Well for me, I honestly envisioned that an atom laser would require shells(*) of material.
However, I suppose with as much condensed atoms the mech are already using, they could probably get it from space like a particle collector or simple the mech's fuel/coolent.
Edit: You know the whole need for a replaceable fuel or atom source got me thinking. Reading the phrase "powered from the vehicle or starship's internal power source," I remembered that Nepleslian used to use a lot of nuclear fusion and sometime fission powered sources. I wonder if that one could take concept
powered from internal power source to a deeper level and the
waste from the fusion/fission reactor actually be the fuel(**) that is fired by the laser as a coherent beam of atoms.
*However, the shell would be heavy condensed matter, so the weapon would then be a heavy not lightweight weapon. So, I am not sure that would be right idea Five was going for.
**Seriously people, you should think about it. from
http://en.wikipedia.org/wiki/Proton-proton_chain
The Tritium/Deuterium fusion reaction produces Helium-3
2 D^1 + 1 H^1 → He^3_2 + γ + 5.49 MeV
I am not going to even cite how many plasma engines shoot out helium. Is there any real difference of helium being used as a weapon if we use it to propel vechiles?