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Diffusing an electron beam throughout a chamber

  1. Sep 26, 2009 #1
    Here's the basic setup: A cathode ray tube is producing an electron beam which leads into a chamber. Would it be possible to put some sort of lens at the entrance of the chamber, so as to diffuse the electrons throughout said chamber.

    Just to be clear, my main question is about the lens, let me worry about producing the electron beam.
  2. jcsd
  3. Sep 26, 2009 #2
    Yes it is possible and such arrangements are actually called electron lenses.A major use of such lenses is to bring a divergent beam of electron to a focus on the screen of a C.R.O.I'm not sure what sort of properties your lens should have.
  4. Sep 26, 2009 #3
    Are there electron lenses that scatter electrons, in the same way there are optic lenses that scatter light i.e. diverging lens?

    And would such a lens, slow down the electrons sufficiently, so as that the electrons could be scattered and diffuse throughout the chamber?
  5. Sep 26, 2009 #4
    I am reasonably confident that such lenses could be designed to meet your requirements.If you had just an electron gun and a single anode then the beam would would have some divergence and this can be manipulated by other electrodes.
  6. Sep 26, 2009 #5
    Magnetic lenses (a quadrupole pair for example) can expand the beam. The size of the expanded beam will depend on the incident electron beam size and divergence. A 1 mm-diameter electron beam with a divergence of 1 milliradian coud be focused to a point say 5 cm after the lens, and the new divergence would be about 0.1/5 = 20 milliradians. An alternate approach is to use a pair of ferrite magnets (or electrostatic deflectors) to quickly (say at roughtly 1000 Hz) simultaneously sweep the beam back and forth, and up and down (at slightly different frequencies).
    Bob S
  7. Sep 26, 2009 #6
    There is a notion of a virtual cathode. Your electron gun will charge positively and the emitted electrons will tend to return. It is a serious problem. You have to collect the electrons and return them, i.e., you have to make a closed circuit. Otherwise your facility will not work.
  8. Sep 27, 2009 #7
    Hmm, what do you think I should read up on in order to have a greater understanding of these issues.
  9. Sep 27, 2009 #8
    Something basic on accelerators, experimental arrangements. I am not good at that, I heard of it many years ago from experimentalists.
  10. Sep 27, 2009 #9
    Do you think the best way to fix these problems would just be to attain a CRT and conduct experiments with the supervision of my physics teacher (Safety First). All the while reading up on CRT's and accelerators in general.
  11. Sep 27, 2009 #10
    Yes, it would be the best way. Think and talk everything over.
  12. Sep 27, 2009 #11
    Yes I think this would be a good idea.Familiarise yourself with a CRT by just playing around with it.Try adjusting the focus and brightness controls.CRTs are pretty safe so supervision is not a big issue but do check with your teacher first.People here might be able to give clearer advice if you explained more precisely what it is you want to do.
    Last edited: Sep 27, 2009
  13. Sep 27, 2009 #12
    Basically, I want to isolate some electrons in a chamber. I plan to do this by allowing the electron beam from a CRT to enter the chamber through an electron lens that will scatter the electrons throughout the chamber.

    By the way, I'm planning to remove the screen of the CRT so as that the beam can feed directly into the chamber. The chamber would be directly connected to the CRT so it would be one large vacuum.

    Also, this is a component of an idea that has patent pending. So if I hint too much on the forum, you still can't win the Nobel prize without me.
  14. Sep 27, 2009 #13
    I think this is about the same information as you gave in your opening post and it seems to me that an electron lens or something similar would indeed be your best bet.I wish you the best of luck with it.:smile:
  15. Sep 27, 2009 #14
    Hello ScienceNerd-
    First, you should decide on what energy electrons you need. If that number is less than several KeV, then perhaps a CRT is not the best source. Also decide on what current you need. The current is basically the required density (electrons/cm^3) divided by the neutralization rate (loss per cm^3) due to residual gas. Finally, if you need lots of low energy electrons, like 100 milliamps at a low energy (say <= several 100 volts). then cut open and use a 5U4 (rectifier) or 6L6 (power tetrode), both good old standby audio amplifier vacuum tubes. The 6L6 might be a better choice if you want to control the output current with a grid.
    Bob S
  16. Sep 29, 2009 #15
    Could any of you suggest some equations I should learn to manipulate, in order to have a more accurate idea of the components I'm going to need.

    Also, how does an electron lens affect the energies of the electrons. Do I have to allow for energy changes when calculating volts and such?
  17. Sep 29, 2009 #16
    Hi ScienceNerd-
    Do you want the electrons to stop in the vacuum chamber, or pass through the chamber and hit the opposite wall? How big is the chamber: 100 cm3, 1 liter, 4 liters? How uniform a distribution of electrons do you want? What kind of electron density do you want (need)? 3 x 109 molecules per cm3?
    Bob S
  18. Sep 30, 2009 #17
    (i)I want the electrons to stop in the vacuum chamber.
    (ii)I suppose for the purpose of testing, it ought to be fairly small. I would think 30cm3.
    (iii)I need the electrons to be as uniform as I can get them without any complex technologies.
    (iv)As for the electron density, I'm not sure. What factors would I need to take into account in order to determine the required electron density?
  19. Sep 30, 2009 #18
    Hello Sciencenerd-
    Would you be willing to cut the glass off an old audio amplifier tube such as a 5U4 or 6L6, put it in a small vacuum chamber, pump it down to say 10-8 torr, and then turn on power to the heater? You can get probably 100 milliamps of 3000 deg C electrons (~.25V +grid V - work function) until space charge chokes off the current.
    Bob S
    Last edited: Sep 30, 2009
  20. Sep 30, 2009 #19
    I would, but I'll have to read up on all them there fancy words first.
  21. Oct 1, 2009 #20
    Is this the apparatus you're talking about? http://media.audiojunkies.com/Shigeki-Yamamoto-A-09S-Tube-Amplifier.jpg [Broken]
    Last edited by a moderator: May 4, 2017
  22. Oct 1, 2009 #21
    Hello Sciencenerd-
    First, make sure you have a suitable vacuum vessel, and can pump it down to roughly 1 x 10-8 torr. Do you know what pressure is suitable for your experiment? Why isn't 10-3 torr suitable? The difference between 10-3 and 10-8 torr is a lot of expense and effort, like having an ion or turbomolecular pump, and maybe also a liquid nitrogen trap.
    Bob S
  23. Oct 3, 2009 #22
    I've been doing some light reading about pressure, but my physics book doesn't mention anything about the unit of pressure Torr.
    What factors would I have to take into account in order to understand the required pressure for my experiment?
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