Particle accelerator design

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1. Jul 24, 2012

papernuke

I've attached a picture of an ion accelerator design I just though of.

The power source can be any constant DC supply; the voltage X would only be limited by the breakdown voltage of the gap at a given pressure.
The pairs of plates are close enough together and large enough that the E field outside a pair of plates is near negligible.
The number of plates is arbitrary; I just chose to draw 5

Would this be an effective design? Would it work at all?

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2. Jul 25, 2012

Bobbywhy

I think it would NOT work, papernuke. Have a look at how the SLAC (google it) linear charged particle accelerator works:

"Linear high-energy accelerators use a linear array of plates (or drift tubes) to which an alternating high-energy field is applied. As the particles approach a plate they are accelerated towards it by an opposite polarity charge applied to the plate. As they pass through a hole in the plate, the polarity is switched so that the plate now repels them and they are now accelerated by it towards the next plate. Normally a stream of "bunches" of particles are accelerated, so a carefully controlled AC voltage is applied to each plate to continuously repeat this process for each bunch.
As the particles approach the speed of light the switching rate of the electric fields becomes so high that they operate at radio frequencies, and so microwave cavities are used in higher energy machines instead of simple plates."

http://en.wikipedia.org/wiki/Particle_accelerator#Electrostatic_particle_accelerators

3. Jul 25, 2012

papernuke

Yeah, I have read about how SLAC and other oscillating particle accelerators work. They make their plates' polarities switch at radio frequencies to match the speed of the oncoming electrons, but that's completely different from my design.

The point of what I drew up was to eliminate the need for RF polarity switching. Why wouldn't it work, though? As electrons pass in between plate pairs, they should be accelerated, and when they're outside of a plate pair, that plate pair should have no effect on the electrons.

4. Jul 25, 2012

Bobbywhy

papernuke, imagine an electron (negatively charged) approaching a negatively charged plate. What force do you think it would feel? Now, imagine an electron moving away from a positively charged plate. What force do you think it would feel?

I can't locate any net or additive acceleration of the electron in this scheme. Maybe I'm missing something due to my very old age, so I remain open to serious corrective evidence.

5. Jul 25, 2012

papernuke

But outside of any given pair of charged plates, Gauss' law shows that the electric field should be zero (or near zero), because
Qenclosed = (charge on + plate)+(charge on - plate) = 0

So outside of a pair of plates, the electron should feel no forces from that pair

6. Jul 25, 2012

Bobbywhy

So, no force means no acceleration.

7. Jul 25, 2012

ZapperZ

Staff Emeritus
So what is it that you are trying to accomplish here? If you just want to show "acceleration", then doing a single capacitor-type arrangement is sufficient.

If you are trying to show that you can achieve the same type of acceleration as an RF structure, then you need to think again WHY we use an RF structure rather than a DC structure (hint: to get the same acceleration as an RF structure, you will need a significantly-longer DC structure).

Zz.

8. Jul 25, 2012

Staff: Mentor

This can be true if you have just two plates. But you have more - and you force those plates to be at the same potential by connecting them via wires. Instead of fixing the charge, you fix the potential.
The flaw is easier to see in terms of potentials: The difference between the first plate and the last one is just X eV.