Influence of the resonant magnetic perturbations on particle transport in LHD

Purpose of these studies is investigations of non-linear plasma response on transport due to stochastic effects. At LHD, perturbation coils create Resonant Magnetic Perturbation (RMP) with the m/n = 1/1 and 2/1 Fourier components. Depending on plasma conditions, the perturbation either enhances or heals the natural m/n = 1/1 magnetic island. In the case of an amplified island the enhanced heat and particle transport across the island causes a rather significant reduction of the confinement. For a healed island there is a rather small decrease of beta with increasing perturbation current. These changes coincide with an increasing width of the open stochastic volume at the plasma edge near the X-point. Additionally systematic experiments were performed, changing the amplitude of the perturbation linearly with IRMP in the range of 0 – 2.7 kA. Two scenarios were realized: first, the discharge was ramped up with an external perturbation already superimposed to the main magnetic field. Second, the external perturbation was applied to the plasma already ignited (typical case of tokamaks with RMPs). As it is shown, there is a clear difference in the size of the 1/1 island and the dependence of ne and Te on the perturbation when comparing those two scenarios. To certain amplitude of the external perturbation the hysteresis effect is observed and the particle transport and confinement are affected much stronger in the discharges with pre-existing magnetic perturbation. Interestingly above certain value of perturbation current a global reduction of Te and ne profiles was observed independent on the chosen scenario. (author)