Ballooning modes in an axisymmetric mirror machine

High-beta ballooning modes are studied in an axisymmetric multiple mirror which is made average-minimum B with end cusps. Electric and magnetic field measurements in the plasma characterize the predominant azimuthal mode number as m = 1. The ballooning character of the mode is determined by measuring the ratio of the mode amplitude near the device center to that near the cusp, and confirmed by measurement of perturbed perpendicular magnetic fields. Theoretical growth rates are calculated numerically using ideal and resistive magnetohydrodynamic (MHD) equations for the rigid m = 1 ballooning mode. Within experimental error it is found that the m = 1 resistive ballooning growth rate scales with radially averaged beta < β> approximately as < β>1/sup //2 for < β>approx. <0.10 (on axis β0approx. <0.20), in agreement with theory. The observed growth rates increase with mirror ratio as expected. The resistive growth rates calculated numerically agree reasonably well with experimental observations