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Linac Design - Electron Beam Acceleration and Collimation

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LAN_403

MArked One

Sat Dec 17 2011, 07:45PM

Registered Member #3688 Joined: Mon Feb 14 2011, 07:39PM
Location: Europe
Posts: 38

Greetings.

I have set out to build a simple linear accelerator for a school chemistry (well, physics really) project.
The Linac will initially be hooked up to a cloud chamber to observe the effects, then to a phosphorus screen to facilitate beam visualization. (that is the extent of the school project)
I have researched electron guns and linac design, but still have some questions:
1: Could multiple electrodes (electrically common at 30-60 KV) spaced apart, be used to accelerate the beam?
2: What forces (apart from particulate collisions and mutual repulsion) affect the divergence of a e-beam, and what calculations can I use to predict this?
3 Apart from reversing the electrode potentials and replacing the cathode with a proton source (hydrogen container with pinhole?), what modifications need to be carried out to convert this to a proton accelerator?

Does anyone know of some online resources where those questions may be answered?

Any help is much appreciated.

Kind Regards,
Ivan Kozlov.

AndrewM

Sat Dec 17 2011, 09:48PM

Registered Member #49 Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362

1. Surely.

2. Do you need a focused beam? Just put a narrow slit or window before your cloud chamber.

3. You can find a good alpha particle source in a smoke detector - americium-2something. I imagine an alpha particle is similar enough to a proton for your demo.

jpsmith123

Sun Dec 18 2011, 08:25PM

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

wrote ...

1: Could multiple electrodes (electrically common at 30-60 KV) spaced apart, be used to accelerate the beam?

What exactly do you mean by this statement?

wrote ...

2: What forces (apart from particulate collisions and mutual repulsion) affect the divergence of a e-beam, and what calculations can I use to predict this?

For a linear electrostatic accelerator, you don't have to worry about the types of dynamic instabilities that plague RF and pulsed induction accelerators. The only other thing I can think of might be an unwanted field (tending to deflect the beam) due to charging of an insulator surface (due to a design weakness where the beam can "see" an insulator somewhere).

wrote ...

3 Apart from reversing the electrode potentials and replacing the cathode with a proton source (hydrogen container with pinhole?), what modifications need to be carried out to convert this to a proton accelerator?

I've never experimented with ion sources, but I'll speculate and say it depends on the kind of ion current you want to accelerate. If you're looking for more than a few microamps, then I think you'll have to build an ion source of some kind.

wrote ...

Does anyone know of some online resources where those questions may be answered?

There's actually quite a bit of stuff available on the internet that might help you. Maybe start with the free ebooks available here:

Steve Conner

Sun Dec 18 2011, 09:10PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

A pretty adventurous high school project, but it has been done! You might get some inspiration from Fred Niell's project.

His accelerator was just a long tube with electrodes in the ends, but it seemed to give results.

jpsmith123

Mon Dec 19 2011, 02:15AM

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

This may be somewhat helpful too:

MArked One

Mon Dec 19 2011, 01:47PM

Registered Member #3688 Joined: Mon Feb 14 2011, 07:39PM
Location: Europe
Posts: 38

Thank you for all the replies!

I'm looking into the sources now, so far it's very interesting.

@jpsmith123, what do you mean by 'pulsed induction accelerators'?
sounds interesting.

Also, to clarify, when I mentioned "1: Could multiple electrodes (electrically common at 30-60 KV) spaced apart, be used to accelerate the beam?", I was trying to escape the problem of generating 100kv+ off the bat. I meant placing electrodes in the tube, and hooking them all up to the same HV generator. Will this approach work, and if yes, does anyone know the electrode spacing equations? Also, what current draw from the HV electrodes am I looking at?

On another note, if I wanted to pulse a proton beam, what modifications must I make (apart from supplying enough ions to the tube, of course!)

Thanks to all for the advice!

Kind Regards,
Ivan Kozlov

jpsmith123

Mon Dec 19 2011, 03:09PM

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

I'm still not clearly understanding your question about electrodes vis-a-vis the accelerating voltage source.

Generally speaking, for a conventional electrostatic accelerator, if you want 100 kev particle energy, then you need a 100 kv power supply. You can break it down into, say, five 20 kv accelerating electrodes, in order to spread out and distribute the electric field evenly to avoid flashover along the accelerating column, but you still need a 100 kv power supply.

BTW on the web page I referred you to is a link for an ebook on particle accelerators which introduces the concept of induction accelerators.

Here is a related review paper:

dude_500

Mon Dec 19 2011, 05:20PM

Registered Member #2288 Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179

MArked One wrote ...

Also, to clarify, when I mentioned "1: Could multiple electrodes (electrically common at 30-60 KV) spaced apart, be used to accelerate the beam?", I was trying to escape the problem of generating 100kv+ off the bat. I meant placing electrodes in the tube, and hooking them all up to the same HV generator. Will this approach work, and if yes, does anyone know the electrode spacing equations? Also, what current draw from the HV electrodes am I looking at?

An ion/electron source, followed by several 20kv electrodes will simply accelerate to 20kv. It'll be at 20KeV by the time you hit the first one, then it'll just keep traveling without accelerating. Acceleration requires field gradient, and there would be no gradient between subsequent 20kv electrodes.

The only way to get more energy out than the power supply has in voltage with a DC linear accelerator is to make a tandem accelerator, which can make particles with twice the energy as the voltage of the supply.

MArked One

Tue Dec 20 2011, 06:55PM

Registered Member #3688 Joined: Mon Feb 14 2011, 07:39PM
Location: Europe
Posts: 38

Ohh, understood.

Looks like I'll have to look into 'real' HV supply design.

Does anyone know what kind of current draw I can expect from the accelerating electrodes? Since they can run off of a van de graaf, I'd expect no more than 100 uA, but how does one calculate such a thing?

jpsmith123

Fri Dec 23 2011, 03:15PM

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

Basically you'll have a cathode, an anode, and optionally some field-grading electrodes in between.

The cathode will emit the electron beam, which will generally terminate on or within the anode (or the beam may be brought out into the atmosphere through a foil or something).

In a system that's working correctly, there should be little or no current associated with the grading electrodes.

If you're using a Van de Graaff generator for power, your current will be limited by the generator's small current capability, which will generally be microamps or tens of microamps depending on the details. Offhand I forget the formula used to estimate the Van de Graaff's current, but it's not too hard to derive. It's basically just an equation for the amount of charge you can carry up the charging belt based on the dielectric constant of the belt, the width, speed and thickness of the belt, and the corona charging voltage, or something like that.

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