Cyclotrons

(Created prior to 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?
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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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