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The ion cannon was the primary defensive weapon of Tarsonis.
General Edmund Duke activated the Ion Cannon when Captain Jim Raynor deserted Arcturus Mengsk. Although the Cannon was defended by the Sons of Korhal and Alpha Squadron, Raynor was successful in destroying it.
It may have been based on the Ion Cannon from Star Wars: Episode V - The Empire Strikes Back given the noticeable similarity of design. While appearing intimidating, the cannon itself bears no actual attacks.