Overview of ASDEX Upgrade results

The ASDEX Upgrade programme is directed towards physics input to critical elements of the ITER design and the preparation of ITER operation, as well as addressing physics issues for a future DEMO design. After the finalization of the tungsten coating of the plasma facing components, the re-availability of all flywheel-generators allowed high-power operation with up to 20 MW heating power at I(p) up to 1.2 MA. Implementation of alternative ECRH schemes (140 GHz O2- and X3-mode) facilitated central heating above n(e) = 1.2 x 10(20) m(-3) and low q(95) operation at B(t) = 1.8 T. Central O2-mode heating was successfully used in high P/R discharges with 20 MW total heating power and divertor load control with nitrogen seeding. Improved energy confinement is obtained with nitrogen seeding both for type-I and type-III ELMy conditions. The main contributor is increased plasma temperature, no significant changes in the density profile have been observed. This behaviour may be explained by higher pedestal temperatures caused by ion dilution in combination with a pressure limited pedestal and hollow nitrogen profiles. Core particle transport simulations with gyrokinetic calculations have been benchmarked by dedicated discharges using variations of the ECRH deposition location. The reaction of normalized electron density gradients to variations of temperature gradients and the T(e)/T(i) ratio could be well reproduced. Doppler reflectometry studies at the L-H transition allowed the disentanglement of the interplay between the oscillatory geodesic acoustic modes, turbulent fluctuations and the mean equilibrium E x B flow in the edge negative E(r) well region just inside the separatrix. Improved pedestal diagnostics revealed also a refined picture of the pedestal transport in the fully developed H-mode type-I ELM cycle. Impurity ion transport turned out to be neoclassical in between ELMs. Electron and energy transport remain anomalous, but exhibit different recovery time scales after an ELM. After recovery of the pre-ELM profiles, strong fluctuations develop in the gradients of n(e) and T(e). The occurrence of the next ELM cannot be explained by the local current diffusion time scale, since this turns out to be too short. Fast ion losses induced by shear Alfven eigenmodes have been investigated by time-resolved energy and pitch angle measurements. This allowed the separation of the convective and diffusive loss mechanisms

Fast-ion redistribution and loss due to edge perturbations in the ASDEX Upgrade, DIII-D and KSTAR tokamaks

Abstract 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 q95 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

Velocimetry analysis of type-I edgelocalized mode precursors in ASDEXUpgrade

When the electron transport barrier remains in its final shape before a type-I edge localized mode (ELM) crash in ASDEX Upgrade, ELM precursors appear as electron temperature fluctuations. In order to relate these precursors to an instability, spatial scales, parity and the cross-phase between electron temperature and radial velocity fluctuations are evaluated by means of velocimetry of measured 2D electron temperature fluctuations. A comprehensive comparison with properties of different instabilities points to microtearing modes. Bispectral analysis indicates a nonlinear coupling of these precursors to a ballooning-type mode prior to the ELM onset

Sawtooth precursor oscillations on DIII-D

The sawtooth oscillation, observed in tokamak plasmas with a central safety factor of less than unity, is a periodic disruptive instability characterized by a slow ramping of central plasma density and temperature, followed by a fast relaxation resulting in flattening of both profiles. Elongated neutral-beamheated discharges on the DIII-D tokamak exhibit multiple precursor oscillations with mode number m/n = 1/1. The dominant m/n = 1/1 mode oscillates at the plasma rotation frequency. A downshifted mode also appears early in the sawtooth ramp. A normalization of electron cyclotron emission imaging data that removes the contribution of slow electron temperature profile evolution reveals that both modes are consistent with an underlying quasi-interchange plasma displacement

Study of the ELM fluctuation characteristics during the mitigation of type-I ELMs

The transitions from type-I to small edge localized modes (ELMs) and back are studied by the electron cyclotron emission imaging (ECEI) diagnostic on ASDEX Upgrade (AUG). ECEI measurements show that the average poloidal velocity of temperature fluctuations of both type-I ELM onsets and small ELMs is the same and is close to 5-6 km/s. Radially, the temperature fluctuations are distributed in the same narrow region of 2 cm in between 0.975 < rho-pol < 1.025 with associ- ated poloidal mode numbers m = 96 +/1 18 and toroidal mode numbers n = 16 +/- 4. The observed fluctuations related to both type-I ELMs and small ELMs vary over the transition simultaneously, however, showing slightly different behaviour. The similarities between type-I ELMs and small ELMs observed on AUG suggest that they both have the same nature and evolve together. In the transition phase a temperature fluctuation mode ('inter-ELM mode') appears, which becomes continuous in the mitigated ELM phase and might cause the ELM mitigation. The mode characteristics (velocities, frequencies and wave-numbers) obtained in the analysis can be further used for the direct comparison in various code simulations

Imaging techniques for microwave diagnostics

Imaging diagnostics, such as Electron Cyclotron Emission Imaging (ECEI) and Microwave Imaging Reflectometry (MIR), exhibit unique characteristics that make them particularly well suited to the validation of theoretical models for plasma instabilities and turbulent fluctuations. A 2-D picture of plasma phenomena is provided unambiguously, from localized, time-resolved measurements. After more than a decade of development and successful demonstrations on RTP [1,2] and TEXTOR [3, 4, 5, 6], ECEI has come into maturity as an electron temperature diagnostic technique, and systems at ASDEX-UG [7] and DIII-D [8] are making regular contributions to plasma physics. The next generation ECEI diagnostic is currently being installed on KSTAR [9, 10]. MIR is a radar reflectometric density fluctuation diagnostic, and hence the perfect complement to ECEI when realized to simultaneously image the same plasma volume. Experiments with MIR at TEXTOR have guided a recent surge in analysis and laboratory experiments aimed at resolving remaining issues [11, 12]. Both techniques are discussed in this tutorial with brief examples of data which illustrate the capabilities of these techniques and motivate future development for application on ITER and burning plasma experiments to come

Visualization of fusion plasma physics via millimeter wave imaging techniques

Advances in microwave technology and innovative ideas have enabled visualization of complex physics of the high temperature plasmas in magnetic fusion devices. ECE Imaging system becomes a powerful tool for MHD physics in various devices and the potential of the MIR system has been reassessed and a system design for KSTAR is in progress