Linear Accelerator

todor943

I am making a proof-of-concept linear accelerator with one stage at home ( if i can call it such ).The vacuum chamber is a 40cm transparent pipe with a radius of 10mm.The particles are accelerated by an electrical field created at the two ends of the tube with a voltage of 20kV.I need an electron source so i have thought of using the crt screen filament but the screen contains some toxic chemicals.I think that vacuum tubes also have a similar filament but I don't know if any part of the vacuum tube or if the filament is toxic too.There is also one implausible concern about x-ray radiation.Do i need any kind of shielding for this small model?I also read that the radiation produced with up to 10kV is negligently low - is that true and what do i have to do if i push it past 10kV?Is safety a concern at all (except for the HV power supply) ?

Bob S

I believe you are thinkiing about accelerating electrons, is that right? You should think about the quality of vacuum you need, how you will get it, and the outgassing of all the materials used. If your vacuum chamber is 40 cm long, how many gas molecules per cubic cm are tolerable? If you can tolerate one ion per square nanometer for the 40 cm flight path, this is equivalent to 2.5 x 10^12 ions per cm^3. Because there are 6 x 10^23 ions (Avagadro's number) in 22,400 cm^3 at 760 mm Hg, this corresponds to about 10^-7 atm, or about 10^-4 torr.

marts

Hi todor943

I did something very similar about a year ago. So you might consider sending me a pm if you need detailed information or photos.

As a filament you might consider simply smashing up a halogen bulb. It will of couse not give of a focused beam but building a small wehnelt cylinder or just using an aperture should solve the problem.

I don't believe that xrays will be a problem as the transparent pipe will already absorb nearly all photons at such low energies. Never the less if you are concerned, why don't you just reduce the accelerating voltage?? It will still be a good proof-of-concept design (20kV are not sufficient to do anything intersting anyway) and xrays will be neglectable.

Bobs vacuum estimation is pretty good, as it corresponds to a mean free path of about a meter. Outgassing shouldn't be to much of a problem (I used standart plexiglas and plumbing equipment as well). But yes you will need quite some vacuum equipment (a turbo-molecular pump and a decent backing pump are essential).

Bob S

You might consider using the CRT gun assembly, which would limit the defocusing of the accelerating beam. You will need a bias supply for the deflector electrodes. What do you plan to use to detect/observe the accelerated beam at the end of your 40cm pipe? If your vacuum weren't quite as good as you expect, you might be able to see the beam-ionization glow in a dark room. RE the radiation, I suspect you will be running at less than 100 microamps (2 watts at 20 kV)(too high a current will burn your screen), so radiation is probably not a problem. I used to check linearity of oscilliscope CRTs at HP (circa 1955) and we didn't worry about radiation (then).