The Plasma Torpedo was a starship based weapon first employed by a Romulan Bird of Prey in 2266, against a string of Federation outposts along the Romulan Neutral Zone.
The plasma torpedo consisted of a small but powerful force field generator which fired a bolt of plasma from a standard torpedo tube. The generator would then establish a very powerful force field around itself and a much larger concentric field with a radius of about one hundred metres. The parent vessel established a field bridge through this outer layer and vented its plasma conduit system into the empty shell between the two fields. The breach was then sealed, and the vessel would break away and let the torpedo continue on its way. A high power sensor on the central body was capable of locking onto the target, and by venting part of the plasma in a specific direction some degree of control could be established over the course of the plasma torpedo.
This was a very complex and cumbersome weapons system with many disadvantages - venting the main plasma system of the starship causes an almost total loss of power while firing. The ship must therefore drop to sublight speed, and whilst firing the cloaking device must be disengaged. The guidance system is extremely limited by the difficulties inherent in operating any sensor system through a high energy plasma field, and any degree of manoeuvring causes a significant drop in eventual yield. Because of these difficulties the plasma torpedo has therefore not been very successful when used against mobile targets such as Starships.
Fixed installations, on the other hand, are a different matter. The plasma torpedo essentially focuses the entire energy output of a starship into one colossal discharge; when used against a Federation asteroid based outpost a single plasma torpedo collapsed the shields and caused near total destruction of the installation even through several kilometres of solid rock. This level of destructiveness was not available with any other weapons system of the time.
The Plasma Torpedo remained in service with the Romulan Warbird as a heavy weapon for use against fixed installations. There is very limited confirmed information available regarding the weapon's use by other races.