U-Shaped Coil Tokamak: Poloidal Field Control

[artis]

It is long known that the toroidal field alone is not sufficient to achieve toroidal force balance within a tokamak due to the field being bent and geometry restricting the field to be of even strength both within the inner and outer circumference of the ring. Induced current is thus used for poloidal field control as a restoring force.

Speaking about the toroidal field I was wondering , is it not possible to use U shaped coils together with the traditional toroidal field coils where the U shaped coil would be mounted on the outer side of the toroidal ring such that It's open part faces inwards , towards the center of the toroid.
This would strengthen the B field in a asynchronous way at the outer part of the toroid where the field is weaker due to geometry.
What are your thoughts about this, has it been tried is there any literature?
I can't seem to find any.

 

[mfb]

Where is the current return of that "U"? If you repeat that pattern around the torus, do you just get lines that go around the torus, i.e. the poloidal coils they already use?

 

[artis]

@mfb see the attached picture, but basically think of it as a toroidal cavity similar to that of the RF cavity where you essentially have a multitude of parallel U shaped coils forming a torus but the middle is empty, Unlike a solenoid, here the field is concentrated at the outer parts instead of the inner parts or middle.

You can also think of them like individual strips of wire filling the geometrical gaps between the toroidal field solenoids/coils since due to geometry the spaces between the coils are not even in the outside vs inside spaces of the toroid chamber.

 

[mfb]

That does nothing. The field inside such a coil is zero, assuming your current return is somewhere far outside. Think of the field geometry you want to get, then remember rot B = 0 outside of coils.

 

[artis]

I think @mfb I get your point, because if the coil is fed externally then it forms a loop on the outside part instead of forming one at the side facing the toroid chamber, so it would have to be fed internally somewhat like an RF cavity is because we know that in a cavity the B field indeed is set up.
So it must be all decided by how the coil circuit is closed which determines where the field lines will be.