Plasma cannons were a type of powerful energy weapon.
In the 21st century, Torchwood Three had one plasma cannon. While chasing alien criminals Jack Harkness tried to stop them with the plasma cannon, destroying them and much of the Severn Bridge. (AUDIO: Submission)
In the 42nd century, Baltazar's ship had plasma cannons. He planned to use them to superheat the Earth's carbon-based life, turning them into a diamond. (TV: The Infinite Quest)
The Godspeed had plasma cannons. It used them against the space barnacles, but the energy made them stronger. (PROSE: The Hungry Night)