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EM wave in plasma

  1. Mar 28, 2006 #1
    Can a monochromatic plane EM wave exists in a plasma when [tex]\omega [/tex] is less than [tex]\omega_p[/tex] (the plasma frequency)? If so, is it attenuated?
    I read in a text that a wave incident on a plasma with [tex]\omega < \omega_p[/tex] is completely reflected, but does this mean such a wave can't exist in a plasma?
    If [tex] \omega > \omega_p[/tex], is the (time averaged) power radiated just the same as a plane wave in vacuum ?

    I get [tex] \mathbb{S} = \frac{cE^2}{8\pi \omega}\sqrt{\omega^2 - \omega_p^2}\textbf{z}[/tex] for omega > omega_p.
    Last edited: Mar 28, 2006
  2. jcsd
  3. Mar 29, 2006 #2


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    Your answer to power flux question (Poynting vector) should convince you that a wave whose frequency is less than the plasma frequency cannot propagate.
  4. Mar 29, 2006 #3
    It does not convince me because my solution is not valid in that domain. For when omega < omega_p the wave vector k becomes complex, so the wave has spatial dependence ~e^(-alpha z) for some real alpha. In a wave guide, this implied attenuation, but here it seems to imply that the wave cannot propogate.
  5. Mar 29, 2006 #4


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    That's precisely because it does not propagate and is what "attenuation" or evanescence means.
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