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RE: vacuum spark gap (not exhaust fan type)

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 gap 
voltage breakdown and switching (On & Off -quench ions may linger)
characteristic.
some gases used: hydrogen, sulfur hexafloride, freon and derivatives, etc.

Reduced breakdown voltage (& repeatability) can be enhanced by including some
radioactive element in the electrode such as 2% thorium in welder tungsten
rods.

Check the text "High Voltage Engineering" (don't recall author-I'll ck & RSVP)
I recall a good chapter on arcs in a vacuum, Pashcen sp? law/curves vs. vacuum.

I use Jennings & Kilovac vacuum relay spark gaps for all my single shot DCTC's
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, effective
    in reducing glass deposition while also reducing x-ray & visible radiation.
  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 electrodes !

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 vacuum.
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

-----Original Message-----
From: Tesla List [mailto:tesla-at-pupman-dot-com]
Sent: Sunday, April 30, 2000 6:45 AM
To: tesla-at-pupman-dot-com
Subject: Re: vacuum spark gap


Original Poster: "Alfred C. Erpel" <aerpel-at-pil-dot-net> 

What I mean is parallel copper (or tungsten) bars in a vacuum (10^-4 torr)
to be used in the primary tank circuit. From what I have been able to read
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 be
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 static
> gaps.  The spark gap must assume a low resistance state in the triggered
> 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