Re: vacuum spark gap (not exhaust fan type)
> Original Poster: "Dale Hall" <<mailto:Dale.Hall-at-trw-dot-com>Dale.Hall-at-trw-dot-com>
> Alfred & All,
> Breakdown voltage in a vacuum is a function of temperature, material,
> electrode physical orientation & texture, separation distance,
> radius of curvature and the degree of vacuum.
> (I believe I recall a Vbkdwn curve turns around with greater vacuum !)
> A gas is NOT necessary for breakdown in a vacuum.
> The above mentioned parameters provide the mechanism and
> I believe the work function of the electrode.
> Including a gas will reduce or eliminate radiation, but also affect the
> voltage breakdown and switching (On & Off -quench ions may linger)
> some gases used: hydrogen, sulfur hexafloride, freon and derivatives, etc.
> Reduced breakdown voltage (& repeatability) can be enhanced by including
> radioactive element in the electrode such as 2% thorium in welder tungsten
> Check the text "High Voltage Engineering" (don't recall author-I'll ck &
> I recall a good chapter on arcs in a vacuum, Pashcen sp? law/curves vs.
> I use Jennings & Kilovac vacuum relay spark gaps for all my single shot
> and achieve very good results (contacts come close but don't contact - jog
> to arc).
> My efficiency is very high, even with voltages as low as 7kVdc.
> I do take advantage of moveable vs. stationary electrodes, however.
> Radiation will become more intense and of concern above ~25kVpk,
> so shielding is be a good idea.
> Two problems to address:
> Assuming tungsten electrodes (recommended)
> tungsten will evaporate onto the glass deposing a detrimental conductive
> path w/time
> so you need as long a glass path as practical and/or even better:
> a metal cup extending from one electrode and around the other,
> in reducing glass deposition while also reducing x-ray & visible
> Deposition is hastened by higher BPS and higher peak current.
> Heat removal becomes a challenge at higher BPS,
> but heat sinking and use of large diameter electrodes
> perhaps coaxial copper/tungsten can be constructed
> to remove the heat effectively.
> One of my Jennings vacuum contactors has ~1" diameter tungsten
> I've though of having someone make just this sort of gap for me:
> A TCBOR / RQ multi cylinder heat sunk gap will probably work great in a
> Fixed electrode (outside ROC) spacing should be made as close as feasible
> each brought out so the number stages may be externally selected.
> The conductor exiting the glass envelope should be of less diameter than
> that of the larger active gap in the evacuated glass to avoid external
> arc over difficulty (no weird esp. sharp bends/edges in the electrodes)
> while being as large a diameter possible to conduct heat.
> If electrodes could have hole channels to support coolant transport
> that would be a plus but likely too difficult w/tungsten !
> unless the tungsten is pressed into a copper rod .....
> What do ya think, Alfred ?
> Regards, Dale
Very interesting response. And it supports what some people say, and
contradicts what others say, just like the returns on my internet search
(the biggest point of contention is that some people say that a spark won't
jump in a vacuum and other people, like you, say it will) . My opinion is
that a sealed vacuum spark gap can be very good in tesla use. When I build
it and have something to report (brag about), I will post the results here.
Regards, Alfred Erpel