From Lexicanum

The Ion Cannon is a high strength weapon created by the Tau that can be mounted on large vehicles or starships. The Ion Cannon itself operates by firing a stream of high-energy ionised particles at a target at very high velocities using an electromagnetic field. These particles react dangerously with solid matter, transferring extreme energy at the atomic level, producing an impact that can vaporise armour as if it was irrelevant.

Sources

• Codex: Tau (3rd Edition)
• Codex: Tau Empire (4th Edition)
• Battlefleet Gothic: Armada (Supplement)

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• Tau Weapons