Re: Marx generator (fwd)

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

Original poster: <sroys@xxxxxxxxxxxxxxxxx>



---------- Forwarded message ----------
Date: Mon, 21 Nov 2005 14:26:32 -0800
From: Jim Lux <jimlux@xxxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: Marx generator (fwd)

At 10:55 AM 11/21/2005, High Voltage list wrote:
>Date: Fri, 18 Nov 2005 13:11:52 +0100
>From: Herwig Roscher <herwig.roscher@xxxxxx>
>To: hvlist@xxxxxxxxxx
>Subject: Marx generator
>
>
>Q1:
>The distance, a voltage of 1 kV can jump, usually is said to be
>0.013". For Marx generators I found values of about 0.04" as well.

0.013" would be 76 kV/inch, which is approximately the "uniform field" 
breakdown voltage.

Your gap will be far from a uniform field.

>Other Marx builders claim:
>
>0.052"          Rapp
>0.045"          Steve Ward
>0.052"          Kronjaeger
>0.050"          Mike Gilchrist
These are roughly 3 times the uniform field gap, implying a strongly 
non-uniform field (e.g. a needle gap or close to it).


>My system jumps only 0.027"/kV. Why? Is this a question of the form
>of the electrodes?

Exactly that.. what are you using for gap electrodes?



>Q2:
>Other Marx builders get:
>
>1.75"/Joule             Rapp
>1.92"/Joule             Steve Ward
>1.83"/Joule             Mike Gilchrist
>
>I only get 0.9"/Joule. Again: Why?

Spark length is dependent on a lot of things other than energy.

Voltage for one
Waveform for another
Shape of the test gap for a third

You might have significant series inductance or resistance.

If all your gaps aren't firing together, you might have a lower voltage, 
longer duration pulse.

What sort of gap are you using for your HV gap?  Many experimenters use 
what is essentially a point/plane gap (i.e. a sharp wire end against a 
ground plane).



>Q3:
>During and after completion of the bank loading there is no sizzling
>noise. Therefor I assume, that there is little or no corona at 20 kV.

Good assumption..
You can also look at it in the dark.

>Is it possible, that at ignition of the bank there is too much corona
>at 500 kV?

Sure, anything is possible

>What are the secrets of Marx tuning? Any help would be appreciated
>very much.

Did you remember to sacrifice a goat while facing precisely 14 degrees west 
of true north?
<grin>

But really, these things are highly idiosyncratic.  Seemingly small changes 
in things like gaps can have huge effects on the output.  Getting all the 
gaps to fire together is real important, so you want a good optical path 
from the first gap (which can be slightly smaller, so it fires first) to 
all the rest. The strong UV from the spark on the first gap helps breakdown 
the rest of the gaps.
Using decent gap electrodes helps.  One of my Marxes used the "twist of 
wire in a loop" electrodes and it was very erratic.  When I went to 
smoothly rounded drawer pulls, it worked a LOT better and more 
consistently.  I think it has more to do with the uniformity of the fields.

Good luck..