Mr. T -- force field technology is nowhere within Earth's grasp in 2258.
(In fact, I don't know for sure that ANY of our races have that particular
tech at the level you're describing.)

Also, the second you put that up, if there were such things, then you
know the station could never be in danger...and suddenly you'd have to start
finding ways to *disable* the system, have it break down, to have any real
jeapordy.

Besides, the jump gate is only used to save energy; any ship big enough
to pose a real threat to B5 could make its own jump point...and you can't
build a force field everywhere.

At any given day, someone on the LA freeway can turn around and see
somebody with a gun pull up alongside. Life is dangerous. B5 reflects that.
The more implausible safeguards you come up with because you want a nice,
safe, riskless society, the more elaborate nonsense you have to go through to
circumvent those safeguards to have any dramatics.

jms