At least you get the idea mag fields only GUIDE radiation not stop it.
As far as propulsion goes, not sure if you would gain much because if if that would work it would depend on getting into a powerful radiation field and would last only as long as you were in that radiation field. You get out of it and you would lose any acceleration gained.
But I am not versed enough in that area to figure out if you could actually get propulsion from such guiding. It might be the radiation just continues along the field lines without making any actual thrust. That would be my guess as to what happens at the ass end of the craft. In the case where the field lines don't encounter any mass I would think the radiation would just keep following the field line and not contribute to thrust.
Just my guess however.
My professional field is in semiconductor manufacturing machines and one of those devices I spent near 20 years on was the ion implanter.
If you can get to a site that shows how they work, the history of that device is the result of two technologies converging on one product, the first being magnetic field steering of ions and we know what happens there, ions and like electrons tend to get curled around field lines, any charged ion does that so you can steer the ion and in this case, a specific AMU number of an ion, its exact isotope number which has an exact mass associated with it.
So the end result is a bunch of yecchy stuff comes out of the ion generator which you would aim for one you want, say Boron 11 as opposed to boron 10.
B11 is a more massive ion than B10 but the main difference there is there is naturally occurring 3 to one more B11's than B10's so the resulting ion beam strength is 3 times more intense so it is in our interest to be able to select a particular isotope number.
So the magnetic steering allows that to be done and what happens is when you adjust the field strength, say it turns out to be 1300 Gauss (which also depends on the incoming velocity of said ions) so B11, being more massive, gets through a small exit slot but B10 being less massive steers into a lower diameter curve which means it bends the most and as a result hits a field stop piece which happens to be graphite which absorbs those ions and thus keeps B10 out of the resultant ion beam.
Then the B11 beam gets accelerated by electric fields to some new velocity which in ion inplanters is how you control how deep into the substrate the ions penetrate, in most cases, silicon wafers but there are others too.
So accelerating to say 100,000 volts of acceleration, the ions bury themselves X amount of microns deep under the surface of the silicon wafer or if it is 200,000 volts of acel, it gets buried deeper, not quite double but quite a bit deeper which is an important parameter for building computer chips for instance.
But the bottom line here is there is a LOT of research already done on the guiding effect of magnetic field lines and any ionized particle gets corkscrewed around the field lines and thus guided by said field lines.
I just found this piece about using mag fields to protect astronauts: