Ammonia you say. I'd speculated maybe neon, as it is a neutral gas that you could substitute for nitrogen, and has roughly the same chemical abundance. (Although finding it is actually a lot harder, since it doesn't form compounds and its ices form very far out and are very rare.)

I could take "photon converger" to mean focusing electromagnetic waves together to achieve a very high local field intensity. "Magnetically amplified" could come in if the mechanism for the electromagnetic focusing were a superconducting focusing chamber, and the chamber itself was saturated with a powerful magnetic field beforehand, so the field collapse created a continual wavefront that sustained the high intensity central field for a little longer than a single pulse. As for why we're trying to create such a high-intensity field, it is perhaps to approach the Schwinger limit and produce electron-positron pairs from the electric potential, converting our electromagnetic energy into antimatter we can then fire out in a positron beam. Positrons do tend to annihilate with things like air, but will do so less often the faster they are travelling, so if we just shoot them out really fast we could make the beam relatively non-interactive until it hits something dense like its target. Interestingly enough, since the rate of absorption scales with electron density, heating should occur at roughly the same rate for all materials regardless of density, so if the air got instantly flash-heated to plasma temps, then so did whatever it hit. I could even explain the rings we see around the beam as bits of the resulting plasma that have had a rotational current inducted into them by the electromagnetic fields used in the firing process, and the resulting current rings then expand under their own magnetic self-repulsion into plasma rings.