Reduced sputtering of the divertor plate for ITER by thermionic electron emission

The erosion rate and lifetime of the high heat-flux components for next generation fusion devices such as the ITER (International Thermonuclear Experimental Reactor) are mainly limited by ion sputtering. The energy of the impinging ions and consequently the sputtering rate is enhanced by the presence of a negative electrostatic field near the high heat-flux components. The sheath potential near (and the sputtering rate of) the divertor plate of the ITER can be very sensitive to electron-emission mechanisms such as secondary electron emission and thermionic electron emission (i.e. the emission of electrons from a hot surface). It is shown that the mechanism of thermionic electron emission could lead to significantly lower incident-ion energies striking the divertor plate for ITER-relevant plasma-edge conditions. The transition temperature of the high heat-flux components for a significant reduction of the sheath potential was found to be in the range of 700-1100 K for materials with a low work function (≃1.5-2.5 eV), and ≃1500-1800 K for refractory metals (e.g. tungsten) with high work functions (≃4-5 eV). It is expected that this transition temperature would be about 1800 K for carbon or TiC