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

What happens if you send a electron beam in a plasma?

  1. Mar 30, 2014 #1
    What happens when a electron travels through the plasma?
    Would it be likely that most of the electrons hit a nuclei or not?
    Would a beam of electrons work like a current through the plasma?
    Can you send electron beams with different voltage, and would it matter? In a hot plasma it's difficult to rise the voltage because the resistance is so low.

    That's the issues I wonder of. Maybe there are other issues or aspects of this. I'm all ear.
  2. jcsd
  3. Mar 30, 2014 #2

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Plasma already has electrons in it - the thermal energy is high enough to prevent atoms from forming.
    The electrons an an additional beam would interact with what it finds there and scatter.
  4. Mar 30, 2014 #3
    So it would not be the same thing as putting two electrodes with different voltage in the plasma. That what i'm looking for, a way to send current through the middle of the plasma without physical contact or almost none.
  5. Mar 30, 2014 #4


    User Avatar

    Staff: Mentor

    Generally the preference is to have the plasma be neutral, otherwise a discharge or plasma instability is more likely.

    In a magnetically confined plasma, four types of heating are used:
    1. Induction heating in which a current is induced in the plasma, much the way a current is induced in a conductor or transformer,
    2. Compressive heating in which the plasma is rapidly compressed (magnetically) by increasing the confining magnetic field,
    3. Microwave or EM heating in which electromagnetic waves heat the plasma, and
    4. Neutral beam heating in which a beam of atoms are injected into the plasma.

    The neutral beam heating involves stripping [deuterium] atoms of electrons [ionization], accelerating the ions (nuclei) toward the plasma, then neutralizing the ions (recombination) prior to injection into the plasma.

    The electrons would scatter off the nuclei and electrons, but the majority of energy transfer would be to the other electrons in the plasma. In a plasma, the electrons and nuclei are electrically (coulombically) coupled, that is they are coupled by the coulomb forces. It's easier to heat the electrons, since they are lighter than the nuclei.
    Last edited: Mar 30, 2014
  6. Mar 30, 2014 #5

    Simon Bridge

    User Avatar
    Science Advisor
    Homework Helper
    Gold Member
    2016 Award

    Confine the electrons to a conductor or some sort of guide that keeps the plasma away.

    I had a vague recall that plasmas can conduct electric currents, looked and found:
    and: http://www.pma.caltech.edu/Courses/ph136/yr2006/0619.1.K.pdf

    ... but I think you want to get electrons physically from one end of a container of plasma to the other.
    Maybe hoping to exploit Debye shielding or something?

    Some questions immediately occur to me:
    What do you mean by "physical contact"?
    What do you hope to achieve by this?
  7. Mar 31, 2014 #6
    In high school chemistry you probably learned about ideal gases. In doing so you probably imagined gas molecules as hard spherical objects that interact via direct billiard ball like collisions. This picture is not how ions and electrons interact in a plasma.

    Ions and electrons are charged and thus they interact over large distances through the Coulomb force. As an electron (or ion) travels through a plasma its trajectory is constantly being "nudged" by the combined interaction of many many many particles in a plasma. In plasma theory, we call these interactions Coulomb collisions and we can define both an effective collision frequencies and an effective mean free path. But its important to keep in mind that these interaction occur over a distance. This is fundamentally different from what occurs in gases.

    What happens to an electron beam as it passes through a plasma really depends on both the plasma conditions and the beam energy. In general, a beam will slow down (transferring its energy to the plasma) but it will also decollimate (we can this velocity space diffusion). However, under certain conditions the beam can actually be accelerated by the plasma up to relativistic energies.

    The plasma potential is really set by what happens near the plasma boundary in what we call the sheath. The dynamics of the sheath depend on both the plasma conditions, but also the wall conditions.

    You can drive a current through a plasma by putting electrodes on two ends of the plasma and then applying a voltage across the electrodes. This is done in many experiments. However you still have to take the sheath potential will calculating the effective potential that the plasma sees.

