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LAN_403

Mates

Thu Sept 18 2008, 07:39PM

Registered Member #1025 Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566

Ok guys,
my progress will be rather slow. However, I'm sending pictures of my first capacitor. There will be 8 of them in the CW stack. The oil filling is planned for the next week...

Proud Mary

Fri Sept 26 2008, 11:49PM

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

Many years ago I saw a plan for an amateur linear accelerator in The Scientific American Book of Projects for The Amateur Scientist C.L. Stong, Simon and Schuster, New York 1960.

The apparatus consisted (from memory!) of three basic parts:

1. A glass tube. At one end was a thermionic filament (kathode) and at the other end a beam window of thin aluminium foil supported on a heavier metal plate drilled with many holes which formed the anode. There may have been voltage equalization rings along the outside of the tube, and an anti-corona sphere at the anode end.

2. A vacuum pump consisting of a home-made mercury diffusion pump backed by a modified refrigerator pump.

3. A small home-made Van de Graaff generator producing 350KV. This was connected to the anti-corona sphere at the accelerator anode.

Substituting hydrogen for air in the tube before pumping it down converted the apparatus into a proton accelerator.

This is all I can remember, but there are plenty of references to the book on the internet, and it would seem sensible to review a well-described and illustrated amateur system such as this that has been proved to work before proceeding with what seems to me to be most unsuitable equipment.

Atomic Parts

Sun Sept 28 2008, 05:37PM

Registered Member #1591 Joined: Wed Jul 16 2008, 06:34PM
Location: Livermore Ca
Posts: 14

Is this what you are talking about?

Mark

Atomic Parts

Sun Sept 28 2008, 05:42PM

Registered Member #1591 Joined: Wed Jul 16 2008, 06:34PM
Location: Livermore Ca
Posts: 14

I have a few more pics of amateur efforts if anyone is interested.

Mark

Proud Mary

Sun Sept 28 2008, 06:19PM

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

Atomic Parts wrote ...

Is this what you are talking about?

I don't think that was quite it, but it was a design of kindred vintage (judging by the style and calligraphy in the diagrams) with a similar level of constructional intricacy, inventiveness, and ambition.

LutzH

Tue Sept 30 2008, 10:39PM

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

I am currently building a 250KV 2ma, prototype FW VM, to test the protective measures for the my eventual 1MV accelerator project. I have done a lot of research on designs and components for this project which I am willing to share to save people from frying their diodes, or even worse their DNA.

If you graph X-ray output vs KVP, if you go from 150KVP to 300 you should have 6-8 times the radiation output. So you have cut the exposure time way down already, but your shielding requirment has increased by almost the same amount!!!

One Suggestion: If you are irradiating only a thin surface layer on the cell cultures you do not need much penetration, if you get a Beryllium window X-ray tube you will have up to 100 times the X-ray dose at the same KVP, as with a glass envelope tube. This is how more than one person has literaly fried the skin off there hand using a seemingly low powered X-ray tube.

For Example at only 50KV at 50 ma, a .5 - 1mm Beryllium window tube will produce a 10,000 to 50,000 R /min dose rate at 1 meter!!! 90% of this dose will not penetrate beyond the skin layers, but it will permanently destroy the skin, so be very careful if you ever fire up a Be window X-ray tube!!! For a thin surface layer in a petri dish this may be acceptable, and reduce the exposure time to seconds. I do not know enough about what you are doing to know if this is an option for you. This is why ALL medical use tubes are required to have at least 1mm Aluminum, not Be filtration, to filter out the soft X-rays.

When building a VM you have to limit the current flow in the stacks to about 1/2 of the single cycle surge rating of the diode. I am using <100ns 20KV/100ma diodes from China (not bad quality), and ceramic capacitors. At very high voltages an output resistor is not practical. So far I am doing the following to protect the VM....

1. Series resistors between all the capacitors in all 3 stacks, for my diodes/caps., this works out to about 2K ohm, each consisting of (2) 1K ohm 5 watt resistors in series.

Note: Some folks put slightly larger value resistors between the diodes and the outside feeding columns of the full wave VM instead, I do not know which is better, but the commercial designs for 200KV+, that I have seen put them between all the capacitor stages in series, in all 3 columns.

2. Double, or even Quadruple, the number of diodes in the first, and last stage of the VM, in terms of current not KV, in other words run several strings of diodes in parallel for higher current.

3. Add resistors to current limit the input to the first stage.

4. Run a resistor voltage divider column between the stacks to equalize potential gradients, and add equipotential rings to the stack connected to the resistor column.

5. Use a segmented accelerator tube with each section being connected to one stage of the VM, and run the tube down the middle between the 3 stacks of the VM. Protective spark gaps for the VM can be conveniently added to the accelerator tube electrodes also.

At 500KV even when accelerating positive ions, you have to supress secondary electrons from the target by biasing an electrode above the target by about 150V-. Failure to do this will result it these electrons being accelerated back up the tube and forming a very powerfull x-ray machine. If you hit 200-500KV, these x-ray photons are very penetrating and very difficult to shield against, so be careful :)

I hope some of this helps, a great resource for VM design is

, this man has done his homework!!! Brilliant work at 60hz !!!

Mates

Wed Oct 01 2008, 07:31PM

Registered Member #1025 Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566

Hi guys, Iâ€™m so busy with setting up a new lab that I have hardly any time for my hobby. At least I finished the power supply for my CW. I'll send the pictures soon...

Lutz, thanks a lot for your comments. I'm happy there is somebody here who understands the problematics and is willing to share some knowledge ;)

I will read you suggestion regarding the linac very carefully and will discuss it soon in details.

Btw: Regarding your X-ray based DNA damage suggestions and warnings. I'm operating routinely 150KV 15mA X-ray unit. So I know... I need much higher energies for my experimental ideas.

jpsmith123

Tue Oct 21 2008, 09:18PM

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

A 5 Mev accelerator would be ambitious to say the least.

I'm also interested in building some kind of several-Mev electron accelerator...I've actually been lazily studying general accelerator design for quite a while now, trying to figure out what kind of design would be best in terms of simplicity, cost, and chance of success.

I've considered betatrons (both conventional and non-conventional), microtrons, variously driven DC types, resonant transformer driven accelerators, conventional RF linacs, "low frequency" linacs, and various short-pulse-driven accelerators such as the "Inductive Voltage Adder", and lastly, single gap "electron diodes" driven by marx generators, blumlein pulsers, and pulse transformers, for example.

I may be wrong, but I ruled out a DC accelerator as being generally impractical *at that kind of voltage level*, especially given my limited resources.

Presently I'm leaning toward some kind of short-pulsed system where much higher voltage gradients can generally be tolerated, leading to a much more compact system.

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