Can a plasma give off black body emission?

  1. This may seem like a strange question, I'm trying to differentiate between the spectra from a plasma and the spectra from a black body, is there a difference?

    I'm thinking the spectrum from a plasma would be that of bremstrahhlung as it is ions flying everywhere, so it would not be possible for it to have the spectrum of a black body.

    but wouldn't a plasma that was enclosed in an area be a black body?

    if someone could help me differentiate it would be really appreciated,

    thanks in advance

    edit in particular this statement:

    This emission mechanism turns out to be same as the black body radiation with finite absorption, which confirms that the principal mechanism of SBSL is bremsstrahlung with slight black body emission nature.

    which makes very little sense to me, how can something be bremsstrahlung AND black body?
     
    Last edited: Aug 14, 2014
  2. jcsd
  3. mfb

    Staff: Mentor

    Plasmas are often very good approximations to black bodies (see stars like the sun as example).

    If you integrate over the bremsstrahlung spectrum with the kinetic energy distributions, you might get something close to a thermal spectrum (not sure).
     
  4. Many thanks for that, yes stars did come to mind! Will look further into the integration you suggested thanks!

    also it occurs to me that a plasma will be a VOLUME emitter, where as the black body will be a surface emitter, therefore will not show exact black body spectra, that sound right?
     
  5. You should reference your statements.
    Other wise, answers become off track quite easily.

    http://journals.jps.jp/doi/abs/10.1143/JPSJ.69.112

    I outlined the statement in red.
    Is that what you are referrring to?

    That should depend upon the density of the plasma, no?

    The species within the plasma do not share only one specific kinetic energy, ie velocity, but have a range from 0 to some high number. Probably a distribution with something to do with a famous physicist, maybe Boltzman, not sure, depending upon the temperature. With a great number of interactions, at different velocities, the EM approaches thermal or blackbody.

    Having said that, though, the spectrum from a mercury gas lamp, or neon sign shows a line spectrum, or close to it, since some of the species are interacting. Here the emmission, absorption of electrons is responsible for the majority of electromagnetic radiation, and this plasma does not show blackbody.

    Not an expert in the subject, but hpefully this will spur some discussion.
     
  6. Yes that's the one, and ok will do, thanks.



    Yes, this did occur to me, for if you imagine a very dense plasma the 'internal' ion emission will only meet the outer and inner ions, it will be the outer ion emission that is observed outside the plasma. (assuming spherical confinement).

    So for a very dense plasma the spectra will be extremely close to BB radiation spectra, from only the surface ions, is that what you believe the paper is referring to when it says "bremsstrahlung with slight black body emission"?

    I don't understand the term "finite absorption", if it was a BB then it would not be finite, so I'm unsure how they can classify it as such. Is it a way of saying 'almost' BB radiation like?


    I think I see where the paper is going, that it is bremsstrahlung in nature but tends towards the characteristics of a BB, would that be a correct interpretation?

    Thanks very much for the reply, very helpful!
     
  7. Update, during my research found an interesting statement which I think helps:


    (from VOLUME 88, NUMBER 19 PHYSICAL REVIEW LETTERS 13 MAY 2002)

    And leads onto (SBSL)

    SO it says something of the design of a BB emitter, that it may not necessarily be a BB emitter in the traditional sense.
     
  8. I found this topic is related to my question and hope someone give me feedback.

    My question is regarding continuous spectra in laser-induced breakdown. There are many figures have shown atomic spectra lines (spikes) on top of the continuous spectra at early stage of laser-inducded breakdown (e.g., figure 2 and 3 in http://www.opticsinfobase.org/ao/abstract.cfm?uri=ao-49-13-C80). I am wondering whether the idea of blackbody radiation still applicable for this case.

    Then my questions is:
    Can we determine the gas temperature using blackbody (Planck's law) although there are continuous spectra with atomic spectra lines?
     
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