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Q & A: Electron beam in a vacuum

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Most recent answer: 03/21/2015
Q:
Thanks for helping me concerning electron beam in vacuum. If a beam of e- loses no energy in vacuum, can we transmit electric energy in this way without loss? I find it quite ridiculous to associate vacuum with superconduction, but I cannot point out the flaw.
- Allan (age 29)
Hong Kong
A:

Yup, you sure can transmit energy in this manner. We always complain about the classification of energy as "electric" or some other kind.

Electron accelerators give a large amount of kinetic energy to the electrons they accelerate. I think the standard line about the linear collider idea is that each bunch of electrons that goes down the accelerator has the same energy content as a cup of yogurt (chemical energy in the yogurt, not rest mass! And the energy of the electron beam is almost entirely kinetic).

A practical worry -- you usually have to get the electrons back to the source to complete a circuit. This step could be lossy. If there’s an electric field pointing in one direction you have to find some way of overcoming it.

Tom

p.s.- This is not superconductivity. It’s not a way to make a circuit in which current flows almost indefinitely. If the electrons have to bounce off of something to make the return trip, they will lose some energy in the bounce.
mike w.


(published on 10/22/2007)

Follow-Up #1: synchrotron losses

Q:
What if we make an electron beam (in a vacuum) and then apply perpendicular magnetic field to force the electrons go in a circle? Then turn off the electron beam. The ring of electrons should remain circulating. You could transfer energy to it by ramping up the magnetic field. And, as far as I understand, the only loss mechanism would be synchrotron radiation. Would electrons going around a circular orbit in a vacuum in magnetic field behave somewhat like a superconducting coil? Would the current persist forever?
- Valentin (age 36)
Mountain View, CA, USA
A:

Yes, this sounds just like a common accelerator. Those radiative losses will keep the current from going forever. Also, the repulsion between the electrons will spread the beam out.

Mike W.


(published on 03/21/2015)

Follow-up on this answer.