|MadSci Network: Engineering|
Excellent question! Your thoughts on the subject are correct, but the trick lies in tricking the electrons (or any other charged particle, e.g. a mass spectrometer). What you do is build the anode with a hole in it. This is almost certainly what they taught you, and you were completely correct in thinking that the electrons or ions would just be reattracted to the anode. However, what they did *not* tell you was that immediately behind the anode was another plate with a hole in it, the holes being aligned. This second plate is usually called the suppressor, and the key thing to making this work is that the suppressor is either grounded OR at a voltage opposite in polarity to the anode. [Question for you: if you're working with electrons, what polarity would you make the anode and the suppressor?] So now the electrons coming off of the anode are accelerated and those that make it through the hole in the anode immediately pass through the hole in the suppressor as well. This suppressor (which can be a plate with a hole in it or a grid) shields the electrons that passed through the hole from the anode and a rough beam is formed so we've sort of "tricked" the electrons. Now, generally you want a fine (collimated) electron beam so you put a second anode/suppressor section in. As before, not all the electrons will hit this second hole BUT those that do tend to be well collimated. You are correct in thinking that you can do this stunt any number of times and the end result will be a high velocity, finely collimated beam of electrons which is generally steered magnetically. The only limitations on this technique are physical size and voltage breakdown between the various plates. But this is the method used. A variant of this technique is sometimes useful when you want to get as many electrons or ions as possible and you don't care about forming a beam. One example would be a vacuum tube. What you do is make the anode a grid so that only a few electrons/ions strike the grid wires; the rest pass right through. Then you use a suppressor grid and make the final anode relatively large. FYI, this is what high-power radio stations do to get the power (several kilowatts/tube) output required. So as you continue your education please don't disdain vacuum tubes; they can be very useful.
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