Numerical study of plasma-wall transition in a magnetic field

A detailed understanding of the interface of a magnetized plasma in contact with a wall is of great importance for furt her progress in plasma-wall interaction research. In particular, a systematic study of configurations with shallow angles of the magnetic field to the wall (a < 5) is necessary for divertor design. Indeed, present attemptsto limit the heat load density to a divertor plate rely on the simplifying assumption that the heat load density scales as sin a also for small angles (less then one degree for ITER). Moreover, a better understanding of this configuration automatically means a better understanding of prob es in magnetized plasmas, which are currently applied for transport and confinement studies in TEXTOR. The present work is a continuation of a joint experimental and theoretical program. In the past, the applicability of a double cylinder probe was extended to determine the ion temperature in magnetized plasmas by using the results of a Monte-Carlo Code [1]. Thegeometry required a six-dimensional code in phase space. The limitation of that code was that the electric field in front of the probe pins was not included in a self-consistent way. Nevertheless, the computed values of the ion temperature were reliable, as confirmed by comparison with results from laser-induced charge-exchange spectroscopy [2]. Pioneering results on the properties of a magnetized plasma in contact with the wall are due to Chodura [3]. By means of a particle-in-cell code (PIC), (three dimensions in velocity space and one dimension in ordinary space), Chodura described the properties of the magnetic sheath (which develops in front of the Debye sheath) when the magnetic field direction changes from perpendicular to grazing incidence relative to the wall. The main disadvantage of PIC codes is their high level of numerical noise, especially in the region near the wall. This problem would become even more severe if a furt her dimension were included in the code in order to account for flows. On the contrary, Eulerian Vlasov codes [4] displaya much lower level of noise, even in regions of small plasma density, such as the Debye sheath in the immediate vicinity of the wall. Therefore, we have implemented a funy kinetic Vlasov code, which is fourdimensional in phase space and accounts for the boundary conditions at the wall. With typical TEXTOR parameters, this code is expected to provide a realistic description of the plasma near the wall. The present report describes numerical results obtained with this Vlasov code. The agreement of our results with those obtained by Chodura is good, and this gives us confidence about the accuracy of our code and its reliability to correctly describe plasma [...