Plasma exhaust is the plasma waste product produced by all fusion reactors in a starship's impulse engines. An excess build-up of EM residue in the exhaust ports can cause restricted plasma exhaust flow. The ignition of plasma exhaust will not cause damage to a starship's nacelles as long as the vessel has its hull plating polarized, and is traveling at half-impulse. When launched, photon torpedoes expel plasma exhaust into the torpedo tube. (VOY: "Future's End, Part II")