Observation of Macroscopic Stability Limits in a Tandem Mirror

A limit to the ratio of central-cell to plug $\ensuremath{\beta}$ of 1.8\ifmmode\pm\else\textpm\fi{}1.0 is measured in the Phaedrus tandem mirror for central-cell $\ensuremath{\beta}'\mathrm{s}$ of 2% to 5%. This is consistent with the limiting ratio of 2.0 predicted for interchange stability, and is the first experimental test of the effect on the interchange limit from varying the plasma pressure along a magnetic field line in a tandem-mirror configuration.