Stability of a plasma confined in a dipole field

Plasma confined in a magnetic dipole field is stabilized by the expansion of the magnetic flux. The stability of low beta electrostatic modes in a magnetic dipole field is examined when the distribution function is Maxwellian to lowest order. It is shown using a Nyquist analysis that for sufficiently gentle density and temperature gradients the configuration would be expected to be stable to both magnetohydrodynamic and collisionless interchange modes. Furthermore, it is shown that when it is stable to the interchange mode it is also stable to ion temperature gradient and collisionless trapped particle modes, as well as modes driven by parallel dynamics such as the ‘‘universal’’ instability. © 1998 American Institute of Physics. @S1070-664X~98!02610-X#