Plasma, even the colder plasmas, are thousands of degrees Celsius wile clearly not to the temperature of air's flash point. Even 1,000Celsius is enough to spontaneously combust many common things.
Anti-Matter? I don't even have to examine the facts to know it's not even enough credit. A nuclear reaction does not even harness a fraction of atom potential power. A Anti-Matter reaction, the canselation of atoms, would either produce no kinetic energy, or enough kinetic energy to set off a miles wide nuclear chain reaction with no survivable end in sight. Which would then produce a even bigger thermal explosion.
There would be no containing the reaction because unlike standard explosion that that apply force on a macroscopic level a fusion/fission brakes down mater on a atomic level. It's not a question of if something can withstand the reaction, it's a question of how far will the sub-atomic 'smoking ashes' be scattered, if there is even that left.
I'm guess I'm not on bored with the whole sci-fi thing in such a near future, I'm wondering how the alien's play with the laws of physics.
Edit from my last post:
As far as detection goes? It's again a question of volume vs how sensitive you can become. Wile it's easily drown out in the earth's magnetic field, solar radiation, or standard radio waves, I assure you that even the keyboard you've been typing on, being run by electricity, is putting out a EM pulse. The alien, using both prodigies amounts of power, and having non-'local' power generation should stick out like a star going supernova.
For that matter the EM field put out my a anti-matter reaction should blanket out any other signal, radar, radio, TV:D,your microwave:twisted: , etc. A nuke puts out a miles wide pulse. The aliens have a bigger, and sustained output to track. I guess detection would just be a matter of havening enough listening posts in the right places to track the source.