Montgomery replies: It isn’t easy to reply to citation-free claims about physics projects that assert “broad successes,” but that is apparently what has to be done when tweaking the controlled fusion program. A first point to be made is that fundamental properties of differential equations are not trivially changed by “ordering” the terms, particularly when the ordering involves dropping those terms with the highest numbers of derivatives. Then even the boundary conditions necessary to determine a solution change. One way to see this would be to try to re-derive the results of our nonideal toroidal steady-state calculation 1 from a perturbation series that starts with ideal MHD.
As far as the alleged differences between MHD and fluid dynamics go, the mechanics of rotating fluids (as in oceans and atmospheres) are actually not that much different. The role of MHD Lorentz forces gets taken over to a large degree by the Coriolis force. The principal results that emerge in geostrophic flow, Ekman circulation, thermal convection, and so on would all be impossible without the inclusion of dissipation coefficients at crucial places in the essential nonideal aspects of the phenomena. 2
I would say that whether flows of meters per second amount to a serious threat for ITER likely depends on the geometry of the flow in a cross section of the toroid. A simple poloidal rotation of that magnitude might be no problem; but a dipolar flow pattern whose streamlines connect the hot geometrical center with the cool perimeter could be a disaster. In our article for the Journal of Plasma Physics , Leon Kamp and I found a transition from the second behavior to the first as the shape of the cross-sectional boundary and the Hartmann number were varied. 1 Is that result in dispute?
