Including the effect of secondary-electron emission at the divertor targets in code modelling

We present a self-consistent method to include in edge plasma simulations the mutual interactions of the secondary electrons emitted by a solid divertor target and the near-target plasma properties. Based on a semi-analytical model, our procedure accounts for the re-absorption of the secondary electrons hitting the wall during their first gyro-period. Theoretical considerations and numerical simulations performed with the B2Solps-5.0 code show that, for parameters in the operational range of present-day tokamaks, the effective electron-induced secondary-electron emission coefficient can be quite small. However, it increases sharply where the electron temperature has a maximum, i.e. near the separatrix. We compare the results of the self-consistent determination of secondary-electron emission with the commonly adopted constant-value assumption. The exact profile of the secondary-electron emission turns out not to have a dominant influence, at least for cases of low temperature at the plates, which have the biggest experimental relevance. The main limitations of our model, in particular the exclusion of roughness effects, are also briefly discussed. (author)