# Question about linacs

Why are the electrodes of a linac connected to an alternating voltage? Within an electrode the electron moves with a constant speed, and once it is outside of the electrode it accelerates uniformly, because of the voltage. But why is there a need for it to be alternating?

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This "it accelerates at a constant speed" is certainly not what you meant to write. – dmckee Jan 18 '13 at 13:59
@dmckee Indeed it's wrong, I looked it up: uniform acceleration is what I meant. – Ylyk Coitus Jan 18 '13 at 14:01
Have a look at the wikipedia diagram en.wikipedia.org/wiki/File:Lineaer_accelerator_en.svg. Now, an electron is first accelerated between the first (-) and second (+) ring, but then it would be decelerated by the third (-) ring. So you want to reverse the voltage by the time the electron gets through the second ring to keep accelerating it. You do this trick - accelerating across many small voltage steps - so you need a smaller voltage supply. The alternative requires a much larger voltage to get the same final beam energy. – Michael Brown Jan 18 '13 at 14:06
@MichaelBrown Your comment amounts to the same stuff I just wrote as an answer, and if you copied into a answer I'd vote for it. – dmckee Jan 18 '13 at 14:10
@dmckee Sorry, I didn't see your answer go up. Yours is good, no need for me to add another one. – Michael Brown Jan 18 '13 at 14:14