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trigger a Marx gen or a CW multiplier MANUALLY

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Fabi

Mon Dec 13 2010, 10:38AM

Registered Member #3510 Joined: Mon Dec 13 2010, 08:31AM
Location: western germany
Posts: 19

Hey guys,

hope i'm in the right forum.

So all what i wanted to ask is how i can trigger a marx gen or CW discharge.
So that when the caps are loaded the gen shouldn't discharge automatically.
It should discharge when i press the "trigger" button.
Can I use a Thyratron tube for that?

Ps. I want to build an X-ray flash device which should not produce x-rays continously.

regs from
Stuttgart, Germany
Fabian Schmidt

cedric

Mon Dec 13 2010, 01:08PM

Registered Member #2941 Joined: Fri Jun 25 2010, 08:08AM
Location:
Posts: 143

you just have to make sure that all the gap (for a marx)have a break down voltage higher than the loading one ,after a system to reduce the first gap sises would be enough to manually trigger it,unless you want to make a fancy noiseless marx you don't need thyratron for that ,on a properly tune marx a triggered spark gap for the first gap work fine...

Fabi

Mon Dec 13 2010, 02:21PM

Registered Member #3510 Joined: Mon Dec 13 2010, 08:31AM
Location: western germany
Posts: 19

Ok
Thank you for your answer

My parts should reach me tomorrow so then i can start to build that thing.

regs
FS

Proud Mary

Mon Dec 13 2010, 03:01PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Hi there, Fabian,

it's not necessary to use a Marx generator to initiate a vacuum discharge. A single capacitor charged to a suitably high voltage will start a 'flash' discharge with an X-ray tube of the right geometry i.e. a needle point cathode and a flat plate anode, or a hollow cathode Grimm-type discharge chamber.

There's a paper with lots of helpful practical details here:

Stella

Fabi

Mon Dec 13 2010, 03:19PM

Registered Member #3510 Joined: Mon Dec 13 2010, 08:31AM
Location: western germany
Posts: 19

thank you

is anywherw a shematic?

btw. i want to drive the tube at least at 70 kV DC

better would be triggered pulses with around 100 kV

radhoo

Mon Dec 13 2010, 04:46PM

Registered Member #1938 Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 701

In case you still want to trigger your Marx, make sure the first spark-gap is big enough, so that no spark will initiate (adjust it per your supply), then either you mechanically bring closer the two electrodes or use a flash-like system with a third electrode to initiate the first discharge.
The other spark gaps are not that important, as once the spark breaks the first gap, it will have sufficient voltage from the other caps to cover the air distance and jump further.

Proud Mary

Mon Dec 13 2010, 06:29PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Fabi wrote ...

thank you

is anywherw a shematic?

btw. i want to drive the tube at least at 70 kV DC

better would be triggered pulses with around 100 kV

Golly, Fabi, it doesn't take the brains of an archbishop to imagine the circuit diagram! First you charge up a capacitor to your chosen voltage. Then, when it is charged, you discharge it through the X-ray tube. That is all there is to do, and why the electrode geometry of flash x-ray tubes is completely different.

Klar?

How will you measure nanosecond X-ray brusts?

Fabi

Mon Dec 13 2010, 07:22PM

Registered Member #3510 Joined: Mon Dec 13 2010, 08:31AM
Location: western germany
Posts: 19

oh thank you

now i've got it

(I'm learning still english) It's not always easy to unserstand more complicaed things like this; the first time.
sorry for that ;)

So you think about a kind of a one-stage-marx generator, right?

I measure the x-ay bursts with a photodiode.

ps: is it easyer/more effective to drive 2 ignition coils antiparallel and add a fullwave rectifier?

FS

Proud Mary

Mon Dec 13 2010, 07:46PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

It's best to use DC - but you can generate X-rays with AC too - in "self-rectifying" mode.

klugesmith

Mon Dec 13 2010, 07:54PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

radhoo wrote ...

In case you still want to trigger your Marx, make sure the first spark-gap is big enough, so that no spark will initiate (adjust it per your supply), then either you mechanically bring closer the two electrodes or use a flash-like system with a third electrode to initiate the first discharge.
The other spark gaps are not that important, as once the spark breaks the first gap, it will have sufficient voltage from the other caps to cover the air distance and jump further.

Spontaneous (or triggered) discharge of a Marx bank could begin with -any- of the spark gaps.
Which one fires first depends on the matching of the gaps,
and the skew in capacitor charging voltages because of RC delays in the charging ladder.
There might be a risetime benefit if a gap in the middle is first to fire.

[edit] If gap #1 is designed to fire first, not because it has a lower breakdown voltage but only because its capacitor charges significantly ahead of the others, then all the other capacitors are under-utilized.

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