Re: Marx spark gaps (fwd)

["High Voltage list" <hvlist@xxxxxxxxxx>]

Original poster: <sroys@xxxxxxxxxxxxxxxxx>



---------- Forwarded message ----------
Date: Wed, 1 Jun 2005 09:18:48 -0600
From: Gomez Addams <gomez@xxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: Marx spark gaps (fwd)


On May 31, 2005, at 9:19 PM, High Voltage list wrote:

>  For a charging voltage of e.g. 20 kV the first gap should break down
> at 20 kV, the second gap at 40 kV, the third gap at 60 kV and so on.
> Consequently the gap distance of the first gap should be about .27",
> that of the second gap about .54", that of the third gap about .81"
> and so on.

That is simply not true, and I have never heard of that practice before 
now.
One sets the stage gaps to be slightly greater than the stage voltage, 
based
on the breakdown voltage for the sphere radius in the local absolute 
air pressure.
You can look up that breakdown voltage in the CRC handbook.

> To my surprise I found, that all of the described Marx generators
> were said to have EQUAL gap distances (with the exception of the
> first gap, which is set a little bit closer sometimes).

  The first (lowest voltage from ground) gap will be set a little bit 
smaller
than the others if a triggered gap is not used.  If a triggered gap (ie;
trigatron, etc) is used for the first gap it is set for the same 
breakdown
voltage as the other gaps, however the actual spacing may be greater 
than the
other gaps, because the field distortion created by the trigger 
electrode
and the hole for same will cause that gap to have a lower breakdown 
voltage
for a given distance.

  Another important trick is that the gaps be so arranged that the 
spheres do
not "shadow" the UV light radiated from each gap to the next, and in as 
close
proximity as physically possible without chance of flashover.  This 
arrangement
allows the UV light from each gap to pre-ionize the adjacent gaps, and 
causes
the bank to erect much faster (in many cases, an order of magnitude 
faster)
than would otherwise be the case.  For the fastest rise time and lowest 
jitter,
it is desirable to have all of the gaps fire as close to simultaneously 
as
possible, and this arrangement promotes that operation.

> Looking for the reason of this unexpected recommendation I found:
>
> "The Firing Probability of Coupling Spark Gaps of the Multiplying
> Circuit according to Marx" by A. Rodewald, Basel
>
> This technical documentation contains a lot of useful information and
> could be downloaded from
>
> http://www.haefely.com/literature/pdf/scientific/e1-04.pdf
>
> I am hoping that you will find it interesting as well.

  That is a good document, but a little more erudite than is necessary 
for most
amateur Marx bank builders.

  - B(G)L


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