New Schemes for Confinement of Fusion Products in Stellarators

The condition of poloidal closure of the contours of the second adiabatic invariant for all reflected particles is studied for stellarators with poloidally closed contours of the magnetic field strength B on the magnetic surfaces through computational stellarator optimization. It is shown that this is possible in a major fraction of the plasma volume and results in excellent alpha-particle confinement. Most importantly the magnetic axis curvature vanishes in all cross-sections with an extremum of B on the magnetic axis. Toroidal mirror traps are investigated with respect to isometry, i.e. the lengths of field lines on a magnetic surface between any two contours of B are equal and the contours of the second adiabatic invariant coincide with the magnetic surfaces. Analytical methods are used to design a vacuum field toroidal mirror trap satisfying this isometry condition. The boundary surface of such a configuration is a well-defined finite-sized toroidal surface. This provides the interface to the VMEC, JMC and MCT codes which allow analyses of equilibrium and neoclassical properties