there has been perpetual generators around for the past 5 years, that can go upto 10Mw, electro magnets can goto 1Mw, a force field only needs 20 gauss, but then you need the width Vs size of craft
https://www.stfc.ac.uk/RALSpace/Areas+of+expertise/Space+Research/Mini+magnetospheres/In+the+news/24654.aspx
NASA should quit crying and going over and over, and team up with ESA and just make a next gen space shuttle
http://science.nasa.gov/science-news/science-at-nasa/2005/24jun_electrostatics/
a force field around a craft works the same as a planets magnetic feild