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Plasma frequency /density

  1. Nov 2, 2012 #1
    I was watching some well know professor giving speech at "CERN" about plasmas and he said like quote: "Plasma frequency , which is a characteristic frequency of the electrostatic oscillations in the plasma , depends on the square root of the plasma density"

    So by that could I assume that the denser the plasma the higher the frequency of oscillations and hence the temperature of the plasma?
    As I understand the oscillations and their frequency is the particles bouncing around and their respective kinetic energy as the higher the frequency the higher the kinetic energy of a given particle ?

    Thanks.
     
  2. jcsd
  3. Nov 2, 2012 #2

    mfb

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    How is that related to temperature of the plasma?

    The plasma frequency is relevant for macroscopic oscillations of fields and particles, not for the random movement (and other energy types) due to the heat.
     
  4. Nov 2, 2012 #3
    Well I thought that compressing a gas/plasma to smaller volumes increases density and that increases particle movement hence the heat of the overall gas/plasma?
     
  5. Nov 2, 2012 #4

    mfb

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    How is that related to your previous question?
    If you heat a plasma by compressing it, you increase the density and the plasma frequency.
    If you heat a plasma by simply adding energy at constant pressure, the plasma expands and you reduce the density and the plasma frequency.
     
  6. Nov 2, 2012 #5
    Thanks mfb you basically confirmed my thought just that I messed up with my description of what i actually wanted to know about.
    So basically heating plasma lowers it's density as it expands and if it wouldn't be confined or even compressed by outer forces then it would expand to a point were it would cool off and turn into a gas state is that right?

    But by increasing pressure you shorten the particle traveling distances and get them closer so that means higher frequency, more faster moving particles and higher temp because of all that action?
     
  7. Nov 2, 2012 #6
    Something tells me you are misinterpreting the meaning of "plasma frequency". A plasma or electron gas can have a plasma frequency without oscillating at that frequency. It a property of plasma's response to external fields. If you dont apply an oscillating external field then your plasma still has a plasma frequency, its just not oscillating at that frequency.

    Like a system can have a resonance frequency, that doesnt mean it is necessarily resonating at that frequency.
     
  8. Nov 2, 2012 #7
    Oh ok , I hope Modus you are right, because now it makes sense.So setting the external applied electromagnetic field to match the plasma frequency would make the "system" resonate , if I understand how resonance behaves in materials like metal bridges or acoustic enclosures then tell me how would a plasma behave? Would the rate of fusion get higher or would it just become more disordered?

    And different density plasmas would have different resonant frequencies I assume.?
    Is the density the only factor that cotribute to the plasma frequency or are there other factors like what type of plasma it is (the chemicals which make it up like hydrogen or some other) ?
     
    Last edited: Nov 2, 2012
  9. Nov 2, 2012 #8

    mfb

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    Wait, where does fusion come from now? If you have some specific application in mind, please specify which. If you manage to increase the speed of the atoms (tricky) and/or get some focus (??), maybe. Plasma oscillations are mainly oscillations of the electrons, not the ions.

    Right

    The effective electron mass and, at high temperature, the temperature: plasma oscillation
     
  10. Nov 2, 2012 #9
    If in a volume of plasma (or in a metal) the electrons get separated from the positive charges (for example by applying an external electric field), an electric field will build up due to the charge separation. This in turn, will pull the charges back together. They will overshoot and the system will oscillate and behave analogous to a harmonic oscillator. The frequency of oscillation is the plasma frequency. Google "drude model" to find a proper derivation. The plasma frequency has nothing to do with fusion.
    Concerning temperature: If temperatures are high, pressure in the electron gas will play a role and you will not get a simple plasma oscillation (the whole plasma volume oscillating), but an electron sound wave ("bulk plasma wave" or langmuir wave) excited by your applied oscillating field.
     
  11. Nov 3, 2012 #10
    Ok I don't want to go completely offtopic asking this but if we have a plasma and we would like to heat it up could we use very fast moving electromagnetic fields that would drag the plasma ions around and create a tangential force on them (if we speak about a tokamak type of plasma confinement that is round)? Or the centripetal force even with huge plasma angular speeds would be too negligible to make a significant increase in temperature?


    Is it true that once the plasma is sufficiently heated and ions stripped from electrons ,the movement of the plasma creates electromagnetic fields on it's own because of the electron and ion opposite charges? So would it be fair to assume that if you could maintain a hot plasma moving around in a spherical like confinement it would create it's own magnetic fields and the higher the movement the stronger the fields would be?
    And how those plasma induced fields act on the plasma itself , do they disrupt the plasma or break it apart?


    @MFB I'm sorry didn't mention where fusion comes in I'm just watching a video lecture of David Campbell ,the Asssistent Deputy Director-General of Iter, he mentions some very interesting things about plasmas and physics in those videos so as I follow i get some questions that I am asking here.
     
    Last edited: Nov 3, 2012
  12. Nov 3, 2012 #11

    mfb

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    Radio frequency heating is a common method to heat plasmas. I don't see a centripetal force there.

    Macroscopic fields won't build up on their own, as the electrons (and ions) move accordingly to cancel them.
    No. You can induce a current (and therefore a magnetic field) in the plasma by external fields, this is done in Tokamaks.
     
  13. Nov 3, 2012 #12
    "You can induce a current (and therefore a magnetic field) in the plasma by external fields, this is done in Tokamaks. "

    I guess this is what they refer to as the "transformer drive" the electromagnet around being the wire and the plasma being the core if we compare to a typical transformer ?
    Also they are planning of going into a "plasma burn" state were the plasma could self sustain with heat from alpha particles, then they could turn off the external heating mechanisms but they would still have to keep the electromagnet that produces the magnetic fields to confine the plasma right?


    Oh and another thing , you say that "Macroscopic fields won't build up on their own, as the electrons (and ions) move accordingly to cancel them."


    But then how does a magnetar or neutron star gets it's huge magnetic field? I guess the answer lies in the magnetohydrodynamics which I have a little problem of understanding in the context of the above stars.
     
  14. Nov 6, 2012 #13
    Anyone? Like mfb? :D
     
  15. Nov 6, 2012 #14

    mfb

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    The forum messes up subscriptions sometimes somehow.

    Right. Even worse, they need a change in the current - and as current is limited, a (current) tokamak has to work in pulses.

    They are not in an equilibrium.
     
  16. Nov 6, 2012 #15
    hmm i quite not follow , what you mean by they are not in equilibrium ? Could you please explain a little more detailed?
     
  17. Nov 7, 2012 #16

    mfb

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    If you wait 10^12 years (or whatever), I think the neutron star will gradually lose its magnetic field, as all internal processes come to a rest. The star keeps spinning, but probably without a (significant) magnetic field, unless I am missing something. The rotation itself might induce a charge separation, no idea if that is relevant.
     
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