Abstract: An Elmo bumpy square (EBS) plasma confinement device is provided by four linear sections formed of a plurality of series connected magnetic mirror confinement segments linked by four 90.degree. sections. Each 90.degree. section is formed of toroidal solenoid-type confinement windings which produce a strong curved solenoidal-type magnet field. Each magnetic mirror segment includes axisymmetric mirror field windings coupled with microwave cavities to form a linear axisymmetric bumpy magnet field, for confinement of a plasma. As in a conventional Elmo bumpy torus, each microwave cavity contains a high-beta, relativistic electron plasma ring which rotates about the confined plasma to provide magnetohydrodynamic stability of the plasma. The electron plasma ring is formed and maintained by microwave heating through the introduction of microwave energy at a specified frequency into each cavity. The 90.degree.