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Name: Unknown
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Date: Around 1993


Question:
Could somebody out there please explain to me how an RF cavity is used to accelerate a particle in a cyclotron? Specifically, I do not understand how a klystron tube is used, and what the frequency has to do with anything. Finally, how or what do the particles absorb, RF energy , a wave, or electricity?



Replies:
You might want to check past issues of Scientific American for details on how particle accelerators work - I am sure I have read something in there. Either there or an issue of "Physics Today" in the past few years might have something. Anyway, here is a very rough answer. The electrons are moving very fast once they get to the RF acceleration section (you have to start them off with quite a bit of energy some other way). When they pass by one of these "klystrons," they do it in a rather short time. The klystron is arranged so that it produces an electric field along the direction the electrons (or other particles) are going, and this electric field oscillates (switches from one direction to the other) with the frequency in question. As long as the electric field is pointed in the forward direction, so that the particles accelerate in it, during the short period of time that the electrons are actually in that vicinity, you will get a net acceleration of the electrons. Basically, accelerating charged particles with a constant field requires enormous voltages (a trillion volts for the new SSC) which cannot be achieved. But oscillating electric fields can do the trick without needing more than a few hundred thousand volts at a time. It does require careful timing of the "bunches" of particles that get accelerated together. The energy comes from the electric field - it is kind of a normal absorption process.

A. Smith



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