What materials are used for the tokamak toroid wall?

[artis]

There are two questions that arose in my mind, first of all tokamaks use toroidal field coils which create a toroidal field within the torus to shape the plasma and confine it, but here is a question, the toroid coils have a static B field produced from a DC current in them, what kind of metal or metal alloy is the torus wall made out of which can satisfy both the chemical parameters necessary for such a wall as well as structural mechanical loads and at the same time have a magnetic permeability which is low or that of air in order to let the field lines from toroidal magnets through because I would imagine if one made the torus out of ordinary steel the torus would have most of the field lines concentrated within itself and the plasma would be left without proper guidance right?Secondly since the tokamak is also a transformer, plasma serves as the secondary winding, essentially a short circuit, but for this to work the toroid chamber cannot be conductive or atleast has to be electrically open at least at one point in order so that the torus chamber itself doesn't become the "conductor" and take up most of the induced current which is made for the plasma instead.

I would love to hear how these matters are set up and resolved, especially the toroid coils and the blanket structure because between the toroidial field coils and the actual plasma there is a lot of material and I can't think of how this material inbetween can not interfere with the B field in a destructive and minimizing way.

 

[phyzguy]

There are stainless steel alloys, called austenitic stainless steels, which are essentially non-magnetic, and have relative magnetic permeabilities very close to 1. I think these materials are typically used for the chamber walls. As for your second question, you are absolutely right that the chamber wall needs to have one or more non-conductive "breaks" in it so that current cannot flow in a loop and so the magnetic field lines can penetrate.

 

[artis]

exactly otherwise the induced current would heat up the torus chamber instead of the plasma inside. I do wonder how many "electrical" breaks there are in the toroidal chamber and how that affects it's structural performance but probably most notably it's radiation performance, I guess they have to join the camber stainless steel with some other material to preserve the vacuum of the chamber as well as keep the lithium blanket in place etc, it is just kind of hard to find answers to these on google because although tokamaks are much discussed but mostly on a popular science level.

if anyone has any resources, video or otherwise I would love to see them

 

[Astronuc]

Austenitic stainless steel 316LN is the reference structural alloy for ITER.
https://www.neimagazine.com/features/featurefabricating-iters-first-wall-4551656/

The 440 blanket modules that completely cover the inner walls of the vacuum vessel protect the steel structure and the superconducting toroidal field magnets from the heat and high-energy neutrons produced by the fusion reactions. As the neutrons are slowed in the blanket, their kinetic energy is transformed into heat energy and collected by the water coolant. In a fusion power plant, this energy will be used for electrical power production.

https://www.iter.org/mach/BlanketDeveloping Structural, High-heat flux and Plasma Facing Materials for a near-term
DEMO Fusion Power Plant: the EU Assessment.
https://arxiv.org/ftp/arxiv/papers/1408/1408.3546.pdf

From earlier - ca. 1991 - https://digital.library.unt.edu/ark:/67531/metadc1090943/

 

[phyzguy]

As I understand it, the ITER vacuum vessel is composed of 9 D-shaped wedges that fit together to make the complete torus. You can see the structure at this site. I think that the electrical breaks are between each of these segments, meaning that these segments are not electrically connected with one another. I'm not sure about this, maybe someone with more knowledge of the structure can comment, but I don't know where else the breaks could be. This picture shows a drawing of one of these d-shaped wedges being fabricated, so you get a sense of the huge size of ITER.