Re: Question: Rotary gap designs - loss

        Re: Question: Rotary gap designs - loss
        Tue, 15 Apr 1997 03:46:17 +0300 (EET DST)
        Kristian Ukkonen <kukkonen-at-cc.hut.fi>
        Tesla List <tesla-at-pupman-dot-com>

On Mon, 14 Apr 1997, Tesla List wrote:

> Thanks Richard,
> I'm under the impression that in a primary circuit, the majority of the
> resistive losses are in the gap arc itself.  However, it is apparently
> not
> worth the time/effort trying to spread this loss over several parallel
> gaps?  
> Kyle

For what it's worth - I recently found a nice publication with lots
of information quite related to TCs:

IEEE Pulsed Power Conference.

These have LOTS of articles about spark-gap design, x-ray/microwave
generation, switching tech (ignitrons, thyrathrons etc.), etc. and
the articles are modern - the books I found from local university
library were '89 ja '93 (that 1993:).

SO: an article "Energy losses in switches" deals with "H, He, N2, air,
SF6, PE, Water as switching di-electric" and states that that the
losses scale as exp(Vpeak*Ipeak, 1.1846) where Vpeak is peak voltage
before breakdown and Ipeak peak current through the gap after
breakdown. Generally the formula for losses in article is
  E = .25 * Vpeak * Ipeak * Tr
where Tr is approximate duration the total source Z the switch sees
matches the driving source Z (total Z = source Z and load Z). They
approximate Tr with formula
  Tr = (88 * sqrt(<rho>/<rho>0))/(exp(Z,1/3)*exp(e,4/3))
where Z is in ohms, e in 10kV/mm and Tr in ns. Strangely, they don't
define <rho>,<rho>0,Z more precisily (!) but the above formula is
for gasses. For liquids :
  Tr = 5 /(exp(Z,1/3)*exp(e,4/3)) 
where e in MV/cm. Propably Z is total Z above, and <rho>0 is density of
gas inside gap at first and <rho> while gas is firing. Just a guess

Interesting paragraph:
"..spark-channel theory developed by Braginskii for gasses. In his
 theory, as with Tr, the spark channel plasma has CONSTANT conductivity
 and the changing spark channel radius causes the channel resistance
 to decrease as function of time. The channel balances the IIR input
 power with the shock front, the ionization of new particles, and
 heating these particles to several eV of temperature. Remarkably,
 the shock front boundary traps the radiation and little power
 gets outside the channel."

So, this paper was in
IEEE pulsed power conference, 1993, pages 463-470
Authors : T.H.Martin, J.F.Seamen, D.O.Jobe

This Martin had some other interesting articles as well. Including
"An empirical formula for gas switch breakdown delay". It states
that "universally"
   <rho> * T = 97800 * exp(E/<rho>, -3.44)
where <rho> is gas density in gm/cc, T is time delay to breakdown
in seconds, E is average e-field in kV/cm. He actually deals with
LOTS of data to verify that interesting formula (varying electrode
shapes, energies etc. - voltages ranging from a new kV to 10MV)

btw: related to RHs water-arc experiments : they seem to use similar
technology in mass drivers. In the publications above there was an
"Electrothermal generation of pressure pulses for mass acceleration"
used water. So, those water-arc explosions are propably quite a known
technology among the rail-gun people. In the article above they use only
352uF 6kV cap with 17,6kJ of energy.  They state that they got 38MPa
pressure pulses. There were references to IEEE transactions on
Proceedings of European Symposeum on electromagnetic launch tech, and
German articles. 

It seems that there's lots of research being done that is quite
related to TCs and spark-gaps. It just takes some effort to find
the publications. Clearly, spark-gaps are not a thing of the past
but have a clear job with modern high-powered pulsed technology.

After a day at the library,

  Kristian Ukkonen.

| Kristian Ukkonen       |  Do as thou wilt shall be the whole of the
law |
| kukkonen-at-alpha.hut.fi  |_____  Chance favours the prepared mind 
| http://www.hut.fi/~kukkonen |  Fear is the mind-killer  |---------