Controlling divertor temperatures with nitrogen contamination - ASDEX.

[Chris Bradley]

I came across this interesting article whilst looking for something on tungsten:

http://www.ipp.mpg.de/~rrf/bda/Publicat ... ini09a.pdf

The ASDEX is the only fully tungsten tokamak, and in this article they are saying how they have been testing tungsten's resistance to a 'full strength' tokamak plasma [as ASDEX is the closest tokamak in regards heating power versus radius to ITER].

During the two years of work on this, they've been cooling the plasma at the divertor with nitrogen. This has had the effect that not only is the heat loading on the divertors reduced, but it also resists ELMs better. The nitrogen does this by radiating in a very lossy way, which is shown by the last figure.

Also, they indicate that ELMs are less severe anyway, with the nitrogen contamination.

By the sounds of it, you can be sure of this nitrogen cooling being included in ITER design...[not sure that means it'll get build, though!]

[Sounds like they may've been reading up on some of Doug's 'controlled contamination' observations/experiments?!]

[edit - found an english language paper on it...]

[Doug Coulter]

Oh, I only talk to guys at CERN, myself ;~). So I doubt they heard of me. But I did hear that in some other tokosaur experiment that they reduced losses by factor of nearly 100 by getting *rid* of the tungsten which helps radiate energy losses as photons amazingly well. The guys went to carbon for beam-limiters and it just got way-way better. At least the lower Z made it harder for the plasma to emit X rays as loss energy. And IIRC, the amount of tungsten getting into their plasmas was really small, a few PPM.

What I find btw, in the ongoing controlled purity experiments is well described more or less as a Penning effect. Things get more stable and light off easier with a little crud in there -- it differs a little with type of the crud, but so far, it's all more or less the same effect but some things are doing more with less of it.
in rough order, it works out like this in increasing sensitivity to crud:
He (sometimes seems between Ne and Ar though, hard to be sure on that one)
Ne
Ar
Air
Water

And water is always the worst enemy of a good vacuum once you get the other stuff handled and leak tight. Enough comes through even a small amount of Viton that I always see some, even if it's down at e-14 mbar. At my best base pressure (call it e-8 mbar or so), it's always the loudest single line-set on the mass spectrometer.

The thing is, that is NOT when it makes the most fusion, to put it in a nutshell. That stable place is where it likes to sit and just waste input power, not make ions hit ions at full speed. It is most surely NOT a sweet spot in the parameter curves if you want Q. By now, I think we've done enough controlled testing to state that as fact or very nearly so. We've also done this with different grid materials, from carbon to tungsten to titanium to tantalum and that does not seem to make much difference to the effect FWIW.