Not just the field needed
Hi RL
I think skepticism is deserved for such big claims, but there's more involved to the drive effect than a strong EM field. The authors have suggested elsewhere that the effect they describe might be involved in the polar outflow observed in pulsars and black holes so they're obviously aware of the issue.
Check out the coverage and links on Wikipedia's article (which might be just a sock-puppet, but a restrained one.)
Adam