Experiments on plasma confinement in internal-ring devices

Internal-conductor devices for low-β toroidal confinement are reviewed. These devices are characterized by high stability against low-frequency oscillations. Experimental observations amply demonstrate this stability property. Plasma confinement approaching the classical limit has been observed in a limited range. It is also shown that the confinement time exceeds the so-called Bohm time by a factor of 200 to 300. The plasma confinement is remarkably sensitive to non-ideal effects such as field error, existence of symmetry-spoiling supports and concentration of impurities.