How the hell can one "easily create external gravitational field".
I know aliens can do it, but us humans maybe not
CRITICAL UPDATE
You can easily create a variety of external gravitational effects not limited to spacecraft propulsion like traction beam, phasing an object through matter in-order to pull it into your spacecraft, gravitational beam weapons, holographic displays, e.t.c by "Careful shaping of the central magnetic" in-order to carry the SHIFTED electrons to their intended target.
If you create a tunnel in which the shifted electrons spiral, everything within the tunnel will become shifted and be easy to manipulate.
You can also create a gravitational beam personal defensive side arm by focusing the SHIFTED electrons into a beam to their intended target.