Re: TC Sparks and Arcs (Was - High Voltage Snubber caps

["Jim Lux" <jimlux-at-jpl.nasa.gov> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

> It is my understanding that the best man made vacuum on earth still has
> millions of atoms in the chamber compared to outer space. This may make
> sparks possible on earth depending on the amount of man made vacuum.
> However, in space with the two isolated and energized terminals a
distance
> apart a spark would be impossible at any voltage because of the better
> vacuum. Is this correct?

Well.. perhaps not the best vacuums on earth... Space is sort of middling
high vacuum... in the same sort of area as in a vacuum tube, depending on
orbit altitude. And, near any sort of object it isn't all that great
because of outgassing, etc. (just like on earth in a vacuum chamber). 
Interestingly, if you want higher vacuums in low earth orbit, you do them
behind the spacecraft (letting the s/c intercept all the atoms and
molecules).

However, yes, in a high vacuum you won't get a spark over.  There will be
field emission (cold cathode emission) in a high vacuum.  And, a practical
HV source will have some physical connection between the two electrodes
along which you could get a surface arc.

> 
> I agree that arcs would be possible in space and this is one of the
> important differences between sparks and arcs. I think of sparks as a
high
> voltage phenomena and arcs as a low voltage phonomena. In other words the
> initiation of the spark requires voltage while the arc requires current.

More of a semantic difference between sparks and arcs... but yes, once you
get a spark channel between the electrodes, the characteristics of the arc
are more determined by the electrode material and the current flowing.  The
voltage drop is also quite low, compared to the breakdown voltage.

> 
> What do you mean by "it would be formed by being "drawn". Are you
suggesting
> a spark has a force? I am not familiar with that concept.

No.. drawing an arc refers to a mechanical process.  Just like in arc
welding, where the voltage is below the minimum sparkover voltage, you have
to touch the electrodes and pull them apart.  At some point in the
separation process, the current flows through tiny bridges between the
electrodes, melting and vaporizing the electrode metal, providing some
source of ions for the arc.  

You can sparkover in a vacuum... That's how the vacuum spark gaps for
overvoltage protection work.  I assume it is some sort of cold cathode
emission to get the electrons into the gap, and then it avalanches in the
usual way to build up the arc.  I don't know for sure though...