EFDA-JET-CP(06)03/23
Fusion Alpha Performance in Advanced Scenario Plasmas based on Reversed Central Magnetic Shear
In a fusion reactor the sufficient confinement of and heating by fusion-born alphas is of great importance. Their wide orbits become particularly worrying in the presence of a reversed qprofile, typical for plasmas with an Internal Transport Barrier (ITB). Thus, it is important to find out whether the benefits of improved thermal particle confinement by ITB are offset by reduced heating efficiency by fusion alphas. In this work, heating by energetic alpha particles is studied using the 5D Monte Carlo guidingcenter code ASCOT. Results obtained with an analytical magnetic background and a circularly symmetric geometry indicate that the overall alpha performance is better for an ITB plasma than for a standard H-mode one. Due to the stagnation orbits, the alpha particles do not escape the core as quickly as was feared which, combined with the typical ITB plasma profiles, seems to lead to more effective use of the fusion fuel. The results obtained with an analytical magnetic background are refined with an authentic JET magnetic geometry. The results are qualitatively similar to those obtained by Constant-of-Motion (COM) 3D Fokker-Planck calculations. However, in the COM approach it is not inherently guaranteed that all orbit topologies, and especially the boundaries between them, are accurately modeled. The strength of guiding-center orbit following approach is in its natural ability to take into account all the possible orbit topologies in phase space. Indeed, ASCOT simulations have been able to resolve e.g. the effect of stagnation orbits on alpha heating, seen in tritium injection experiments. Because alpha heating is localized at the stagnation points, their location can be important, for instance, for the stability of an ITB.