more on breakdown voltages

To: tesla-at-grendel.objinc-dot-com
Subject: more on breakdown voltages
From: EDHARRIS-at-MPS.OHIO-STATE.EDU
Date: Thu, 16 Mar 1995 23:01:40 -0500 (EST)
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	Just the results of Ed's playtime:

	I bought some Kroger Ziplock bags of 1.75mil thickness - presumably 
they're polyethylene. I made an aluminum foil plate capacitor out of one 
side of the bag and hooked it up to my 8kV DC supply.

	It lasted 40 seconds before breakdown!!! That's about 4.5kV per mil 
- way over the "bulk" value for the breakdown of just about anything.

	I found a reference which may interest some of you. It's called 
"High Power Switching" by Ihor Vitkovitsky. Here's it's table of contents:

Gas spark gap switches
Vacuum and low pressure discharges
Solid Dielectric breakdown switching
Liquid dielectric switches
etc...

He deals with some switches which have been deleloped to deal with peak 
power levels of 10^12 Watt. Perhaps there is some relevance to T-coils?

Here's a couple of interesting empirical formulas from it:

Breakdown field for air:
(shows simple 30kV/cm is not always applicable)

E=24.6 + 6.7*SQRT(P/d)

E  = dc breakdown field (kV/cm)
P  = pressure in atmospheres
d  = electrode spacing in cm

Breakdown field for solids:
(shows for a given area, breakdown goes up with decreasing thickness)

E=K*V^(-1/10) 

E =  dc breakdown field (MV/cm)
V =  Volume of dielectric in cm^3
K =  rough constant depending on material

K= 2.5 for polyethylene and teflon
 = 3.3 for Lucite (PMMA)
 = 3.6 for Mylar

They also give a empirical formula for the relative lifetime of the solid 
dielectric used in "pulsed" applications based on it's breakdown field 
and "working" field values:

Life ~ (Ebreakdown/Eworking)^8