Statistical description of collisionless α-particle transport in cases of broken symmetry: from ITER to quasi-toroidally symmetric stellarators

Abstract

The confinement of α-particles is vital for any future fusion reactor. Unfortunately, the inevitable appearance of inhomogeneities in the magnetic field activates the non-collisional transport by virtue of ripple trapping and ripple induced stochastization. While a large and growing body of literature is devoted to the mitigation of these channels of losses for future reactor designs, far too little attention has been paid to characterize the statistical nature of the underlying stochastic process, which is generally assumed to be diffusive. Here the effect of breaking the toroidal symmetry on collisionless α-particle transport is analyzed numerically with a guiding center orbit following code MOCA for several configurations: a perfectly toroidally symmetric ITER-like tokamak and four stellarators with different levels of quasi-toroidal symmetry. Statistical characterization of banana widths, bouncing times and banana center evolution put into question the classical convection/diffusion approach to adequately describe collisionless α-particle transport as the magnetic configuration departs from toroidal symmetry.