Please reference the conceptual drawing below of this spacecraft propulsion idea. In the vacuum of outer space, if a strong electron beam enters a strong magnetic field, as shown in the drawing, it should be deflected away from the magnet as it travels through that magnetic field. The deflection of this electron beam due to Lorentz Force should exert a perpendicular force against the magnet and the magnet should be repelled away from the electron beam at a perpendicular angle to the beam. With the electron gun and magnet being fastened to the stern of a spacecraft as shown below, then the spacecraft should be propelled in the direction of the Lorentz Force.
If this electron beam is say equal in power to one commonly used in welding and if the magnet is say a very strong Neodymium permanent magnet (or a strong electromagnet), will the deflection of this electron beam create a Lorentz Force strong enough to propel this spacecraft?
This drawing only shows one electron gun-magnet pair to make it easier to convey this conceptual idea, but ideally there will be two pairs fastened to the spacecraft's stern. The second pair (not shown) would be flipped 180 degrees when compared to the first pair in order to create a Lorentz Force that is perpendicular to the other Lorentz Force and the combination of these two Lorentz forces should make the spacecraft move straight ahead.
To address the issue of this spacecraft building up an enormous positive charge due to the discharge of electrons, this spacecraft could incorporate the technology used in NASA's Heliopause Electrostatic Rapid Transit System (HERTS). That system is designed to use its positively-charged tethers to repel protons flowing in the Sun's solar wind to generate its propulsion, and the collected electrons from the solar wind are discharged via its electron gun.
So, using this NASA technology (with NASA's permission of course), this spacecraft's electron guns could possibly run continuously for a long period of time. Also, one key advantage this spacecraft may have over a NASA HERTS spacecraft is that this spacecraft may be able to return to Earth if the generated Lorentz forces from its electron gun-magnet pairs are greater than the repulsion force of the protons in the Sun's solar wind pushing against the spacecraft's positively charged tethers.