Modeling of parasitic plasma under the divertor roof baffle

In this paper we present results of numerical simulations of parasitic plasmas created by radiation under the divertor. The extended version of a 2d3v PIC-MCC code [K. Matyash, R. Schneider, A. Bergmann, W. Jacob, U. Fantz, P. Pecher, J. Nucl. Mater. 313-316 (2003) 434] is used for this purpose. The two mechanisms of formation of such plasmas are taken into account in the model: photoionization of the neutral gas in the volume, or electron-impact ionization by photoelectrons emitted from the surface. The photon fluxes necessary for creation of such plasmas are estimated. The magnitude of fluxes obtained within this model agrees with experimentally measured values and with estimates found in B2-Eirene calculations.