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300kV Marx generator, driving flash x-ray tube.

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plazmatron

Tue Sept 16 2008, 11:46PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

Since the summer is over, I have finally gotten around to photographing the Marx Generator, and Flash x-ray setup.
I will be doing a thorough write up on Flash radiography, with special attention to using common valves as the x-ray tube shortly. So, people who were trying to follow the work, will have a little more to go on!

This is an overview of the Marx Generator. It is 10 stage, and capable of generating a 300kV impulse at 3pps!
It is driven by the good old royer circuit, and is powered by a Lead-acid battery.

The Marx Generator firing, and producing a lovely 30cm spark.

The flash x-ray tube, rated for 250keV

This is an image of a fluorescent screen, placed opposite the output window, of the tube, with an old HDD in the beam path.
The image is grainy, as it was taken in the dark. The bright specks, are caused by scattered x-radiation.

Backyard Skunkworks

Wed Sept 17 2008, 12:32AM

Registered Member #1262 Joined: Fri Jan 25 2008, 05:22AM
Location: Maryland, USA
Posts: 451

Nice pics, great setup!

Proud Mary

Wed Sept 17 2008, 09:01AM

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

Great stuff, Leslie!

With the winter nights coming on, my own interests have taken me into the realm of modulation of the control grid of the PD500, to produce pulse modulation of the beam at Low, Medium and High frequencies.

Varying the duty cycle of the modulation allows one to remain within the 30W anode dissipation of PD500, whilst using very much higher voltages and instantaneous currents. This project is still in its infancy, and I will share details when I have produced something reliable.

I also have some schemes for external magnetic 'pulling' with the hope of imposing some shape and directionality to the PD500 beam, - relative fluence mapping to be done with my Precitron Janus Dual Channel Electrometer fed from a pair of parallel plate chambers. (see pic)

mikeselectricstuff

Wed Sept 17 2008, 10:25PM

Registered Member #311 Joined: Sun Mar 12 2006, 08:28PM
Location:
Posts: 253

Hmm - I have one of those tubes, and a marx generator....
Just wondering how did you handle insulating the connection to the tube?

Proud Mary

Wed Sept 17 2008, 10:47PM

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

mikeselectricstuff wrote ...

Hmm - I have one of those tubes, and a marx generator....
Just wondering how did you handle insulating the connection to the tube?

Oil immersion is a must to stop flashover, envelope puncturing, and excess anode seal temperature.

plazmatron

Wed Sept 17 2008, 11:13PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

For insulating common valves, I glued mine into plastic cups, and filled them with oil. There are pics here: http://www.fineartradiography.com/hobbies/x-ray/vac.html

Oddly when using real flash x-ray tubes, they can be driven in air upto 300kV, since the voltage transient will travel through the tube, much quicker than the time it takes for a leader to form and generate a flashover. So long as nothing metal or alive

is in the vicinity of the tube, there should be no problems, unless you push it way past its rated voltage!
I have mine simply mounted on a piece of plexiglass. The cathode is the large metal end, with the metal window. The anode is the little prong way down the hole in the tube. I connect to the anode using a small spring soldered to a length of HT cable

Leslie

uzzors2k

Thu Sept 18 2008, 08:56PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

Awesome, I've been waiting for some more info on your setup for a while now. I recently got an x-ray cassette, but I still couldn't take any x-rays. I have the development process down, it's just that the fluorescent screen doesn't light up at all while irradiated. I have no idea what's wrong as my Geiger still clicks from 3 meters and through one wall, so there must be ionizing particles present. I'm trying to finish my 80kV multiplier, so I can try some CW experiments to see whether it's the tube I'm using or the voltage source.

Proud Mary

Fri Sept 19 2008, 03:47PM

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

Uzzors wrote ...

I have no idea what's wrong as my Geiger still clicks from 3 meters and through one wall, so there must be ionizing particles present. I'm trying to finish my 80kV multiplier, so I can try some CW experiments to see whether it's the tube I'm using or the voltage source.

A GM counter is not really suitable for this kind of work. To characterize X-ray pulses you would be better to use an integrating dosimeter of some kind. The cheapest and simplest solution would be a Cold War quartz fibre dosimeter which can be bought (together with their charger units) quite cheaply on ebay. The more sensitive Cold War quartz fibre dosimeters have a full-scale deflection of 100cGy, so should show a significant deflection over, say, 100 shots, from which you can obtain an average for each one.

A blacked-out PIN photodiode can also form the basis of a simple integrating X-ray dosimeter of great efficiency in the range 10 - 60keV or so, where most your output can be expected, but you will have to calibrate it yourself!

plazmatron

Fri Sept 19 2008, 05:05PM

Registered Member #1134 Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351

Uzzors wrote ...

I recently got an x-ray cassette, but I still couldn't take any x-rays. I have the development process down, it's just that the fluorescent screen doesn't light up at all while irradiated. I have no idea what's wrong

There should be a visible flash, on the screen in the dark.
What is the `erected` capacitance of your marx gen? Mine is 470pF (10 caps at 4700pF each)
You do need considerable current, to throw at those tubes, so a total erected C of 470pF is a good place to start.

As Harry says, A cold war dosimeter would be the better choice, for detecting. Especially around marx gens, where the EMP is significant enough to produce false readings on some geiger counters.

Leslie

Proud Mary

Fri Sept 19 2008, 06:52PM

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

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

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