Re: vacuum spark gap

If you have access, you might want to check out Farrall, George F., "Vacuum
Arcs and Switching" in the August 1973 issue of IEEE Proceedings (vol 61,
No 8), pp 1113-1136

High Vacuum doesn't have a breakdown strength in kV/cm, per se, because the
spark formation process is dominated by other things. Some researchers have
reported breakdown fields on the order of 3-5 MV/meter (30-50 kV/cm,
comparable to air) in gases with number densities of 1E16-1E19/cubic meter
(STP is 260E23/m^3, so that's in your pressure range).  It appears from a
cursory reading of the above reference that the dominant factor is the
smoothness and condition of the electrodes.  Small roughnesses (say from a
previous arc melting a tiny spot) or insulating inclusions (gas bubbles,
etc.) cause field concentrations, and if the radius is sufficiently small,
the field strength is high enough for field emission, which gets enough
electrons out there to get the gap to break down. If a spot on the
electrode gets hot enough, it will spit out electrons thermionically (like
in a vacuum tube), greatly reducing the breakdown voltage, but your vacuum
is nowhere near what is in a typical tube.

For long gaps, the breakdown is usually on the surface of the container.

The quench is very rapid (on the order of microseconds)

Xrays are a hazard any time you have HV and vacuum. Fortunately, they are
quite soft, and easily shielded (get some thin lead sheet and you'll
massively over shield).  BTW, a standard geiger counter isn't very
sensitive to low energy xrays, so just because your geiger isn't clicking
doesn't mean that you aren't being dosed.  

> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: vacuum spark gap
> Date: Sunday, April 30, 2000 2:45 PM
> Original Poster: "Alfred C. Erpel" <aerpel-at-pil-dot-net> 
> What I mean is parallel copper (or tungsten) bars in a vacuum (10^-4
> to be used in the primary tank circuit. From what I have been able to
> from searches, some people say this won't work and others say it will. I
> believe (not know) that it will work. What I don't know is what is the
> breakdown voltage of a vacuum (volts/mil). Also what danger might there
> about radiation being generated?
> > Original Poster: "Alfred C. Erpel" <aerpel-at-pil-dot-net>
> > Original Poster: "Lau, Gary" <Gary.Lau-at-compaq-dot-com>
> >
> > I don't believe that a "vacuum spark gap" actually involves a vacuum.
> > Rather it employs a vacuum cleaner motor to suck or blow air across
> > gaps.  The spark gap must assume a low resistance state in the
> > condition, and this requires plasma - super hot ionized air.  I don't
> think
> > a spark gap would work in a vacuum, and it would work poorly in low
> pressure
> > air.
> >
> > Gary Lau
> > Waltham, MA USA
> >> Hello,
> >>
> >>     I am going to build a vacuum spark gap. I have a vacuum
> >> pump capable of
> >> 10^-4 torr. I have all the machine shop equipment, vacuum
> >> grade grease etc. to
> >> do this properly. It will need to switch less than 500
> >> watts.
> >>     What non-mechanical issues might there be? X-ray
> >> concerns or any other type
> >> of radiation? What percentage of energy might be expected to
> >> be absorbed by the
> >> gap and/or radiated? What is the dielectric strength of a
> >> 10^-4 torr vacuum?
> >>
> >> Regards,
> >> Alfred Erpel