Mirror effect on radius of separatrix in field reversed configuration

The separatrix radius can be increased by application of external mirror field. This is concluded by a thought experiment and confirmed numerically. The radius depends not only on the average beta ratio <β> but also on configuration of the applied external field and the separatrix length. With restriction to a class of external field, an approximate formula giving the radius as a function of <β> and mirror ratio of the external field is proposed for configurations such that the separatrix ends are near the externally applied mirrors. Dependence of the radius on the separatrix length is also studied numerically. Its maximum value is explained with use of a thin plasma model. (author)