating equations where the electrode shape and size are considered (William North). The field strength is taken from the website of Jim Lux and supported by various papers on the subject:

Field Strength = p * ( B / ( C + ln ( p * d)))

where
p = pressure in Torr (mm Hg). For air, this value is 760
B = 365 Vcm-1 Torr-1
C = ln( A / ln ( 1 + 1 / gamma))
d = gap width

where
gamma = 0.095 (secondary ionization coefficient)
A = 14.6 cm-1 Torr -1

Electrode geometry equations used for the arc voltage are from the L.A.N.L. report "High-Power Microwave-Tube Transmitters, chapter 7, by William North".

As I said, there's a lot that Javatc does in this area and I can't list it all here. I use Javatc to get decent data before I build. It's just an option for coilers.

Take care,
Bart


Vinnie wrote:
Hello People

While everyone is on this subject.
How does one determine the number of gaps and spacing between each segment in a cylindrical R.Q.S.G? It seems like from what I read that if you don't have enough gaps that's bad and if you have too many that's also bad. Is there anyway to determine what is actually needed? Is it determined by your input voltage or you cap or a combination
of both?

Thanks for your time.

Vinnie

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