Manipulate earth's existing fields.
The most cost effective defense to radiation is to polarize the Van Allen belts and turn them into a wave-shield.
incoming gamma radiation needs to be diverted/struck by compatible wavelengths.
Think of a pair of polarized sunglasses which block uv rays because the glass doesn't permit certain wavelengths to continue in a straight-line trajectory.
Polarizing exist gamma radiation into a denser, coherent field is at this time problematic..however in about 300 years I will probably have the problem figured out. Or perhaps maybe next week if I take shorter lunches.
I may have to speed up the earth's rotation or toy with the core a bit though. Would anyone really object to having 8 hour days and nights if it meant survival from a hostile sun?
;-)