A multiphasic chamber was a device constructed by the crew of the USS Equinox in 2376 for the purpose of confining nucleogenic lifeforms and preventing them from escaping to their native realm. Designed as a stasis chamber, the device operated by surrounding a lifeform with a multiphasic force field, which inhibited its ability to escape the normal spacetime continuum via interspatial fissure.
When the Equinox crew first used the device to study one of the lifeforms, they discovered that it could only survive outside of its realm for a brief period of time, and inadvertently caused its death. Upon examining the remains, they discovered that they could be converted into a source of power with which the crew could augment their warp drive, significantly shortening their journey back to the Alpha Quadrant. They subsequently modified the chamber with a polaron grid and a submolecular resequencer, for the purpose of converting the alien cell structures into a crystaline compound.
After encountering the crew of the USS Voyager, the Equinox crew attempted to conceal their criminal experiments by flooding their research lab with thermionic radiation. When Captain Janeway became suspicious, however, she sent The Doctor to investigate. Examining the equipment in the lab, The Doctor uncovered the Equinox crew's activities, and Janeway subsequently confronted Captain Ranson. (VOY: "Equinox")