Poloidal variation of high-Z impurity density due to hydrogen minority ICRH on Alcator C-Mod
نویسنده
چکیده
In the Alcator C-Mod tokamak, strong, steady-state variations of molybdenum density within a flux surface are routinely observed in plasmas using hydrogen minority ion cyclotron resonant heating. In/out asymmetries, up to a factor of 2, occur with both inboard and outboard accumulation depending on the major radius of the minority resonance layer. These poloidal variations can be attributed to the impuritys high charge and large mass in neoclassical parallel force balance. The large mass enhances the centrifugal force, causing outboard accumulation while the high charge enhances ion-impurity friction and makes impurities sensitive to small poloidal variations in the plasma potential. Quantitative comparisons between existing parallel high-Z impurity transport theories and experimental results for r/a < 0.6 show good agreement when the resonance layer is on the high-field side of the tokamak but disagree substantially for on-axis and low-field side heating. Ion-impurity friction is shown to be insufficient to explain experimental results and and the accumulation of impurity density on the inboard side of flux surface is shown to be due to a poloidal potential variation from magnetic trapping of nonthermal cyclotron heated minority ions. Parallel transport theories are extended to account for cyclotron effects and revised models agree with experimentally measured impurity density asymmetries to within ~50%.
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تاریخ انتشار 2011