D-shape cross-section of plasma in fusion reactors

[Andropov]

I was curious about why the plasma inside a modern fusion reactor (ITER, DIII-D) is modeled with a D-shaped cross section.

From what I've read, it is more stable that way, but WHY is it more stable than a similar plasma flow with a circular cross-section?

Thanks in advance (and sorry if I posted this in the wrong section, I hope not)

 

[Astronuc]

The 'D' configuration is nearly half of an 'O', and the plasma is 'wrapped' around the axis perpendicular to the plane of the toroid. A toroidal plasma has a gradient in the magnetic field in the radial direction, with the toroidal/azimuthal field strongest at the inner surface on the horizontal plane formed by the major radius. Ideally, one minimizes the peak magnetic field intensity, so one can maximize the mean field intensity.

 

[Andropov]

Ok, thanks! That was exactly what I wanted to know.

 

[the_wolfman]

Ideally, one minimizes the peak magnetic field intensity, so one can maximize the mean field intensity.

This really isn't that big of a concern, especially in the context of stability. In fact, gradients in the magnetic field lead to magnetic shear which stabilizes many instabilities. To a certain extent variations in the magnetic field are desirable.

I'm not sure if there is an intuitive explanation as to why D shaped plasmas have improved stability. But its true that they have improved stability with regards to a number of different types of instabilities. I'll look at Freidberg's book and see if he gives a simple explanation.