Abstract. At the TEXTOR tokamak an external resonant magnetic perturbation is applied with the Dynamic Ergodic Divertor to control the edge transport properties. The approaches to analyze the impact of such kind of edge stochastisation on transport apply mostly a shell like picture which includes a dependence of transport from magnetic field topology in the radial direction only. In this paper multiple experimental evidence is presented that contrary to these approaches the perturbation applied forms a poloidally heterogenous edge layer in which the transport characteristics are determined by the poloidally alternating field line behavior. A thorough analysis of density and temperature profiles and their gradients for base mode spectra with poloidal/toroidal mode numbers of m/n = 12/4 and m/n = 6/2 is worked out in comparison to the modeled magnetic field topology and results from three dimensional transport modeling with EMC3/EIRENE. Hereby two poloidally adjacent transport domains are identified for the first time in such detail. A domain representing a helical scrape off layer (SOL) is formed by field lines with short connection and therefore prevailing parallel transport to the wall elements. Here, the field lines are clustered into extended flux tubes embedded into a long connection length ergodic domain with diffusive transport characteristics and enhanced radial transport.

Two-dimensional images of electron temperature perturbations are obtained with electron cyclotron emission imaging (ECEI) on the DIII-D tokamak and compared to Alfvén eigenmode structures obtained by numerical modeling using both ideal MHD and hybrid MHD-gyrofluid codes. While many features of the observations are found to be in excellent agreement with simulations using an ideal MHD code (NOVA), other characteristics distinctly reveal the influence of fast ions on the mode structures. These features are found to be well described by the nonperturbative hybrid MHD-gyrofluid model TAEFL.

Neutral beam injection into reversed magnetic shear DIII-D and ASDEX Upgrade plasmas produces a variety of Alfvénic activity including toroidicity-induced Alfvén eigenmodes and reversed shear Alfvén eigenmodes (RSAEs). These modes are studied during the discharge current ramp phase when incomplete current penetration results in a high central safety factor and increased drive due to multiple higher order resonances. Scans of injected 80 keV neutral beam power on DIII-D showed a transition from classical to AE dominated fast ion transport and, as previously found, discharges with strong AE activity exhibit a deficit in neutron emission relative to classical predictions. By keeping beam power constant and delaying injection during the current ramp, AE activity was reduced or eliminated and a significant improvement in fast ion confinement observed. Similarly, experiments in ASDEX Upgrade using early 60 keV neutral beam injection drove multiple unstable RSAEs. Periods of strong RSAE activity are accompanied by a large (peak dS n =S n % 60%) neutron deficit. Losses of beam ions modulated at AE frequencies were observed using large bandwidth energy and pitch resolving fast ion loss scintillator detectors and clearly identify their role in the process. Modeling of DIII-D loss measurements using guiding center following codes to track particles in the presence of ideal magnetohydrodynamic (MHD) calculated AE structures (validated by comparison to experiment) is able to reproduce the dominant energy, pitch, and temporal evolution of these losses. While loss of both co and counter current fast ions occurs, simulations show that the dominant loss mechanism observed is the mode induced transition of counter-passing fast ions to lost trapped orbits. Modeling also reproduces a coherent signature of AE induced losses and it was found that these coherent losses scale proportionally with the amplitude; an additional incoherent contribution scales quadratically with the mode amplitude.

Controlled experiments on the suppression of the m/n = 2/1 tearing mode with electron cyclotron heating and current drive in TEXTOR are reported. The mode was produced reproducibly by an externally applied rotating perturbation field, allowing a systematic study of its suppression. Heating inside the island of the mode is shown to be the dominant suppression mechanism in these experiments. An extrapolation of these findings to ITER indicates that the projected system for suppression of the tearing mode could be significantly more effective than present estimates indicate, which only consider the effect of the current drive but not of the heating inside the island.

We present here the first phase-space characterization of convective and diffusive energetic particle losses induced by shear Alfvén waves in a magnetically confined fusion plasma. While single toroidal Alfvén eigenmodes (TAE) and Alfvén cascades (AC) eject resonant fast ions in a convective process, an overlapping of AC and TAE spatial structures leads to a large fast-ion diffusion and loss. Diffusive fast-ion losses have been observed with a single TAE above a certain threshold in the fluctuation amplitude.

