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4hv.org :: Forums :: High Voltage
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300kV Marx generator, driving flash x-ray tube.

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plazmatron
Fri Sept 19 2008, 07:05PM
plazmatron Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
Harry wrote ...

I wonder if the addition of a peaking capacitor might improve the transfer efficiency of the energy from the Marx into the tube:

Theoretically yes, but 300kV caps are hard to come by! The usual method shaping the impulse from a Marx, is to use Wave shaping resistors, or to build a choke charging Marx (if you want really short impulses).

Link2
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uzzors2k
Sat Sept 20 2008, 09:45AM
uzzors2k Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Lacking current is probably the problem then. I have 20 stages of 470pF, so only 23pF when "erected". I was thinking of experimenting with irridating semiconductors just a few weeks ago actually, I'll give it a go once I rebuild my SSTC or finish the multiplier.
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Proud Mary
Sat Sept 20 2008, 01:58PM
Proud Mary Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Uzzors wrote ...

Lacking current is probably the problem then. I have 20 stages of 470pF, so only 23pF when "erected".

Setting aside the complex rise of current as emission begins in the tube, if your erected capacitance is but 23pF, and the inter-electrode capacitance of the tube plus other stray capacitance is also of the order of 23pF, (a not unlikely figure) then your anode voltage is reduced to 50% of the Marx maximum before any conduction has even started.

As an example and talking point only, this means that if your Marx output is 100kV, then your peak anode voltage will be 50kV when it is split with the system capacitance of the tube.

As a general rule, very roughly two thirds of the energy of an X-ray tube's output will be in the lower one third of the emission spectrum, so that would mean that 66% of your output would be below about 16keV.

What is more, the discharge will follow the CR time constant of the tube and its supply, so that your high peak will occur only momentarily at the start of the discharge pulse, so that only a very small proportion of the X-ray energy produced will be at 50keV.

If we assume that the glass envelope causes severe attentuation of the beam below 10keV, you will see that though X-rays may well be produced, that only a small amount of their energy will ever leave the tube if your peak pulse anode voltage is only 50kV.

As an afterthought on my earlier comment on the general unsuitability of GM tubes for this kind of work, you should consider the possibility that the clicking of your remote instrument is not caused by X-radiation at all, but by rectification/direct detection by the counter audio circuit of the strong EMI pulse caused by the Marx discharge.
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uzzors2k
Sat Sept 20 2008, 06:45PM
uzzors2k Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Harry wrote ...

...then your anode voltage is reduced to 50% of the Marx maximum before any conduction has even started.

Excellent point, I hadn't even considered that. I did some tests with just the marx generator alone because I doubted the production of x-rays, but the Geiger would only flash when in very close vicinity. It still flashed however, which isn't comforting, some proper EM shielding would be a good idea. I'm abandoning this Marx setup for x-rays.
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plazmatron
Sun Sept 21 2008, 02:47PM
plazmatron Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
Uzzors wrote ...

I'm abandoning this Marx setup for x-rays.

I hope your`e going to build a bigger one then! wink
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Proud Mary
Sun Sept 21 2008, 03:56PM
Proud Mary Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Leslie's flash X-ray tube looks to have a very low inter-electrode capacitance indeed, much less than 1pF, I dare say.

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Fabio
Mon Sept 22 2008, 09:09PM
Fabio Registered Member #122 Joined: Fri Feb 10 2006, 12:55PM
Location: Milano Italy
Posts: 148
Can you post more detailed pics about this tube?


I have a high vacuum pump, i have some getters, many glass tubes, various glass to metal seals and a basic equipment for lampworking, i also have some really basic glassblowing skills and i' already made two simple Xray tubes and some other glass items..... i have everything necessary for build a similar tube and i don't think that can be so difficult to make! amazed


Ciao!
Fabio.
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plazmatron
Wed Sept 24 2008, 08:18PM
plazmatron Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
Fabio wrote ...

I have a high vacuum pump, i have some getters, many glass tubes, various glass to metal seals and a basic equipment for lampworking, i also have some really basic glassblowing skills and i' already made two simple Xray tubes and some other glass items..... i have everything necessary for build a similar tube and i don't think that can be so difficult to make! amazed


There is an diagram of the tube at:

Link2

The cathode is ring shaped, and covered in small wire points to act as emitters.

A far easier way of building a Flash xray tube, would be to have a single needle shaped cathode, and an angled target/anode, in a long tube.

Of course you can ground the anode, if you set your Marx up to produce -250kV, which would make your setup somewhat safer.

Like this:


1222287483 1134 FT54047 Fxr


Leslie
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Fabio
Wed Sept 24 2008, 09:40PM
Fabio Registered Member #122 Joined: Fri Feb 10 2006, 12:55PM
Location: Milano Italy
Posts: 148
Thank you for the info, my previous tube isn't particulary good and now i know why, i used a big rounded chatode (to be honest, the chatode electrode is only a fresh getter) and a small tungsten anode instead (made with a welding rod).

Anyhow, in spite of the simple construction, i got some weak rays from this tube, the emission can be revealed some meters away with a geiger counter and few centimeters away with a barium platinocyanide screen in a dark enviroment.


Th

Th

Th


Ciao!
Fabio.
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plazmatron
Thu Sept 25 2008, 03:35AM
plazmatron Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
Fabio wrote ...

Thank you for the info, my previous tube isn't particulary good and now i know why, i used a big rounded chatode (to be honest, the chatode electrode is only a fresh getter) and a small tungsten anode instead (made with a welding rod).


They are very nice! I wish I had some decent Vacuum equipment!
A pointed cathode will do a way better job, than the getter. You must try it and let us all know how you get on!
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