Stability condition for quasi-interchanges in a plasma with a constant-pitch magnetic field and an external gravitational potential

The necessary and sufficient condition for stability of interchanges with k vector · B vector = 0 in a plasma with a constant-pitch field and an external gravitational potential differs from that for which k vector · B vector → 0 using the energy principle and the normal mode analysis, where k and B are the wavevector of the perturbation and the magnetic field, respectively. The dispersion relation obtained by means of the normal mode analysis shows that the discontinuity of the stability criterion results from the existence of the two types of quasi-interchanges for small k vector ‧ B vector ≠ 0 The growth rates for these modes are of the same order of magnitude, so the necessary and sufficient condition for stability of quasi-interchanges is Suydam's condition having an external gravitational potential in a constant-pitch field. Violation of this criterion leads in first instance to unstable quasi-interchanges and pure interchanges arise only if this criterion is strongly violated. (author)