Re: Marx generator (fwd)

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Original poster: Steven Roys <sroys@xxxxxxxxxx>



---------- Forwarded message ----------
Date: Wed, 23 Nov 2005 11:16:55 +0100
From: Herwig Roscher <herwig.roscher@xxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: Marx generator (fwd)


Mike and all the other helpful list members,

> >My system jumps  0.027"/kV
> The voltages per inch above sound about right - I get 4.5 feet in
> bipolar mode from 1MV (approx), which is 0.054"/KV
- Whowww! But this is twice as big as my value!

> anything within 20% is probably fine, but your output does seem on the
> low side.
- What could be the reason?

> How are you triggering ? are you relying on gap breakdown or using a
> forced trigger ?
- I'm using a forced trigger with one of the modern car ignition
coils for distributorless ignition. This ensures, that the bank is
triggered from bottom to top. There are other advantages as well.

> 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.
- IMO(!) the gap distance should be so, that the bank doesn't ignite,
before it is loaded completely. Any larger distance should increase
the losses (similar to the spark gap of Tesla machines). Do you
agree?

Of course the triggered gap has a larger distance!  I'm using brass
balls of 0.4" diameter for the spark gaps.

>  Does the erecting of a Marx coil happen all at once or does one gap
> fire then the next and so on ??
- After one gap was ignited, it overvolts the next one and so on.
This happens within some µs.

> 0.013" would be 76 kV/inch, which is approximately the "uniform field"
> breakdown voltage. Your gap will be far from a uniform field.
- Among other things, because of the small ROC.

>> My system jumps only 0.027"/kV.  Is this a question of the form of
>> the  electrodes?
> Exactly that.. what are you using for gap electrodes?
- Brass balls of 0.4" diameter.

> You might have significant series inductance or resistance.
- I'm using solid wire about AWG 13. Of course the resistors have got
thinner connections.

> If all your gaps aren't firing together, you might have a lower
> voltage, longer duration pulse.
- How could I check this? Of course I'm a little bit afraid of using
electronic measuring equipment in the neighborhood of a Marx
generator.

> What sort of gap are you using for your HV gap?
- Thick sharpened(!) copper wires.

> Many experimenters use what is essentially a point/plane gap (i.e. a
> sharp wire end against a ground plane).
- I'll try this arrangement.

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

>  so you want a good optical path from the first gap to all the rest.
> The strong UV from the spark on the first gap helps breakdown the rest
> of the gaps.
- All the gaps are in the same plane, e.g. each gap can "see" its
neighbor.

Thank you all for your assistance.









Bye		Herwig

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Greed is the root of all evil !
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