Impurity ion dynamics near magnetic islands in the standard and high-mirror configurations of Wendelstein 7-X

Summary form only given. At the contemporary stage of fusion development the question of particle transport is very important. One of the main problems for the operation of fusion reactor is the penetration of impurity ions which are sputtered from the divertor plates or vacuum vessel into the core of the plasma. Even a small fraction of heavy ions in the plasma leads to the significant increase of bremsstrahlung radiation losses thus increasing the plasma temperature needed for the ignition threshold. In the present paper the radial transport of tungsten ions is studied for two magnetic field configurations of Wendelstein 7-X (W7-X) stellarator being constructed in the Max-Planck-Institut fur Plasmaphysik, Greif- swald, Germany. W7-X coil system consists of 50 modular and 20 planar non-circular coils combined in M=5 magnetic periods. There are also 10 control coils used to control the magnetic islands radial position. It is known that due to lack of axial symmetry in stellarators some group of particles is trapped in local magnetic wells. These particles show a fast bouncing motion superposed onto a slow uncompensated radial drift. The existence of such a group of particles leads to the large neoclassical losses in stellarators compared to the tokamak case. But for the optimized 3-D structure of W7-X magnetic system the significant reduction of drift losses is expected. Due to collisions a passing particle can change its magnetic moment which results in trapping in the magnetic well and subsequent loss from the system. In this work the Coulomb scattering is modelled via the discrete Monte Carlo collision operator. The guiding center equations for the ensemble of N=1000 test particles are solved using the 6-th order Runge-Kutta method. The statistical properties of the ensemble of tungsten ions are studied by calculating the mean square displacement and diffusion coefficient. It is shown that the existence of magnetic islands at the plasma periphery leads to the enhancement of particle transport.