Stochastic broadening of the separatrix of a tokamak divertor

Abstract

The plasma in a modern tokamak is bounded by a separatrix between magnetic field lines that form toroidal magnetic surfaces, on which the plasma is confined, and open field lines that divert the plasma exhaust to so-called divertor plates. This separatrix is sharp in an ideal tokamak, but we show that magnetic perturbations create a stochastic region of open field lines inside the ideal separatrix which contains approximately 6 times the magnetic flux as does the strike points of these lines on the divertor plates. Since magnetic field lines are a 11/2 degree of freedom Hamiltonian system, the behavior of the field lines in a tokamak divertor is archetypal for the similar Hamiltonian systems.