Asteroid use

Alternatively, you could provide cosmic ray protection
by positively charging the metalic exterior of of the
rotating habitat to repel ionizing protons while
beneath the metalic exterior, providing a small
magnetic field by internal coils to deflect the
significantly less massive electrons.
Maybe,but the Ames study looked at this option and concluded that passive shieldingwas a better solution. I'd see more of a case for active shieldingon a ship or a mobile habitat than a stationary habitat.
Recent advances in superconducting materials may make active shieldingsignificantly more attractive than at the time (1970's) the Ames studieswere conducted. I haven't looked into this closely, but I do knowthat there are some projects in development to use magnetic fields workingwith the solar wind for propulsion. Shielding should be easier thanpropulsion since you don't need as much control.
Al Globus