Beam-free-all
From FreeSpace Wiki
beam-free-all Sets all beam weapons on the specified ship to be active
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Used to free all the beams on a given ship in the mission. This SEXP will only work for ships that are already in a mission - Using it on a ship that has yet to arrive will have no effect. For SCP, there is also a Mission Specs option to have all beams freed by default for all ships in the mission, which will also cover arriving ships.