[AMPS] Arcing in air vs. arcing in a vacuum

[Michael Tope]

Jon,

I think the phenomenon you are discussing is know as "multipaction". I am
not familiar with the details of the physics, but suffice it to say that your friend
is correct that ionization is more prevalent at low barometric pressures. I have
seen this demonstrated on a airborne HF antenna mast in an altitude chamber.

I am not certain, but I believe the trend reverses itself under conditions of high vacuum.
In this case, I suspect that the absence of a free ion supply vis-a-vis the absence 
of low pressure gas forces the conducting surfaces of the electrodes to supply 
the ions needed for conduction, which is probably akin to making a vacuum tube
conduct without heating the cathode. 

I think this explains why one of the engineers I work with at JPL was so happy when 
W.L. Gore finally built a vented SMA connector for him. Seems that small amounts
of trapped gas in a space-borne connector can cause multipaction at very low power
levels. If the connector is vented properly, space high vacuum will establish itself 
soon after launch and such problems are avoided. 

Anyway, that's all I know - enough to be dangerous.

Mike, W4EF......................

Original Message-----
From:	Jon Ogden [SMTP:jono@enteract.com]
Sent:	Monday, May 17, 1999 7:35 AM
To:	amps@contesting.com
Subject:	[AMPS] Arcing in air vs. arcing in a vacuum


Hi all,

I got a question.  I had a discussion with a ham friend of mine on our 
way to Dayton this weekend.  We were talking about the *gas* theories in 
tubes and tube arcing.  His comment was that from his experience, he has 
determined that the voltage breakdown potential of a vacuum is LESS than 
that of the voltage breakdown potential in air.  He said that while doing 
some EMC testing of a product he has designed for his company, he found 
this to be the case.  He had to simulate lightning strikes at high 
altitude and thinner air.  The breakdown potential was much less at 
higher altitude (thinner air) than at the thicker air of sea level.  The 
arcs happened at less voltage at high altitude.  So he has made the 
conclusion that if the breakdown potential of thin air is less than 
regular air, the breakdown potential of a vacuum is less still.  I don't 
know if this jump can be made.

Can someone help explain this?  All my instincts tell me that a vacuum 
has the highest breakdown withstanding potential.  I seem to remember 
hearing once though that thin air ionizes easier than "thick" air so 
perhaps that explains it.

Thanks a lot.

73,

Jon
KE9NA


--------------------------------------------------------------------------
The Second Amendment is NOT about duck hunting!


Jon Ogden

jono@enteract.com
www.qsl.net/ke9na

"A life lived in fear is a life half lived."


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