(covers information from an alternate timeline)
A polaric generator
A polaric generator, or polaric ion generator, was a type of generator which was used to cut through subspace by opening subspace fractures.
In 2371, B'Elanna Torres and Harry Kim invented such a device to retrieve Captain Kathryn Janeway and Tom Paris who were lost in a subspace fracture on a planet in the Delta Quadrant. They activated the device in engineering to demonstrate it to Chakotay and Tuvok. The green beam of the device broke subspace and opened a subspace fracture but only for a few moments. It was not possible to open the same fracture a second time with this device due to the damage to subspace. Their own rescue attempt ended up causing the polaric ion explosion that created the fractures, as well as destroying all organic material on the planet and leaving it in ruins, resulting in Janeway's death. However, when Janeway was sent back in time by a day for the second time, she realized what was happening and closed the fracture, creating an alternate timeline where the explosion never occurred. (VOY: "Time and Again")