Finite Larmor radius stabilization of ballooning modes in an axisymmetric tandem mirror

The stability of an axisymmetric tandem mirror to ballooning modes is examined including the effect of finite ion gyroradius stabilization. The central cell of the tandem mirror is treated as a long-thin system with isotropic pressure, while the end plug is modeled by a stabilizing boundary condition. For the particular equilibrium considered, the finite Larmor radius corrections to ideal magnetohydrodynamic theory indicate a strong stabilizing effect on high–n ballooning modes even for quite modest values of the gyroradius, k⊥ ρi∼a/L≪1. The dependence of βc on k⊥ ρi is computed, and physical mechanisms associated with this stabilizing influence are discussed.