Conceptual study of ferromagnetic pebbles for heat exhaust in fusion reactors with short power decay length

Ferromagnetic pebbles are investigated as high heat flux (q∥) plasma facing components in fusion devices with short power decay length (λq) on a conceptual level. The ability of a pebble concept to cope with high heat fluxes is retained and extended by the acceleration of ferromagnetic pebbles in magnetic fields. An alloying concept suited for fusion application is outlined and the compatibility of ferromagnetic pebbles with plasma operation is discussed. Steel grade 1.4510 is chosen as a well characterized candidate material to perform an analysis of the heating process. Scaling relationships as a function of q∥ for maximum and optimal pebble diameter, allowed exposure time, and removal time safety margin are obtained numerically for spherical pebble geometry. The acceleration of ferromagnetic pebbles in a tokamak resulting from magnetic gradients is studied and operation parameters for an ITER-based reactor are outlined. Counter-intuitively, it is found that ferromagnetic pebbles perform better for narrow λq profiles, making them an attractive heat exhaust concept for next step devices and thus an option to be investigated in detail. The key results of this study are that very high heat fluxes are accessible in the operation space of ferromagnetic pebbles, that ferromagnetic pebbles are compatible with tokamak operation and current divertor designs, that the heat removal capability of ferromagnetic pebbles increases as λq decreases and, finally, that for fusion relevant values of q∥ pebble diameters below 100 μm are required.