EM radiation temperature vs particle temperature

[artis]

I just realized I'm having a problem in understanding this.
So let's take an example the CMB is around 160 Ghz and the blackbody temperature within this frequency range is 2.7K which is rather cold as it is close to absolute zero.

Then let's take another example, Iter plasma will achieve about 150 000 000 degrees Celsius. One of the heating mechanisms will be RF heating.
I read that there are Gyrotrons already made for Iter that work at 170Ghz.

Here is what I don't understand, a black body radiating at 170Ghz is close to absolute zero temperature wise, yet pumping 170Ghz radiowaves into a plasma can push its temperature to millions of degrees Celsius?
Clearly the black body radiation frequency of a 150 million celsius object isn't 170Ghz but way way higher.
Ps. I guess there is also a simpler example , a LED, where the actual temperature of the LED die is lower than that of the emitted EM radiation.

 

[hutchphd]

The common definition of temperature only applies to systems of many particles; see Thermodynamic temperature on Wikipedia.

 

[Dale]

Here is what I don't understand, a black body radiating at 170Ghz is close to absolute zero temperature wise, yet pumping 170Ghz radiowaves into a plasma can push its temperature to millions of degrees Celsius?

The RF energy is not thermal. They do not deliver that energy by warming up a bit of material and letting it radiate thermally. The RF is work, not thermal energy transfer. It has very little entropy.

You don’t have to go to ITER for this. Your microwave oven will suffice. Those work at 2.45 GHz and yet can heat your food well beyond 1 K. They also draw a lot of power.

 

[Andy Resnick]

I just realized I'm having a problem in understanding this.
So let's take an example the CMB is around 160 Ghz and the blackbody temperature within this frequency range is 2.7K which is rather cold as it is close to absolute zero.

Then let's take another example, Iter plasma will achieve about 150 000 000 degrees Celsius. One of the heating mechanisms will be RF heating.
I read that there are Gyrotrons already made for Iter that work at 170Ghz.

Here is what I don't understand, a black body radiating at 170Ghz is close to absolute zero temperature wise, yet pumping 170Ghz radiowaves into a plasma can push its temperature to millions of degrees Celsius?
Clearly the black body radiation frequency of a 150 million celsius object isn't 170Ghz but way way higher.Ps. I guess there is also a simpler example , a LED, where the actual temperature of the LED die is lower than that of the emitted EM radiation.

It's only possible to assign a temperature to EM radiation *iff* the spectral content of the radiation corresponds to a black-body spectrum (at some temperature T).

 

[artis]

ok so @Dale and @Andy Resnick and others , so when we normally talk of a black body radiating a black body spectrum we are talking about a substance that is dense aka consists of many atoms.

So a terrestrial fusion plasma is a very transparent low density substance and adding to that the RF energy supplied I suppose heats more electrons than ions so is not doing thermal work on the systems as much as it is supplying direct kinetic energy to a rather small group of particles as compared to the number of particles that would be in a dense substance?
Is this a bit similar to how the Cern's LHC RF cavities themselves being only MW rated can with multiple passes bring up the proton bunches to TeV range ?Can I compare this to AC transformer action where you can exchange low voltage/high current for high voltage/low current in a sense that the plasma RF heating power can bring say a 1 Atmosphere plasma to a higher temperature while the same RF heating power could bring a 5 Atmosphere dense plasma to a lower overall temperature sort of like putting more load on a loaded secondary winding reduces the voltage?I know this is maybe a weird example but I hope you are getting what I want to understand, thanks.

 

[hutchphd]

I believe there are two conceptual problems here:

1. The concept of temperature is really defined only for a system with many degrees of freedom. As such it is measure of the average energy of constituent particles: often it is loosely used to describe the speed of a gas or plasma particle . It is really an average measure
2. Heat Capacity and temperature are different. A denser material will therefore often require more heat to raise its temperature

Neither of these concepts is particular to the physics of plasma.
The assignment of a temperature to EM radiation implies the "photon gas" is in thermal equilibrium. Because they do not interact, the standard way to achieve the required interaction of "photons" is to allow them to interact with matter at a known temperature (e g. the walls of the container)

 

[Dale]

So a terrestrial fusion plasma is a very transparent low density substance and adding to that the RF energy supplied I suppose heats more electrons than ions so is not doing thermal work on the systems as much as it is supplying direct kinetic energy to a rather small group of particles as compared to the number of particles that would be in a dense substance?

