Science Porn

[SteelCurtain]

boiyoiyoiyoing

[pocaracas]

poca, tell us more about how do you get a density profile from *one* antenna at the edge? Inquiring minds want to know

Also, is eta just some sort of number density of what and R the distance from the edge? And why does it have plateaus! And what does the H in H mode stand for?

Also, did many of the kids cry when you were done? srsly, I'd like to see how you sold that material to 12 year olds!

I just noticed that I didn't answer the questions!!

1. One antenna probes a line of sight along the midplane of the tokamak. It is then assumed that it is poloidally symmetric, that is it has the same density along a particular magnetic field line.... imagine my line going around this here cross-section:


2. eta_e is the electron density and the units are electrons per cubic metre.
2.1. R=4 is the wall (the antenna is a bit further out, but nothing's supposed to be there).
2.2. Plateaus in density appear due to a transport barrier that is present at the edge... plasma tends to move outward (you know... from hot to cold, in this case, core to edge), so it gets more or less equalized up to that barrier.
2.3. H stands for High confinement, as opposed to Low confinement. The profile I showed in my last post is an L-mode one.

3. No kids were harmed during the presentation, so none cried. 2 or 3 did struggle to stay awake.... but some 5 looked really awed by it... of course, this sort of pic helps:

[bonus!]

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I had to explain everything in simple language... from magnetic fields due to electric currents, to the reverse, to atoms and electrons flying around and ionizing, and isotopes and nuclear fusion in tokamaks and in the sun...

[Iroscato]

I don't understand any of these words, so I'm just gonna assume Poca and Alex are having some sort of wizard battle.

[Alex K]

Your density profile looks very nice, poca Good stuff!

I don't get several things: what is the spectrogram's x axis if you say that for each probe frequency you look at its maximum. Maximum in what? I think I misunderstood something.

A more technical question, In your dispersion formula, I miss a complex variable prescription what to do with the pole of the greens functiony thingy in the integrand. Or does that just give you something finite automatically?

[pocaracas]

Your density profile looks very nice, poca Good stuff!

I don't get several things: what is the spectrogram's x axis if you say that for each probe frequency you look at its maximum. Maximum in what? I think I misunderstood something.

Ah... right...
What is a spectrogram if not a succession of DFFTs?
For each probing frequency, we get a DFFT, so the X-axis of the spectrogram is the probing frequencies.
The maximum I refer to is that of each such DFFT, which is, in principle, the beat frequency resulting from the beating of the reference wave and the other wave that went in the plasma and came back. This beat frequency has a straightforward relation to the wave's group delay, so I plot it already converted.

A more technical question, In your dispersion formula, I miss a complex variable prescription what to do with the pole of the greens functiony thingy in the integrand. Or does that just give you something finite automatically?

Oh... heck... I got mixed up with the names there... damn me and this copying that formula from someone else's paper!!
w_p, in that case, is not the plasma frequency, like I said earlier, but the probing frequency.
The integral, which is calculated as a sum, stops at the last point before the one relative to itself.... actually, it's the d_phi that matters, so it's more like half a point before... We use the algorithm described in section III of this paper, but with a slight correction on the A_i,j, because that thing is numerically unstable.

Since we never reach the pole, we never run into any trouble!

[Longhorn]

Your density profile looks very nice, poca Good stuff!

Entirely scientific question: is that a nerdy pickup line?

[pocaracas]

Your density profile looks very nice, poca Good stuff!

Entirely scientific question: is that a nerdy pickup line?

[Alex K]

Why do you get beats? Don't you get the same frequency back with a constant phase shift?

[pocaracas]

Why do you get beats? Don't you get the same frequency back with a constant phase shift?

the phase shift is dependent on the time the wave travels in the plasma until it gets to the reflecting layer... along the way, it meets layers which have a non-zero index of refraction which must be accounted for... and these are not always in the same position... or else, why have such a diagnostic at all?

[Alex K]

Why do you get beats? Don't you get the same frequency back with a constant phase shift?

the phase shift is dependent on the time the wave travels in the plasma until it gets to the reflecting layer... along the way, it meets layers which have a non-zero index of refraction which must be accounted for... and these are not always in the same position... or else, why have such a diagnostic at all?

That sounds difficult... is that is why you integrate over those frequencies in your formula? To account for all the stuff happening in between? Or is that thinking too naively...