while yes that is possible however you need the magnetic field to be strong enough to deflect blaster bolts or defelct other light sabers. That's where it ends up having to be extremely powerful and can suck in coins.
If you just want it to keep the plasma in place then ok but if you want to actually do anything then it becomes unrealistic.
It may be possible to create a cutting plasma blade, what for all intents and purposes is a lightsaber, but cannot deflect blaster bolts, which themselves have yet to be invented. This is still a very useful sword, capable of cutting through anything, yes? If all plasma blades are of the same charge they will still repel each other, while cutting directly through any other kind of sword with ease. In effect you have a light saber that doesnt deflect bullets. This seems an actual possibility, one that could be realized at some point in the future. Its still a lightsaber in all other respects. Or am i missing something?