A delta wave augmenter was a device that induced a deep, restful sleep in humans and humanoids. The Fifth Doctor constructed one on Deva Loka using the sonic screwdriver as a component, and used it to help Nyssa sleep after her collapse. (TV: Kinda)
The Doctor later used his delta wave augmenter on Nyssa again as part of his plan to defeat Nephthys. (PROSE: The Sands of Time)
On Kirith, Raphael used a delta wave augmenter to stop dreaming about Darien, but it was only effective for a few months and the dreams returned. (PROSE: Timewyrm: Apocalypse)
By the time of the Thirteenth Doctor, the Doctor's TARDIS was equipped with a delta wave augmenter. The Doctor used parts of it to try to communicate with the ghosts on the Powell Estate. (AUDIO: Salvation)