Description
In this contribution, we explore the effects of prompt re-deposition due to Larmor gyration on electron emission from hot tungsten plasma-facing components under conditions relevant to divertor plasmas in ITER. Field-assisted thermionic emission (within the Schottky regime), secondary electron emission and electron backscattering are treated by the 2D3V particle-in-cell code SPICE2, which also calculates self-consistently the value of the surface electric field that dictates the strength of the Schottky correction to the work function. The simulations target plasma conditions anticipated during the impact of partially mitigated edge-localised modes on the ITER divertor targets, with the magnetic field at a grazing angle of incidence with respect to the plasma facing components.
Previously, a semi-empirical model was presented describing the magnitude of the escaping electron current in the presence of a virtual cathode [1]. This model is relevant to contemporary tokamaks, where the virtual cathode typically develops already for surface temperatures below the tungsten melting point and where classical thermionic emission is dominant. However, the predictions for ITER suggest that under certain conditions, the electron emission at the tungsten melting point may not be sufficient to form a virtual cathode and thus a complementary model for electron emission in a classical monotonic magnetized sheath is required. Such a model is presented in this work - it is based on four main elements: (i) a model of prompt re-deposition in presence of strong surface electric field developed by Costin [2]; (ii) simplified models of secondary electron emission and electron backscattering as a function of plasma temperature, as presented in [3-4]; (iii) a new semi-empirical relation between the total electron emission yield and surface electric field; (iv) a novel approximate model for the magnitude of the surface electric field in the absence of electron emission.
[1] M. Komm et al., Nucl. Fusion 60 (2020) 054002
[2] C. Costin, Sci. Rep. 11 (2021) 1874
[3] P. Tolias et al. Nucl. Mat. Energy 25 (2020) 100818
[4] P. Tolias et al. Nucl. Fusion 62 (2022) 026007