Existence and calculation of sharp boundary magnetohydrodynamic equilibrium in three-dimensional toroidal geometry

The problem of sharp boundary, ideal magnetohydrodynamic equilibria in three‐dimensional toroidal geometry is addressed. The sharp boundary, which separates a uniform pressure, current‐free plasma from a vacuum, is determined by a magnetic surface of a given vacuum magnetic field. The pressure balance equation has the form of a Hamilton–Jacobi equation with a Hamiltonian that is quadratic in the momentum variables, which are the two covariant components of the magnetic field on the outer surface of the plasma. The condition of finding a unique solution on the outer surface is identical with finding phase‐space tori in nonlinear dynamics problems, and the Kolmogorov–Arnold–Moser (KAM) theorem guarantees that such solutions exist for a wide band of parameters. Perturbation theory is used to calculate the properties of the magnetic field just outside the plasma. Special perturbation theory is needed to treat resonances and it is explicitly shown that there are bands of pressure where there are no solutions.