Re: pure water capacitor?

To: tesla-at-pupman-dot-com
Subject: Re: pure water capacitor?
From: "Bert Hickman" <bert.hickman-at-aquila-dot-com> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)
Date: Wed, 16 Feb 2000 20:17:16 -0700
Approved: twftesla-at-uswest-dot-net
Delivered-To: fixup-tesla-at-pupman-dot-com-at-fixme

Mark,

As you've noted, pure water is a reasonably good insulator with a
comparatively high dielectric constant. In fact, deionized water has
been tried in experimental high voltage capacitors for high power pulsed
power systems with limited success. And, it's often used to create low
impedance, high-power, transmission lines or low-impedance pulse forming
networks at many research facilities. However, water's insulating
properties under high-voltage stress only exist for a short time
(microseconds or tens of microseconds), and then the water breaks down. 

As you've also noted, keeping the "universal solvent" from partially
disolving anything it's housed in is extremely difficult. And inserting
metallic capacitor plates simply provides another ready source of
metallic ions. Keeping the water to an acceptable level of deionization
requires continual circulation through deionizing equipment. And, unless
you also scrupulously de-gas the water, slight amounts of entrapped gas
will come out of solution, forming microscopic gas bubbles on the
plates. These then become ionization sites, promoting easier breakdown
of the water. 

Kinda' makes even a LDPE/foil/mineral oil cap look mighty attractive in
comparison... :^)

Safe coilin' to you!

-- Bert --

Tesla List wrote:
> 
> Original Poster: "Mark Broker" <broker-at-uwplatt.edu>
> 
<SNIP>
  The only problem I can see is
> contamination: everything has to be 110% clean before adding water.  This
> is do-able.
> 
> Are there any reasons why I shouldn't try to use this?
> 
> For the record, I'm looking at a 15kV/120mA+ NST.
> 
> Thanks
> 
> Mark