The first thing I though of was a huge sword made from a steel girder, a.k.a. “steel beam”, so “beam sword”.
Anyway, pretty much right out, since we’re focusing on sci-fi, not space fantasy, which is where beam swords go.
aside: The most reasonable-ish beam swords I’ve ever seen balance wise were in Armored Core, where it was only powered up for the duration of a swing, and even then it would put a serious dent in the power reserves of most mechs. Nice that they actually put some thought into how much power such a thing would expend.
Anyway, I don’t have a problem with something reasonable that acts in a way reminicent to a “beam sword”, but the problem is, as with many unrealistic tropes like dual-wielding, the end result probably isn’t going to be what the person was asking for in the first place. For example a plasma arc, from what I understand requires a massive amount of power, a HUGE high-pressure inert gas supply, a very significant chunk of hardware, and you end up with a plasma jet on the order of several inches long. This is not remotely weaponizeable. An acetelene torch would probably be far easier to weaponize, and even then would have extremely poor performance.
If you’re talking about an open-air electrical arc, we already have that with the electroshock CBM, could add that effect to a craftable weapon, but again, not what OP asked for.