Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Linear Accelerator

  1. Oct 24, 2011 #1
    1. The problem statement, all variables and given/known data
    Could someone please fill in the gaps in my knowledge about Linear Accelerators.

    The drift tubes are spaced equally apart. BUT the electrons speed up in between the tubes so surely the distance between the tubes should increase as does the length of the drift tubes

    What ensures no force is felt inside the tubes? and what causes the force on the electrons between the tubes?

    If I get this I will hoepfully understand Linac's but may come across a few more questions later.

    Thanks for any help

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Oct 24, 2011 #2
    There is no electric field E in the hollow of the tubes but there is an electric field E between the tubes.
    Remember that electric force = Eq
    Last edited: Oct 24, 2011
  4. Oct 24, 2011 #3
    would it not be better so put it like this (I've been doing a lot of reading about).

    no force acts inside the tube as the electrodes are equally attracting in all directions so the electrons continue moving in their initial directions.

    Half way through the tube the AC voltage on the drift tube changes and the electron is repelled from the tube it is in and towards the next tube.

    Only when it is outside the drift tubes does the electron experience an electric field that causes it to accelerate?

    is that a better way to think of it?
  5. Oct 24, 2011 #4


    User Avatar

    I guess you are talking about a multi MeV electron accelerator.

    You should note that accelerators consist of an initial stage (or series of) in which there may be stages where the drift tubes are of different lengths. However, at some large fraction of c, it is no longer true that the electrons will speed up (or at least, to a degree that causes them to desynchronise with the fields). In other words, at high relativistic energies, they begin to gain mass instead of 'laboratory-speed' as they accelerate through the fields, thus the drift tubes of the final stages of an accelerator can be of similar length.

    This is as far as I understand it, and that I presume this is on the point you are raising.
  6. Oct 24, 2011 #5
    If we assume non-relativistic speeds are my points above correct?
  7. Oct 24, 2011 #6


    User Avatar

    Like grz said, there is no e-field in the tubes. At the fringes of the tube, then you get a field. By adjusting the geometry, you can also achieve axial focussing. (This is independent of whether the accelerated particles are relativistic.)
  8. Oct 24, 2011 #7
    just to check why do you get a field at the fringes?
  9. Oct 24, 2011 #8


    User Avatar

    The ends of the tubes are a discontinuity. So the e-fields between the tubes will take on convex forms. They can be further shaped so that any particle riding through will (integration wise) have more work done on it by e-fields inwards than outwards, thus focussing it.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook