Re: Marx generator (fwd)

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

Original poster: <sroys@xxxxxxxxxxxxxxxxx>



---------- Forwarded message ----------
Date: Mon, 21 Nov 2005 19:12:21 +0000
From: Mike Harrison <mike@xxxxxxxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: Marx generator (fwd)

On Mon, 21 Nov 2005 11:55:37 -0700 (MST), you wrote:

>Original poster: <sroys@xxxxxxxxxxxxxxxxx>
>
>
>
>---------- Forwarded message ----------
>Date: Fri, 18 Nov 2005 13:11:52 +0100
>From: Herwig Roscher <herwig.roscher@xxxxxx>
>To: hvlist@xxxxxxxxxx
>Subject: Marx generator
>
>
>List,
>
>I have just finished building my first Marx generator and I am about 
>tuning it.
>
>Capacitors			3.33 nF
>Resistors			2 MOhm
>Stages 			25
>Loading voltage	20 kV
>Spark gaps		0.28"
>Erected C			133 pF
>Bang voltage		500 kV
>Bang size			16 Joule
>Discharge length	13.4"
>
>However I would like to ask the more experienced builders for some 
>help:
>
>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.
>
>Other Marx builders claim:
>
>0.052"		Rapp
>0.045"		Steve Ward
>0.052"		Kronjaeger
>0.050"		Mike Gilchrist
>
>My system jumps only 0.027"/kV. Why? Is this a question of the form 
>of the 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?
>
>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. 
>Is it possible, that at ignition of the bank there is too much corona 
>at 500 kV?
>
>What are the secrets of Marx tuning? Any help would be appreciated 
>very much.
>

As long as you have enough capacitance (about >1nf), additional capacitance should not make much
difference to the output lenghth - it just makes it brighter and louder 

The voltages per inch above sound about right - I get 4.5 feet in bipolar mode from 1MV (approx),
which is 0.054"/KV although I may  have pushed the input a bit higher than 25KV that time..!
This is by no means a precise science though, especially where fast risetimes are concerned -
anything within 20% is probably fine, but your output does seem on the low side.  

If you run it in the dark you will easily see if you have a large amount of corona loss..! I only
see significant corona on about the top quarter of mine (www.electricstuff.co.uk/marxthree.html).
  
How are you triggering ? are you relying on gap breakdown or using a forced trigger ? 
If the former, maybe not all the caps are fully charging.

0.28" sounds very short for a 20KV gap, especially of force-triggered I'd have thought double that
would be more appropriate, depending on geometry of course.