A plasma configuration is stable against a given perturbation if the change in potential energy due to the perturbation is positive. This change in potential energy is expressed as an integral through the volume of the plasma. The system is obviously stable if the integrand is positive everywhere so that the change in energy is positive in every volume element. Laing has shown that there do exist obviously stable configurations;his work is described in this lecture and some numerical examples are given. The chief characteristic of all these obviously stable configurations is that the total axial current carried by the plasma is zero.
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