Modification of the macroscopic stability of a tandem mirror by partial line-tying

Increased stability is observed during experiments with gun injection in the Phaedrus tandem mirror. This is manifested by three effects. First, a magnetic mirror cell with bad average curvature, adjacent to a plasma gun, confines plasma with betas up to 20%. Second, the central cell, also with bad curvature, is stable at betas up to 20%, exceeding the minimum-B interchange stability limit by a factor of 20. Third, the quadrupole magnetic field required for interchange stability is reduced. These effects are shown to agree with a theory of line-tying stabilization, supplemented by minimum-B stabilization.