Here's what I'm proposing - a small turbine directly coupled to a permanent magnet that's spinning at, say, 150k RPM. The magnet is enclosed one or several wire loops parallel to the axis of rotation.
Generated EMF is proportional to the magnetic flux and its rate of change. Without an iron core, the flux would be a lot smaller, but it would be in part compensated by the much faster rate of change. There would also be no core losses as well as no eddy current losses. Is something like this practical? Why is it said that driving generators from turbines without any gear reduction is inefficient?
Are iron cores added to generators to increase their specific power or their efficiency? Is it both?