ONSAGER RELAXATION OF TOROIDAL PLASMAS

The slow relaxation of isolated toroidal plasmas towards their thermod ynamical equilibrium is studied in an Onsager framework based on the e ntropy metric. The basic tool is a variational principle, equivalent t o the kinetic equation, involving the profiles of density, temperature , electric potential and electric current. These profiles enter two fu nctionals reflecting the entropy of the field plasma-plasma system and the entropy production rate. These functionals are symmetrical. By th emselves, they would drive an Onsager evolution of the system. However , the variational principle also contains an antisymmetrical functiona l reflecting the trajectory effects. These effects are eliminated, so that the Onsager relaxation is automatically established, in situation s of low collisionality where the trajectories are integrable and clos e to the magnetic surfaces (e.g. in axisymmetric tokamaks). In such si tuations the Onsager character of the slow relaxation is a mere conseq uence of the Hamiltonian nature of the field-plasma system. In the col lisional or non-integrable cases, an Onsager evolution may be still de rived from the variational principle, but the plasma layers around suc cessive magnetic surfaces must be independent enough, in the sense tha t unconfined trapped particles are forbidden unless they are detrapped long before they depart significantly from the magnetic surfaces. New minimization procedures are proposed to obtain entropy and entropy pr oduction rate functionals expressed in terms of the profiles of densit y etc, which drive the Onsager relaxation of the profiles. Onsager rel axations are possible in the presence of a turbulent field, either in an integrable situation (e.g. well separated magnetic islands) or in a non-integrable case (overlapping islands). The variational principle then involves the characteristic frequencies of the turbulent field, o n the same grounds as the profiles of density, etc.