I would not say that because a microwave oven uses the same principle to heat dense substances. So it is not the density of the substance that is relevant.

Remember the first law of thermo: $\Delta U = Q - W$. We can increase the internal energy through heat or work done on the system (negative work done by the system). The RF energy is $W$, not $Q$.

 

[jartsa]

So a terrestrial fusion plasma is a very transparent low density substance and adding to that the RF energy supplied I suppose heats more electrons than ions so is not doing thermal work on the systems as much as it is supplying direct kinetic energy to a rather small group of particles as compared to the number of particles that would be in a dense substance?

Terrestrial fusion plasma is a good absorber of microwaves, therefore it must also be a good radiator of microwaves.

So a tokamak must emit almost as much microwaves as a tokamak shaped black-body at the same temperature.

I suggest you calculate how much such object emits microwaves. I guess it is quite small amount.

 

[DrStupid]

The assignment of a temperature to EM radiation implies the "photon gas" is in thermal equilibrium. Because they do not interact, the standard way to achieve the required interaction of "photons" is to allow them to interact with matter at a known temperature (e g. the walls of the container)

In this regard it is not different from an ideal gas.

 

[artis]

Well the photon gas analogy to ideal gas I guess only works for the CMB as it is homogeneous everywhere to a high degree and the space around it has literally no walls with which to interact.

Ok @hutchphd I get your point, temperature for a ordinary gas is the average kinetic energy of the atoms/molecules in that gas with some tail end of higher velocities and some with lower.

@Dale ok but it takes more work to bring a denser gas up to the same temperature and less work for a less dense one, given the surface area stays the same aka losses stay the same it would either need stronger RF power input to get to the same temp or a longer time for the lesser power input but if also losses increase then only a stronger input power could maintain the same temp , is my reasoning correct?

 

[vanhees71]

In this regard it is not different from an ideal gas.

Well, and ideal gas of "massless particles" with no conserved charge, i.e., the entire thermodynamics in thermal equilibrium is determined by the temperature $T$ only.

 

[Dale]

@Dale ok but it takes more work to bring a denser gas up to the same temperature and less work for a less dense one, given the surface area stays the same aka losses stay the same it would either need stronger RF power input to get to the same temp or a longer time for the lesser power input but if also losses increase then only a stronger input power could maintain the same temp , is my reasoning correct?

Sure, but all of that is about the amount or the rate of energy transfer. None of it is about the direction of energy transfer. Here the direction of energy transfer goes against the temperature gradient because it is work, not heat.

 

[jartsa]

Terrestrial fusion plasma is a good absorber of microwaves, therefore it must also be a good radiator of microwaves.

So a tokamak must emit almost as much microwaves as a tokamak shaped black-body at the same temperature.

I suggest you calculate how much such object emits microwaves. I guess it is quite small amount.

If we heated a body with a very special color, extremely far from black-body color, to a ridiculously high temperature, the body could radiate a large amount of microwaves, and nothing else.

That microwave radiation could be equivalent to microwave radiation of Iter gyrotrons, except for not being coherent.

 

[artis]

@jartsa can you elaborate on what you mean by "special color"? because the way I see it as you mean a very narrow frequency within the visible spectrum but then again a narrow frequency can be made into any spectrum from RF to X rays etc ?

ps. @Dale well from what you said I take that my stance is correct when I said that the more dense a substance is (the more atoms confined within a certain space) the more work has to be supplied to raise their average kinetic energy to a certain point.
Like for example we can supply TeV of kinetic energy to a small bunch of protons in the LHC but we could never realistically supply the same kinetic energy to a proton or neutral plasma the size of an average Tokamak as it would require enormous amounts of power to be supplied within a small amount of time in order to overcome losses, right?That being said I do wonder about one thing, a gas temperature is said to be it's average molecule/atom kinetic energy with tail ends of higher and lower energies.
Now is there a trend that the less particles you have in a given system the more precise/narrow their kinetic energy range irrespective of how you supply that kinetic energy aka whether by RF or infrared heating etc?

 

[jartsa]

can you elaborate on what you mean by "special color"? because the way I see it as you mean a very narrow frequency within the visible spectrum but then again a narrow frequency can be made into any spectrum from RF to X rays etc ?

When hot, my special colored object emits no radiation, except microwaves.

So therefore also: When hot, my special colored object reflects all radiation, except microwaves. So it's almost perfectly white.