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Re: 20 joules at 100 bps vs 4 joules at 500 bps

Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx>

At 08:26 PM 8/2/2005, you wrote: 
Original poster: "Antonio Carlos M. de Queiroz" <acmdq@xxxxxxxxxx>

Tesla list wrote:

Original poster: "Dmitry (father dest)" <dest@xxxxxxxxxxx> 

for a nonuniform field you don`t need so high voltages - it would be enough
much smaller.
http://www.kronjaeger.com/hv/hv/msr/spk/index.html


The 30 kv/cm is for electric field. If the geometry is not two parallel
plates, breakdown occurs when the electric field somewhere hits this
value. I have the exact formula for two ideal spheres implemented in
the Inca program. A good (approximate) simple formula for two identical
spheres can be found in the "North report" (look in the archives).
Needles cause some weird effect, probably related to the same phenomenon
that causes small gaps to not obbey the same rule. That thing of sparks
never occurring with less than 300 V. (Why? Is this really true?)


Really is true... there IS a minimum sparking voltage. As you keep dropping the pressure, eventually you get a glow discharge, but never a spark, or possibly current flow from field emission. We depend on this all the time for high power RF stuff operating in low pressures (like Martian atmosphere... CO2 and Argon at about 5-10 torr...)


Now.. as to why this is so....

For a given gap, as you reduce the pressure, the breakdown voltage keeps decreasing. However, at some point, the mean free path of that ionized atom that's going to start the breakdown gets to be more than the distance between the electrodes, so it just contributes to current flow (as in a GM tube or ionization chamber for particle detection). To get a spark, you need to have that avalanche.