THE VALIDITY OF THE EXTENDED ENERGY PRINCIPLE

A recent analysis of plasma stability based on modifications of the ex tended energy principle for magnetohydrodynamic stability led to concl usions that are too optimistic. The original interpretation of this pr inciple is indeed applicable. The present analysis demonstrates explic itly the fallacy of using the wrong functional for deltaW in the exten ded energy principle. It then shows that the original energy-principle functional deltaW(B) is also obtained for a model in which a surface mass is incorporated to provide pressure balance. This work therefore indicates, but does not prove, that the eigenfunctions that are obtain ed from a minimization of the extended energy principle with the prope r kinetic-energy norm provide a good representation of what would be a chieved with an exact treatment.