Hmmm... time to reinject some life into this thread! (hopefully)
I first tried to solve this problem many years ago in childhood I once thought I had come up with a feasable solution which was:
one sided mirrors, I thought as light could pass through one side not the other you could bounce a laser infinately between two mirrors (the bottom
being one sided) forever increasing in intensity.
However these days, I think the design of a light based weapon would be more akin to a fusion generator with exposed lasers - the main problems
1) the time you could sustain the reaction
2) the source of the initial energy
anyway, basic design would have a central telescoping tube with a parabolic disk mirror at the top (wider than the tube) and multiple (though probably
not continuos lasers around the tube) - giving it a nice swing feel, but no stab ability
These lasers would be reflected back to a gold plate creating x-rays which would then be used to condense a deutrium/tritrium containing sphere
(crystal) - creating power of an approximate order of 10 times the input power, hopefully allowing the laser to be sustained for 10 times the initial
power input through some means of harnessing the energy and storage
(maybe initially you could plug another charged battery onto the end to provide the required starting power)
also the handle would require a near perfect insulator as containing a (minute) sun could proove to be a challange, and the weapon would require
refeuling after each use (both a new crystal and initialising battery)
Much of this based on the technology being developed for nuclear fusion
Just stay away from mirrors!!
Another area I would consider investigating would be electron guns and... plasma (as we all suggested)
Electrons have a mass, therby providing something for the other sabers to contact. though obvious problems such as creating a near vacuum with gas
corresponding to the desired colour in the appropriate location without an external shielding glass tube - again I'm thinking a central telescoping
tube connecting cathode and anode, with vents (providing your gas and expelling nearby air) and suction (to recollect the gas for further use, and
hopefully lower the pressure around the saber itself) would have to be implemented.
again this would provide a swing feel with the anode end being weighted appropriately, but no real stab ability
Just stay away from magnets!!
Not that it would be much use, but what better icon to show yesteryears products of imagination, are todays products of reality!
Here's to hoping one day (before George Lucas passes) the lightsaber in some functional form is created!