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A relatively simple xray tube.

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LAN_403

jpsmith123

Sat Jan 30 2010, 06:49AM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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Posts: 843

Well I came across an interesting paper, so I thought I'd share the information in case anyone else may be interested.

The details of the paper are as follows:

Rev. Sci. Instrum. 68 (6), June 1997

High repetition rate compact source of nanosecond pulses of 5â€“100 keV x-ray photons

A. Khacef, R. Viladrosa, C. Cachoncinlle, E. Robert, and J. M. Pouvesle
GREMI, CNRS/UniversiteÂ´ dâ€™OrleÂ´ans, B.P. 6759, 45067 OrleÂ´ans Cedex 2, France

A powerful, compact, and repetitive flash x-ray system based on a cable transformer technology powered by ceramic capacitors in a Blumlein-like configuration has been developed. Open circuit voltages in excess of 100 kV can be achieved while commutation occurs at low voltage (<20 kV). The x-ray emission from a low impedance x-ray diode with a hollow cathode configuration was observed under a wide range of experimental conditions. The critical parameters limiting the flash x-ray performances are mainly the pressure in the x-ray diode and the anodeâ€“ cathode space. This true table top device is able to produce doses up to 1 R per shot, measured at the output window, of x-rays between 5 and 100 keV. The pulse widths were about 20 ns and the maximum repetition rate was about 60 Hz. Operation is possible in air or in other gases (He, Ne, Ar, Kr, Xe, H2,N2) at pressures varying from 10-3 mbar for xenon to about 1 mbar for helium. Â© 1997 American Institute of Physics. [S0034-6748(97)05006-5]

Here's a diagram of the tube, and a graph of the output vs pressure:

Note that the output is relatively insensitive to the operating pressure over a fairly wide range, and this range is well within the capability of a cheap mechanical vacuum pump.

The paper discusses the anode and cathode diameters, spacings, etc. that gave the best performance.

It seems this system is also the subject of U.S. patent 5,651,045, although the patent doesn't go very much into the details of the tube design.

LutzH

Sat Jan 30 2010, 09:31AM

Registered Member #1721 Joined: Sat Sept 27 2008, 08:44PM
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Thanks for this post, I have not seen this one. This design will scale to 1MV, and it is used commercially for most x-ray flash systems, like the ones made by Titan (Now Named L-3 Comm) or something. Very simple, very reliable, and very nice to see :)

Proud Mary

Sat Jan 30 2010, 12:34PM

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

It seems to me that the chief concern of the patentees is not with the flash X-ray tube - in which there is little that is new - but with their particular arrangement of the vector inversion pulse forming lines to ensure their mechanical stability and uniform electrical properties, though it is not easy to see what especial advantage their scheme with recessed PVC dielectric plates would have over dielectric oil - except, perhaps, as they suggest, compactness.

For the amateur constructor, I would expect the biggest single obstacle for most would be the very high cost of co-axial pulse cable with suitably high withstand voltage.

jpsmith123

Sat Jan 30 2010, 04:43PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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AFAIK the cables have to withstand only the charging voltage.

IIRC, RG8 cable can tolerate 50 kv pulses...so if you use say 6 lengths of RG8 and a 50 kv charging voltage, you could theoretically get 300 kv (minus losses) at the output (series connected) end.

Of course if you want long pulse lengths then you would need long cables and the thing would become bulky and cumbersome and expensive.

Proud Mary

Sat Jan 30 2010, 08:11PM

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

jpsmith123 wrote ...

AFAIK the cables have to withstand only the charging voltage.

Surely, they must withstand 2V due to the superposition of reflected and incoming waves at open-ended coaxial transmission line, consistent with reflection coefficient gamma = (R-Zo)/(R+Zo).

I'm sure there's still a lot to be found out about X-ray production by Grimm-type hollow cathode glow discharge devices, but one can experiment with them without the very tricky construction of a wave-erection transmission line Marx, though as Lutz says, it would be good to see.

jpsmith123

Sat Jan 30 2010, 09:21PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
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Posts: 843

Well in the body of the paper the authors state:

"The typical charging voltage was about 15 kV and the high-voltage open circuit pulse was about 100 kV with 60 ns
duration full width at half-maximum ~FWHM." (This is using 6 cables connected in parallel at the input and in series at the output).

One of these days I'm going to try to do an EM simulation of this type of transmission line transformer to see exactly what's going on.

Proud Mary

Sat Jan 30 2010, 09:39PM

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

Oh right, I see.

LutzH

Mon Feb 01 2010, 01:39AM

Registered Member #1721 Joined: Sat Sept 27 2008, 08:44PM
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Posts: 136

Hello:

I used to own the Russian version of this tube (250KV normal use, 320KV Max), until I sold it to a tube collector in Europe. I ran mine with a small marx bank by using 30KV 2700pF high quality ceramic doorknobs. It worked very well although my pulse length may have stretched into the 1 microsecond realm.

I had an idea to go faster by using a coaxial de-ionized water capacitor, but I never needed x-ray pulses this short. I still have a bunch of the new 30KV doorknobs if someone wants to build a precision marx bank. Immersing it in the commercial fluorocarbon fluids works great for insulation, but oil will do in a pinch. You can x-ray an airgun pellet going through a lightbulb, even with only a simple marx bank driving it.

The military used these to study shaped charges and nuclear core implosions, which in the case of nukes led to self forging fragment technology, this made smaller American football shaped nukes possible.

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