(covers information from an alternate timeline)
A plasma inducer was a device that malfunctioned on the USS Enterprise-D in the anti-time past before the ship's maiden voyage to Deneb IV and, given Jean-Luc Picard's preexisting knowledge of the problem, likely also malfunctioned on the ship at some point in the original timeline.
In 2367, while checking on the modifications Geordi La Forge had made to the Enterprise’s systems, Doctor Leah Brahms noted that the plasma inducers were interlinked with the warp field generator. (TNG: "Galaxy's Child")
In the anti-time past, Captain Picard used his knowledge of his own past to assist Miles O'Brien regarding a problem with the the warp plasma inducers on the USS Enterprise-D by providing O'Brien with specifications to bypass the secondary plasma inducer. (TNG: "All Good Things...")