Tearing mode formation after internal crash events like sawteeth or fishbones is one of the most important MHD processes that results in a big island structure and associated confinement degradation. The process implies magnetic reconnection at the rational surface, which has been investigated in great detail in the ASDEX Upgrade tokamak. Using direct local measurements, it is found that the crash leads to the formation of an ideal kink mode with large saturated amplitude at the resonant surface immediately after the sawtooth crash. This kink mode transforms into a tearing mode on a much longer timescale than the crash itself. The ideal kink mode, formed at the resonant surface after the crash, provides the driving force for the magnetic reconnection. The conversion of the ideal kink mode into a tearing mode after the internal crash is similar for various values of plasma rotation and normalized pressure.

VL - 57 UR - http://pubman.mpdl.mpg.de/pubman/item/escidoc:2398199:6/component/escidoc:2401513/Igochine_Tearing.pdf IS - 3 U1 -FP

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U5 - c6df82450ccb1327528ac6de28bc9dfe ER - TY - JOUR T1 - Impact of ideal MHD stability limits on high-beta hybrid operation JF - Plasma Physics and Controlled Fusion Y1 - 2017 A1 - Piovesan, P. A1 - Igochine, V. A1 - Turco, F. A1 - Ryan, D. A. A1 - Cianciosa, M. R. A1 - Liu, Y. Q. A1 - Marrelli, L. A1 - Terranova, D. A1 - Wilcox, R. S. A1 - Wingen, A. A1 - Angioni, C. A1 - Bock, A. A1 - Chrystal, C. A1 - Classen, I. A1 - Dunne, M. A1 - Ferraro, N. M. A1 - Fischer, R. A1 - Gude, A. A1 - Holcomb, C. T. A1 - Lebschy, A. A1 - Luce, T. C. A1 - Maraschek, M. A1 - McDermott, R. A1 - Odstrcil, T. A1 - Paz-Soldan, C. A1 - Reich, M. A1 - Sertoli, M. A1 - Suttrop, W. A1 - Taylor, N. Z. A1 - Weiland, M. A1 - Willensdorfer, M. A1 - ASDEX Upgrade Team A1 - DIII-D Team A1 - EUROfusion MST1 Team VL - 59 UR - http://www.euro-fusionscipub.org/wp-content/uploads/WPMST1CP16_15388_submitted.pdf IS - 1 U1 -FP

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U3 - FP120 U5 - 9459e9933ae840264ddb8f8a49bbc0dd ER - TY - JOUR T1 - Overview of progress in European medium sized tokamaks towards an integrated plasma-edge/wall solution JF - Nuclear Fusion Y1 - 2017 A1 - Meyer, H. A1 - Eich, T. A1 - Citrin, J. A1 - Classen, I. A1 - Hogeweij, D. A1 - Jaulmes, F. A1 - Kappatou, A. A1 - van den Brand, H. A1 - Vanovac, B. A1 - Vijvers, W. A. J. A1 - Westerhof, E. A1 - et al. AB - Integrating the plasma core performance with an edge and scrape-off layer (SOL) that leads to tolerable heat and particle loads on the wall is a major challenge. The new European medium size tokamak task force (EU-MST) coordinates research on ASDEX Upgrade (AUG), MAST and TCV. This multi-machine approach within EU-MST, covering a wide parameter range, is instrumental to progress in the field, as ITER and DEMO core/pedestal and SOL parameters are not achievable simultaneously in present day devices. A two prong approach is adopted. On the one hand, scenarios with tolerable transient heat and particle loads, including active edge localised mode (ELM) control are developed. On the other hand, divertor solutions including advanced magnetic configurations are studied. Considerable progress has been made on both approaches, in particular in the fields of: ELM control with resonant magnetic perturbations (RMP), small ELM regimes, detachment onset and control, as well as filamentary scrape-off-layer transport. For example full ELM suppression has now been achieved on AUG at low collisionality with n = 2 RMP maintaining good confinement H H(98,y2) =approx 0.95. Advances have been made with respect to detachment onset and control. Studies in advanced divertor configurations (Snowflake, Super-X and X-point target divertor) shed new light on SOL physics. Cross field filamentary transport has been characterised in a wide parameter regime on AUG, MAST and TCV progressing the theoretical and experimental understanding crucial for predicting first wall loads in ITER and DEMO. Conditions in the SOL also play a crucial role for ELM stability and access to small ELM regimes. VL - 57 IS - 10 U1 - FP U2 - PEPD U5 - f781e58d912e0c330cdf9b05c806267b ER - TY - JOUR T1 - Measurement of turbulent electron temperature fluctuations on the ASDEX Upgrade tokamak using correlated electron cyclotron emission JF - Review of Scientific Instruments Y1 - 2016 A1 - Freethy, S. J. A1 - Conway, G. D. A1 - Classen, I. A1 - Creely, A. J. A1 - Happel, T. A1 - Köhn, A. A1 - Vanovac, B. A1 - White, A. E. VL - 87 UR - http://pubman.mpdl.mpg.de/pubman/item/escidoc:2320841:2/component/escidoc:2418533/Freethy_Measurement.pdf IS - 11 U1 -FP