The newly installed electron cyclotron emission imaging diagnostic on ASDEX Upgrade provides measurements of the 2D electron temperature dynamics with high spatial and temporal resolution. An overview of the technical and experimental properties of the system is presented. These properties are illustrated by the measurements of the edge localized mode and the reversed shear Alfvén eigenmode, showing both the advantage of having a two-dimensional (2D) measurement, as well as some of the limitations of electron cyclotron emission measurements. Furthermore, the application of singular value decomposition as a powerful tool for analyzing and filtering 2D data is presented.

For fusion reactors, based on the principle of magnetic confinement, it is important to avoid so-called magnetic islands or tearing modes. They reduce confinement and can be the cause of major disruptions. One class of magnetic islands is that of the perturbation field driven modes. This perturbation field can, for example, be the intrinsic error field. Theoretical work predicts a strong relationship between plasma rotation and the excitation of perturbation field modes. Experimentally, the theory on mode excitation and plasma rotation has been confirmed on several tokamaks. In those experiments, however, the control over the plasma rotation velocity and direction, and over the externally applied perturbation field was limited. In this paper experiments are presented that were carried out at the TEXTOR tokamak. Two tangential neutral beam injectors and a set of helical perturbation coils, called the dynamic ergodic divertor (DED), provide control over both the plasma rotation and the external perturbation field in TEXTOR. This made it possible to set up a series of experiments to test the theory on mode excitation and plasma rotation in detail. The perturbation field induced by the DED not only excites magnetic islands, it also sets up a layer near the plasma boundary where the magnetic field is stochastic. It will be shown that this stochastic field alters both the rotational response of the plasma on the perturbation field and the threshold for mode excitation. It therefore has to be included in an extended theory on mode excitation.

The suppression of (neoclassical) tearing modes is of great importance for the success of future fusion reactors like ITER. Electron cyclotron waves can suppress islands, both by driving noninductive current in the island region and by heating the island, causing a perturbation to the Ohmic plasma current. This Letter reports on experiments on the TEXTOR tokamak, investigating the effect of heating, which is usually neglected. The unique set of tools available on TEXTOR, notably the dynamic ergodic divertor to create islands with a fully known driving term, and the electron cyclotron emission imaging diagnostic to provide detailed 2D electron temperature information, enables a detailed study of the suppression process and a comparison with theory. Tearing modes, and, in particular, neoclassical tearing modes (NTMs), have a deleterious effect on the performance and stability of tokamak plasmas. Larger tokamaks, like the proposed ITER tokamak, are more susceptible to the formation of NTMs. It is therefore important to develop techniques to control or suppress them and to gain understanding of the suppression process. Islands can be stabilized by driving a (helical) current perturbation inside the island region. Gyrotrons are an ideal tool for the localized generation of this current through the injection of radio frequency waves into the plasma. This current can either be directly driven noninductively by electron cyclotron current drive (ECCD) [1] or indirectly by heating the island by electron cyclotron resonance heating (ECRH) [2 -4], causing a helical perturbation to the Ohmic current due to the temperature dependence of the plasma conductivity. ECCD is thought to be a more efficient way to suppress (neoclassical) islands. The tearing mode suppression by heating is often neglected. In this Letter, it will be shown that on TEXTOR the physical mechanism at work during heating can be clearly identified. It is demonstrated that also heating gives a sizeable suppression of the islands.A set of tearing mode suppression experiments on the TEXTOR tokamak is described, that focuses on the suppression by heating (ECRH). In TEXTOR, suppression by ECRH dominates over ECCD [5] due to the low current drive efficiency (low T e ).TEXTOR is a medium sized limiter tokamak with a circular plasma cross section (R 0 1:75 m, a 0:46 m) and is ideally suited for island suppression studies due to the unique combination of available tools. With the dynamic ergodic divertor [6], islands can be created and controlled with (in contrast to other tokamaks) a fully known driving term. The gyrotron can be used to generate highly localized EC waves inside the island. Finally, the process of suppression can be observed in detail by the 2D electron cyclotron emission imaging (ECEI) diagnostic [7].The dynamic ergodic divertor (DED) on TEXTOR is a perturbation field experiment consisting of 16 helical coils on the high field side, aligned with the q 3 field lines. Figure 1(a) shows the vacuum field used for the experiments described in this Letter...