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Edge localized modes (ELMs) are magnetohydrodynamic (MHD) instabilities that cause fast periodic relaxations of the strong edge pressure gradient in tokamak fusion plasmas. Magnetic pick-up coils allow the extraction of toroidal mode numbers n during the ELM cycle including the nonlinear crash on ASDEX Upgrade, providing a good comparability to nonlinear 3D MHD codes. This paper investigates how the mode numbers before and during the ELM crash change with a variation of plasma parameters. It is found that the toroidal structure size during the crash is similar to the one existing slightly before and always has a low n = 1–7 range. Furthermore, in the nonlinear phase of the ELM n does not show a clear trend with most peeling-ballooning relevant parameters such as normalized pressure gradient, bootstrap current density or triangularity, whereas a strong decrease of n with edge safety factor q95 is observed in agreement with nonlinear modeling in the here investigated high collisionality region. A simple geometric model is presented, which is capable of explaining the q scaling by existence of ballooned structures that minimize n. [ABSTRACT FROM AUTHOR]

The results on high frequency coherent modes developing at the plasma edge after L–H transitions are presented. These edge coherent modes (ECMs) observed in the I-phase, during ELM-free and ELMy H-mode phases have common characteristics, such as their frequency of 50–200 kHz, radial position and scaling of the frequency with the pressure gradient. The ultra-fast swept reflectometer with the sweep time of 1 μs has provided measurements of the mode dynamics and localisation. The ECMs propagate in the electron diamagnetic direction in the laboratory frame and are localized in the region between the pedestal top and the steepest pressure gradient. [ABSTRACT FROM AUTHOR]

In ASDEX Upgrade, the propagation of cold pulses induced by type-I edge localized modes (ELMs) is studied using electron cyclotron emission measurements, in a dataset of plasmas with moderate triangularity. It is found that the edge safety factor or the plasma current are the main determining parameters for the inward penetration of the Te perturbations. With increasing plasma current the ELM penetration is more shallow in spite of the stronger ELMs. Estimates of the heat pulse diffusivity show that the corresponding transport is too large to be representative of the inter-ELM phase. Ergodization of the plasma edge during ELMs is a possible explanation for the observed properties of the cold pulse propagation, which is qualitatively consistent with non-linear magneto-hydro-dynamic simulations. [ABSTRACT FROM AUTHOR]

The impact of three-dimensional (3D) tokamak geometry from external magnetic perturbations on edge instabilities has been examined in high confinement mode plasmas with edge localised modes (ELMs) in ASDEX Upgrade. The 3D geometry has been probed using rigidly rotating MP fields. The measured distortions of the plasma boundary are compared to single-fluid ideal magnetohydrodynamic (MHD) equilibria using VMEC and MARS-F applying ideal and resistive MHD, whereas VMEC uses only ideal MHD. Both codes accurately reproduce the measured radial displacements of the edge density and temperature profiles in amplitude, toroidal phase and their dependence on the applied poloidal mode spectrum. The induced 3D geometry distorts the local magnetic shear, which locally reduces the stabilising effect from field-line bending at certain most unstable field lines. Around these field lines, we observe additional stable ideal MHD modes with clear ballooning structure in-between ELMs. After their immediate appearance, they saturate and then grow on timescales of the pedestal pressure recovery. The subsequent ELMs show strongly localised magnetic perturbations of the initial crash and accompanied energetic electrons around the same most unstable field lines. These are strong signatures that filaments at the ELM onset preferentially erupt on these most unstable (‘bad’) field lines with their unfavourable 3D geometry where preceding ballooning modes are observed. [ABSTRACT FROM AUTHOR]

The steep edge gradient region of tokamak plasmas in the high confinement regime is known to drive instabilities, which cause transport. Several diagnostics are used to allow for a high degree of characterization of low-frequency modes appearing in between type-I edge localizes modes (ELMs). These modes are dominantly observed in electron cyclotron emission (ECE) and ECE imaging measurements as a modulation of radiation temperature (). In the radial magnetic field ( measurements, the frequency range of 4 kHz to 12 kHz is observed. The position of the mode is determined to be at the upper part of the steep gradient region, the poloidal mode velocity is changing from 1.5  ±  0.5 km s−1 to 2.5  ±  0.5 km s−1 and the toroidal mode number is 13 to 14. A comparison with the measured velocity leads to the conclusion that the phase velocity of the mode is smaller than 3 km s−1 or zero. The poloidal structure of the modes is found to agree with the poloidal structure size associated with n  =  13 as estimated from the equilibrium calculations. The modes are compared between two different heating phases during one discharge, and are found to differ in duration, velocity, frequency and toroidal mode number. The possibility of non-linear interaction between these modes and other, high frequency modes existing in the narrow pedestal, is assessed via bicoherence analysis. The presented analysis gives an unprecedented picture of the mode, its position, its structure and its velocity, calling for comparison with non-linear modelling. [ABSTRACT FROM AUTHOR]

Electron cyclotron emission imaging (ECEI) provides measurements of electron temperature (Te) and its fluctuations (δTe). However, when measuring at the plasma edge, in the steep gradient region, radiation transport effects must be taken into account. It is shown that due to these effects, the scrape-off layer region is not accessible to the ECEI measurements in steady state conditions and that the signal is dominated by the shine-through emission. Transient effects, such as filaments, can change the radiation transport locally, but cannot be distinguished from the shine-through. Local density measurements are essential for the correct interpretation of the electron cyclotron emission, since the density fluctuations influence the temperature measurements at the plasma edge. As an example, a low frequency 8 kHz mode, which causes 10%–15% fluctuations in the signal level of the ECEI, is analysed. The same mode has been measured with the lithium beam emission spectroscopy density diagnostic, and is very well correlated in time with high frequency magnetic fluctuations. With radiation transport modelling of the electron cyclotron radiation in the ECEI geometry, it is shown that the density contributes significantly to the radiation temperature (Trad) and the experimental observations have shown the amplitude modulation in both density and temperature measurements. The poloidal velocity of the low frequency mode measured by the ECEI is 3 km s–1. The calculated velocity of the high frequency mode measured with the magnetic pick-up coils is about 25 km s–1. Velocities are compared with the E × B background flow velocity and possible explanations for the origin of the low frequency mode are discussed. [ABSTRACT FROM AUTHOR]

In addition to the relaxation of the pedestal, edge localised modes (ELMs) introduce changes to the divertor and scrape-off layer (SOL) conditions. Their impact on the inter-ELM pedestal recovery is investigated, with emphasis on the electron density (ne) evolution. The typical ELM cycle occurring in an exemplary ASDEX Upgrade discharge interval at moderate applied gas puff and heating power is characterised, utilising several divertor, SOL and pedestal diagnostics. In the studied discharge interval the inner divertor target is detached before the ELM crash, while the outer target is attached. The particles and power expelled by the ELM crash lead to a re-attachment of the inner target plasma. After the ELM crash, the outer divertor target moves into a high recycling regime with large ne in front of the plate, which is accompanied by high main chamber neutral fluxes. On similar timescales, the inner target fully detaches and the high field side high density region (HFSHD) is formed reaching up to the high field side midplane. This state evolves again to the pre-ELM state, when the main chamber neutral fluxes are reduced later in the ELM cycle. Neither the timescale of the appearance of the HFSHD nor the increase of the main chamber neutral fluxes fit the timescale of the ne pedestal, which is faster. It is found that during the ne pedestal recovery, the magnetic activity at the low field side midplane is strongly reduced indicating a lower level of fluctuations. A rough estimation of the particle flux across the pedestal suggests that the particle flux is reduced in this period. In conclusion, the evolution of the ne pedestal is determined by a combination of neutral fluxes, HFSHD and reduced particle flux across the pedestal. A reduced particle flux explains the fast, experimentally observed re-establishment of the ne pedestal best, whereas neutrals and HFSHD impact on the evolution of the SOL and separatrix conditions. [ABSTRACT FROM AUTHOR]

The upgrade of the edge charge exchange recombination spectroscopy diagnostic at ASDEX Upgrade has enabled highly spatially resolved measurements of the impurity ion dynamics during an edge-localized mode cycle (ELM) with unprecedented temporal resolution, i.e. 65 μs. The increase of transport during an ELM induces a relaxation of the ion, electron edge gradients in impurity density and flows. Detailed characterization of the recovery of the edge temperature gradients reveals a difference in the ion and electron channel: the maximum ion temperature gradient is re-established on similar timescales as , which is faster than the recovery of . After the clamping of the maximum gradient, Ti and Te at the pedestal top continue to rise up to the next ELM while ne stays constant which means that the temperature pedestal and the resulting pedestal pressure widen until the next ELM. The edge radial electric field Er at the ELM crash is found to reduce to typical L-mode values and its maximum recovers to its pre-ELM conditions on a similar time scale as for ne and Ti. Within the uncertainties, the measurements of Er align with their neoclassical predictions for most of the ELM cycle, thus indicating that Er is dominated by collisional processes. However, between 2 and 4 ms after the ELM crash, other contributions to flow, e.g. zonal flows or ion orbit effects, could not be excluded within the uncertainties. [ABSTRACT FROM AUTHOR]

In order to understand the mechanisms that determine the structure of the high confinement mode (H-mode) pedestal, the evolution of the plasma edge electron density and temperature profiles between edge localised modes (ELMs) is investigated. The onset of radial magnetic fluctuations with frequencies above 200 kHz is found to correlate with the stagnation of the electron temperature pedestal gradient. During the presence of these magnetic fluctuations the gradients of the edge electron density and temperature are clamped and stable against the ELM onset. The detected magnetic fluctuation frequency is analysed for a variety of plasma discharges with different electron pressure pedestals. It is shown that the magnetic fluctuation frequency scales with the neoclassically estimated velocity at the plasma edge. This points to a location of the underlying instability in the gradient region. Furthermore, the magnetic signature of these fluctuations indicates a global mode structure with toroidal mode numbers of approximately 10. The fluctuations are also observed on the high field side with significant amplitude, indicating a mode structure that is symmetric on the low field side and high field side. The associated fluctuations in the current on the high field side might be attributed to either a strong peeling part or the presence of non-adiabatic electron response. [ABSTRACT FROM AUTHOR]