The plasma response from an external n = 2 magnetic perturbation field in ASDEX Upgrade has been measured using mainly electron cyclotron emission (ECE) diagnostics and a rigid rotating field. To interpret ECE and ECE-imaging (ECE-I) measurements accurately, forward modeling of the radiation transport has been combined with ray tracing. The measured data is compared to synthetic ECE data generated from a 3D ideal magnetohydrodynamics (MHD) equilibrium calculated by VMEC. The measured amplitudes of the helical displacement around the outboard midplane are in reasonable agreement with the one from the synthetic VMEC diagnostics. Both exceed the predictions from the vacuum field calculations and indicate the presence of a kink response at the edge, which amplifies the perturbation. VMEC and MARS-F have been used to calculate the properties of this kink mode. The poloidal mode structure of the magnetic perturbation of this kink mode at the edge peaks at poloidal mode numbers larger than the resonant components vertical bar m vertical bar > vertical bar nq vertical bar, whereas the poloidal mode structure of its displacement is almost resonant vertical bar m vertical bar approximate to vertical bar nq vertical bar. This is expected from ideal MHD in the proximity of rational surfaces. The displacement measured by ECE-I confirms this resonant response.

VL - 58 SN - 0741-3335 UR - https://arxiv.org/abs/1603.09150 IS - 11 U1 -FP

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U3 - FP120 U5 - a1e652bc93cbf7fd81c614e73a717c8b ER - TY - JOUR T1 - Influence of externally applied magnetic perturbations on neoclassical tearing modes at ASDEX Upgrade JF - Nuclear Fusion Y1 - 2015 A1 - Fietz, S. A1 - Bergmann, A. A1 - Classen, I. A1 - Maraschek, M. A1 - M. García-Muñoz A1 - Suttrop, W. A1 - Zohm, H. A1 - ASDEX Upgrade Team AB - The influence of externally applied magnetic perturbations (MPs) on neoclassical tearing modes (NTM) and the plasma rotation in general is investigated at the ASDEX Upgrade tokamak (AUG). The low n resonant components of the applied field exert local torques and influence the stability of NTMs. The non-resonant components of the error field do not influence MHD modes directly but slow down the plasma rotation globally due to a neoclassical toroidal viscous torque (NTV). Both components slow down the plasma rotation, which in consequence increases the probability for the appearance of locked modes. To investigate the impact of externally applied MPs on already existing modes and the influence on the rotation profile, experimental observations are compared to modelling results. The model used here solves a coupled equation system that includes the Rutherford equation and the equation of motion, taking into account the resonant effects and the resistive wall. It is shown that the NTV torque can be neglected in this modelling. To match the experimental frequency evolution of the mode the MP field strength at the resonant surface has to be increased compared to the vacuum approximation. This leads to an overestimation of the stabilizing effect on the NTMs. The reconstruction of the entire rotation profile via the equation of motion including radial dependencies, confirms that the NTV is negligibly small and that small resonant torques at different resonant surfaces have the same effect as one large one. This modelling suggests that in the experiment resonant torques at different surfaces are acting and slowing down the plasma rotation requiring a smaller torque at the specific resonant surface of the NTM. This additionally removes the overestimated influence on the island stability, whereas the braking of the island's rotation is caused by the sum of all torques. Consequently, to describe the effect of MPs on the evolution of one island, all other islands and the corresponding torques must also be taken into account. VL - 55 IS - 1 U1 - FP U2 - PDG U5 - d7ef3c45d2c39d6b80277fd1403be7f0 ER - TY - JOUR T1 - Fast-ion redistribution and loss due to edge perturbations in the ASDEX Upgrade, DIII-D and KSTAR tokamaks JF - Nuclear Fusion Y1 - 2013 A1 - M. García-Muñoz A1 - Akaslompolo, S. A1 - Asunta, O. A1 - Boom, J. A1 - Chen, X. A1 - Classen, I.G.J. A1 - Dux, R. A1 - Evans, T. E. A1 - Fietz, S. A1 - Fisher, R. K. A1 - Fuchs, C. A1 - Geiger, B. A1 - Hoelzl, M. A1 - Igochine, V. A1 - Jeon, Y. M. A1 - Kim, J. A1 - Kim, J. Y. A1 - Kurzan, B. A1 - Lazanyi, N. A1 - Lunt, T. A1 - McDermott, R. M. A1 - Nocente, M. A1 - Pace, D. C. A1 - Rhodes, T. L. A1 - Rodriguez-Ramos, M. A1 - Shinohara, K. A1 - Suttrop, W. A1 - VanZeeland, M. A. A1 - Viezzer, E. A1 - Willensdorfer, M. A1 - Wolfrum, E. A1 - the ASDEX Upgrade A1 - DIII-D Team A1 - KSTAR Teams AB - The impact of edge localized modes (ELMs) and externally applied resonant and non-resonant magnetic perturbations (MPs) on fast-ion confinement/transport have been investigated in the ASDEX Upgrade (AUG), DIII-D and KSTAR tokamaks. Two phases with respect to the ELM cycle can be clearly distinguished in ELM-induced fast-ion losses. Inter-ELM losses are characterized by a coherent modulation of the plasma density around the separatrix while intra-ELM losses appear as well-defined bursts. In high collisionality plasmas with mitigated ELMs, externally applied MPs have little effect on kinetic profiles, including fast-ions, while a strong impact on kinetic profiles is observed in low-collisionality, low q 95 plasmas with resonant and non-resonant MPs. In low-collisionality H-mode plasmas, the large fast-ion filaments observed during ELMs are replaced by a loss of fast-ions with a broad-band frequency and an amplitude of up to an order of magnitude higher than the neutral beam injection prompt loss signal without MPs. A clear synergy in the overall fast-ion transport is observed between MPs and neoclassical tearing modes. Measured fast-ion losses are typically on banana orbits that explore the entire pedestal/scrape-off layer. The fast-ion response to externally applied MPs presented here may be of general interest for the community to better understand the MP field penetration and overall plasma response. VL - 53 UR - http://stacks.iop.org/0029-5515/53/i=12/a=123008 U1 - FP U2 - PDG U5 - 297ecfd209412ec1f2b5deab45ac5511 ER -