    There are five methods. The fifth is alpha (or fusion product) heating. While not a major source of heating for current MFE experiments. Alpha heating is central is to idea of a sustained fusion reaction.

    Its better to call the first form of heating ohmic heating, not induction. There are a number of ways to drive current in a plasma. As the current resistively decays it will heat the plasma, just like current flowing through a wire.

    Good for you. I encourage to to continue to wonder, but I also encourage you to start building a foundation of plasma physics.

    Chen's text book "Introduction to Plasma Physics and controlled fusion" is a good starting point. It contains introductory material on both Sheaths and Coulomb Collisions. Its designed as a text book for an upper lever undergraduate course or an introductory graduate level course.

    I also recommend Dr. Callen's personal web page:

    He has links to his course notes of many graduate level plasma courses. (If you're interested in Coulomb Collisions check out the 725 notes).
  8. Mar 31, 2014 #7
    I had not read the articles yet. The second was more like a book. Thank you for them.

    I'm nor sure if physical contact in a hot and reacting plasma would destroy electrodes, but to me it's likely. Better to avoid physical contact with 100 milj plasma and even hotter nucleus and neutrons.

    What I hope to achieve with an electron beam is heating without melting electrodes and electrical heating that works even in the hot low resistance plasma. What I also hope to achieve is to heat a narrow cylindrical volume. Let's say the whole volume is 4 m long and 2 m diameter. I want to heat most in the middle, say a 20 centimeters across and 4 m long volume.

    But any beam will do, as long as it's a bit focused. There are other ways of heating as I learned, and some seams to be in beam form, like D2 injection or microwaves if you can focus them
    Last edited by a moderator: May 6, 2017
  9. Mar 31, 2014 #8
    Thanks for those encouraging words. I have no time to comment your hole post, but actually I have ordered that book. Should be coming soon. I do need such books I understand, to help me understand:smile:
  10. Apr 1, 2014 #9
    the_wolfman I don't understand all of it, but too me plasma seam to be a bit like foam that is very fluent, but stick together a bit and fill the space which both gases and foam does. I also understand that the electron in a electron beam can collide both with nucleus and other electrons, and the later are not so well defined in shape but quite blurry and therefore get hit more often then suggested.

    I know what Coulomb forces is. Repelling due to same charges of two or more particles, so I think coulomb collision has to do withe these repelling forces, but it isn't, is it? You say that even particles some distance away get affected. To me its like hitting a membrane or a bubble.
    That the beam slows down is easy to understand of your explanation. But accelerate I don't understand and maybe it's not so important or?

    Thanks a lot for the facts. I have to read more about this and I will. I'm eagerly waiting for Mr postman that will bring me "Introduction to Plasma Physics and controlled fusion":)
  11. Apr 2, 2014 #10
    Its better to think of a plasma as a conducting gas. A lot of interesting things happen because plasmas are very good conductors, but plasmas are very similar to gases in many respects.

    Coulomb collision happen because multiple particles are interacting via the Coulomb force. The collisions happen at a distance because the Coulomb force interacts between two particles that are separated by some distance. The Coulomb force is not a contact force.

    The plasma resistance to the beam decreases with increasing beam energy. This means that there is a critical energy above which the plasma resistance can no longer "balance" the applied potential the drives the beam. Above the velocity, the applied field would accelerate the beam, weakening the resistance, causing the beam to accelerate even faster, etc. This effect is due to the nature of Coulomb collisions, and it would not occur in a neutral gas where billiard ball collisions dominate. Is it important? Well if you want to shoot a beam though a plasma, you should understand this. If not, you run the risk of accidentally creating a relativistic electron beam that burns a hole through your experiment.
  12. May 24, 2014 #11
    When an electron beam propagates through a plasma, you will get a streaming instability. Double humped distribution functions are not stable.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook