This is actually entirely possible. I would like to propose a very simple mechanism that would naturally result in a light saber.
All you would actually need is a very powerful, focused laser beam. If this laser beam is powerful enough, it will naturally plasmize the air it strikes. This is called the blooming effect.
The thing about this beam of resultant plasma is... It's opaque.
Now, often, we don't want the blooming effect. Scientists have gone to remarkable lengths to avoid the blooming effect, pulsing powerful lasers on and off every femtosecond, so that the plasma has time to get out of the laser's way and the laser isn't blocked by it.
But... For a light saber... The blooming effect would be perfect. A really, really powerful laser would make a beam of plasma, which would in turn block the laser, limiting it's range to about the length of a light saber blade.
I will also note that this system would absolutely be able to make good use of a focusing crystal.
The resultant device would require more power than we can fit into a device of this size in our current stage of development... But everything else is already quite attainable for this device. We could absolutely make a light saber if we had all the required resources for the next thirty years.
There are perhaps two drawbacks to this method:
1) I think you probably knew this, but a light saber is quite impractical. If you had a power source capable of causing thermal blooming and small enough to fit in your hand, you could slightly modify your light saber's laser to pulse on and off every femtosecond, and suddenly the light saber's blade has near infinite length and is a deadly plasma gun. Attach an electrode to the plasma arc and suddenly it isn't just a plasma gun, it's a thunderbolt gun.
2) There is one aspect of light sabers that this system doesn't quite replicate. When two blades constructed in this way collide, they would likely pass right through each other. At most there would be a small blast of plasma as they collided, but they certainly would not bounce off of each other. This issue, however, I suspect is actually impossible to solve if you want to stay within the realm of real physics.
That being said, a light saber is actually scientifically plausible. I doubt the makers of star wars had any clue of that fact, but nevertheless, it's doable if you have the resources.