Actually the Rogowski coil is used for measurements right now, but is not very efficient for energy conversion. The technology that is efficient is derived form microwave technology which turns electron beams into microwaves. there are a number of devices, but one common one is the gyrotron. Yes, electrons moving to follow the ions are a concern, but we think that we can design the collection coils so that the easiest route for the electrons is through the coils, not through the plasma.
It would be easier to understand how such high efficiencies are achieved if the particles velocity were uniform. Since they are not; it is hard to understand why the slower particles would not be decelerated to a standstill and reversed by the magnetic field created in the coil by earlier arriving particles.
Classically it would seem to me that efficiencies above 68% aren’t attainable, due to the above reasoning. I obviously don’t get something.