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Can a plasma give off black body emission?

by rwooduk
Tags: black, body, emission, plasma
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rwooduk
#1
Aug14-14, 06:22 AM
P: 132
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?
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mfb
#2
Aug14-14, 01:27 PM
Mentor
P: 12,022
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).
rwooduk
#3
Aug14-14, 03:46 PM
P: 132
Quote Quote by mfb View Post
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).
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?

256bits
#4
Aug14-14, 05:06 PM
P: 1,494
Can a plasma give off black body emission?

You should reference your statements.
Other wise, answers become off track quite easily.

A mechanism of light emission from a single bubble sonoluminescence (SBSL) was formulated by assuming that the source for the light emission is bremsstrahlung in partially ionized gases. 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. Also the spectrum was measured and the observed results were compared with the calculated ones. Calculated and experimental results yield common spectral behavior in the visible region: the spectral radiance shows power-law dependence on wavelength with an exponent of -2.5. The SBSL spectrum which is characterized by the continuous one with no major peaks has been confirmed experimentally. The bubble dynamics model proposed also takes account the afterpulse characteristics of SBSL.
http://journals.jps.jp/doi/abs/10.1143/JPSJ.69.112

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

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

which makes very little sense to me, how can something be bremsstrahlung AND black body?
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.
rwooduk
#5
Aug15-14, 06:06 AM
P: 132
Quote Quote by 256bits View Post
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?
Yes that's the one, and ok will do, thanks.



Quote Quote by 256bits View Post
That should depend upon the density of the plasma, no?
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?


Quote Quote by 256bits View Post
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.
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!
rwooduk
#6
Aug15-14, 06:57 AM
P: 132
Update, during my research found an interesting statement which I think helps:


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

This could happen because the observation of a spectrum which accurately matches a blackbody does not absolutely require that the conditions generating the blackbody be the same as those invoked in the traditional derivation of an equilibrium blackbody source. .
And leads onto (SBSL)

In this case insight into the light-emitting mechanism cannot be obtained without additional information. On the other hand, if the blackbody spectrum is actually produced by
conditions that match the traditional conditions (matterlight equilibrium, and a high density of states, then the observation of an SL blackbody spectrum leads to interesting
consequences regarding the bubble’s interior.
SO it says something of the design of a BB emitter, that it may not necessarily be a BB emitter in the traditional sense.


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