By injecting neutral beams close to the loss cone of a deep, axisymmetric mirror cell and locally heating electrons, an ambipolar potential can be established having a minimum at the midplane and a strong off-midplane peak on one side of the midplane. Such a mirror cell could serve as a plug for a tandem-mirror machine, thus eliminating the problems associated with non-axisymmetry of ion orbits. A minimum-B cell located outside this axisymmetric cell could then be added as an MHD anchor.