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Recent compact toroid research

The hope to find a path towards a fusion reactor of small size and (relatively) easy technical handling laid the background for the study of high-beta concepts in the past. While it became apparent that straightforward progress in the line of theta pinches was not achievable, many activities ceased to continue. One of the few exceptions is the investigation of field-reversed configurations (FRCs) which has persisted on a small scale over the years. With a series of experiments starting in the mid-seventies a breakthrough occurred when two major obstacles were overcome by Es'kov and coworkers (1978) in maintaining a reasonable fraction of the bias flux and by Minato and colleagues (1982) in controlling the rotational instability. Since then a very spectacular progress has been achieved leading to a multiplication in the lifetime, the demonstration of axial plasma translation, and a substantial improvement of both experimental knowledge and theoretical understanding in equilibrium and confinement physics of the FRC. The compact toroid is defined as a plasma with singly connected, toroidal magnetic topology which is sustained by internal plasma currents. No toroidal field coils link the plasma. Today, compact toroids are investigated predominantly within the framework of these two variants: the FRC with purely poloidal fields and the Spheromak which has poloidal and toroidal fields of similar magnitude. Both concepts are supported by related studies, e.g. on relativistic particle rings.