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U3 - FP120 U5 - 57e49e2764675999a511e414acf84a11 ER - TY - JOUR T1 - Plasma response measurements of external magnetic perturbations using electron cyclotron emission and comparisons to 3D ideal MHD equilibrium JF - Plasma Physics and Controlled Fusion Y1 - 2016 A1 - Willensdorfer, M. A1 - Denk, S. S. A1 - Strumberger, E. A1 - Suttrop, W. A1 - Vanovac, B. A1 - Brida, D. A1 - Cavedon, M. A1 - Classen, I. A1 - Dunne, M. A1 - Fietz, S. A1 - Fischer, R. A1 - Kirk, A. A1 - Laggner, F. M. A1 - Liu, Y. Q. A1 - Odstrcil, T. A1 - Ryan, D. A. A1 - Viezzer, E. A1 - Zohm, H. A1 - Luhmann, I. C. A1 - ASDEX Upgrade Team A1 - EUROfusion MST1 Team AB -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 - Conversion of the dominantly ideal perturbations into a tearing mode after a sawtooth crash JF - Physics of Plasmas Y1 - 2014 A1 - Igochine, V. A1 - Gude, A. A1 - S. Günter A1 - Lackner, K. A1 - Yu, Q. A1 - Orte, L. B. A1 - Bogomolov, A. A1 - Classen, I. A1 - McDermott, R. M. A1 - N C Luhmann Jr. A1 - ASDEX Upgrade Team AB - Forced magnetic reconnection is a topic of common interest in astrophysics, space science, and magnetic fusion research. The tearing mode formation process after sawtooth crashes implies the existence of this type of magnetic reconnection and is investigated in great detail in the ASDEX Upgrade tokamak. The sawtooth crash provides a fast relaxation of the core plasma temperature and can trigger a tearing mode at a neighbouring resonant surface. It is demonstrated for the first time that the sawtooth crash leads to a dominantly ideal kink mode formation at the resonant surface immediately after the sawtooth crash. Local measurements show that this kink mode transforms into a tearing mode on a much longer timescale (10(-3)s - 10(-2)s) than the sawtooth crash itself (10(-4)s). The ideal kink mode formed after the sawtooth crash provides the driving force for magnetic reconnection and its amplitude is one of the critical parameters for the length of the transition phase from a ideal into an resistive mode. Nonlinear two fluid MHD simulations confirm these observations. (C) 2014 AIP Publishing LLC. VL - 21 SN - 1070-664X IS - 11 U1 - FP U2 - PDG U5 - 078f2a924419611736dda178056583e8 ER - TY - JOUR T1 - Structure and dynamics of sawteeth crashes in ASDEX Upgrade JF - Physics of Plasmas Y1 - 2010 A1 - Igochine, V. A1 - Boom, J. A1 - Classen, I. A1 - Dumbrajs, O. A1 - Gunter, S. A1 - Lackner, K. A1 - Pereverzev, G. A1 - Zohm, H. KW - PLASMA KW - RECONNECTION KW - TOKAMAK AB - The crash phase of the sawteeth in ASDEX Upgrade tokamak [Herrmann et al., Fusion Sci. Technol. 44(3), 569 (2003)] is investigated in detail in this paper by means of soft x-ray (SXR) and electron cyclotron emission (ECE) diagnostics. Analysis of precursor and postcursor (1,1) modes shows that the crash does not affect the position of the resonant surface q=1. Our experimental results suggest that sawtooth crash models should contain two ingredients to be consistent with experimental observations: (1) the (1,1) mode structure should survive the crash and (2) the flux changes should be small to preserve the position of the q=1 surface close to its original location. Detailed structure of the reconnection point was investigated with ECE imaging diagnostic. It is shown that reconnection starts locally. The expelled core is hot which is consistent with SXR tomography results. The observed results can be explained in the framework of a stochastic model. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3529363] VL - 17 SN - 1070-664X UR -