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Additional electron cyclotron resonance heating (ECRH) is used in an ion-temperature-gradient instability dominated regime to increase R/LTe in order to approach the trapped-electron-mode instability regime. The radial ECRH deposition location determines to a largedegree the effect on R/LTe. Accompanying scale-selective turbulence measurements at perpendicular wavenumbers between k⊥?=?4-18?cm-1 (k⊥ρs = 0.7-4.2) show a pronounced increase of large-scale density fluctuations close to the ECRH radial deposition location at mid-radius, along with a reduction in phase velocity of large-scale density fluctuations. Measurements are compared with results from linear and non-linear flux-matched gyrokinetic (GK) simulations with the gyrokinetic code GENE. Linear GK simulations show a reduction of phase velocity, indicating a pronounced change in the character of the dominant instability. Comparing measurement and non-linear GK simulation, as a central result, agreement is obtained in the shape of radial turbulence level profiles. However, the turbulence intensity is increasing with additional heating in the experiment, while gyrokinetic simulations show a decrease. [ABSTRACT FROM AUTHOR]

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--3s -- 10--2s) than the sawtooth crash itself (10--4s). 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. [ABSTRACT FROM AUTHOR]

In the ASDEX Upgrade tokamak, a radiation measurement for a wide spectral range, based on semi-conductor detectors, with 256 lines of sight and a time resolution of 5µs was recently installed. In combination with the foil based bolometry, it is now possible to estimate the absolutely calibrated radiated power of the plasma on fast timescales. This work introduces this diagnostic based on AXUV (Absolute eXtended UltraViolet) n-on-p diodes made by International Radiation Detectors, Inc. The measurement and the degradation of the diodes in a tokamak environment is shown. Even though the AXUV diodes are developed to have a constant sensitivity for all photon energies (1 eV-8 keV), degradation leads to a photon energy dependence of the sensitivity. The foil bolometry, which is restricted to a time resolution of less than 1 kHz, offers a basis for a time dependent calibration of the diodes. The measurements of the quasi-calibrated diodes are compared with the foil bolometry and found to be accurate on the kHz time scale. Therefore, it is assumed, that the corrected values are also valid for the highest time resolution (200 kHz). With this improved diagnostic setup, the radiation induced by edge localized modes is analyzed on fast timescales. [ABSTRACT FROM AUTHOR]

Novel flow rotation measurements based on charge exchange recombination spectroscopy at the inboard midplane of the ASDEX Upgrade tokamak reveal the existence of an asymmetric flow structure at the H-mode edge, which is shown to arise due to a poloidal impurity density asymmetry. A quantitative evaluation of the impurity density at the inboard side demonstrates that the impurities redistribute along the flux surface, resulting in a poloidal dependency of the impurity density. The poloidal and toroidal impurity flows measured at the high-field side (HFS) and low-field side (LFS) are compared to theoretical predictions based on the parallel momentum balance, which includes friction, inertia, pressure and electric force. Both a fluid and a kinetic approach are used, showing good agreement with each other. The measured impurity flow structure is described by the model quantitatively when a finite poloidal main ion flow of ∼2 km s−1 arises, which is in keeping with the standard neoclassical prediction. The interplay of all terms, in particular the inclusion of the impurity inertia term, is important in reproducing the observed flow structure and results in an impurity accumulation at the HFS. The existence of a poloidal impurity density asymmetry in the edge transport barrier slightly reduces the drift parameter v/D, however, the experimental value is consistent with standard neoclassical theory. This demonstrates that despite the asymmetry in the impurity density, the impurity particle transport is at the neoclassical level. [ABSTRACT FROM AUTHOR]

Tokamak scenario development requires an understanding of the properties that determine the kinetic profiles in non-steady plasma phases and of the self-consistent evolution of the magnetic equilibrium. Current ramps are of particular interest since many transport-relevant parameters explore a large range of values and their impact on transport mechanisms has to be assessed. To this purpose, a novel full-discharge modelling tool has been developed, which couples the transport code ASTRA (Pereverzev et al 1991 IPP Report 5/42) and the free boundary equilibrium code SPIDER (Ivanov et al 2005 32nd EPS Conf. on Plasma Physics vol 29C (ECA) P-5.063 and http://epsppd.epfl.ch/Tarragona/pdf/P5_063.pdf), utilizing a specifically designed coupling scheme. The current ramp-up phase can be accurately and reliably simulated using this scheme, where a plasma shape, position and current controller is applied, which mimics the one of ASDEX Upgrade. Transport of energy is provided by theory-based models (e.g. TGLF (Staebler et al 2007 Phys. Plasmas14 055909)). A recipe based on edge-relevant parameters (Scott 2000 Phys. Plasmas7 1845) is proposed to resolve the low current phase of the current ramps, where the impact of the safety factor on micro-instabilities could make quasi-linear approaches questionable in the plasma outer region. Current ramp scenarios, selected from ASDEX Upgrade discharges, are then simulated to validate both the coupling with the free-boundary evolution and the prediction of profiles. Analysis of the underlying transport mechanisms is presented, to clarify the possible physics origin of the observed L-mode empirical energy confinement scaling. The role of toroidal micro-instabilities (ITG, TEM) and of non-linear effects is discussed. [ABSTRACT FROM AUTHOR]

The influence of rotation on the (m, n) = (3, 2) neoclassical tearing mode onset and the marginal point at the ASDEX Upgrade is investigated. In this context, the different trigger mechanisms are identified and the influence of not only the rotation but also the rotation gradient and the differential rotation between the resonant and the triggering surface on the neoclassical tearing mode (NTM) stability is analysed. The existence of an upper NTM onset threshold can be observed in correlation with the rotation normalized to the Alfvén velocity. It can also clearly be verified that at the ASDEX Upgrade the NTM onset threshold increases with co- and counter-current directed rotation and also with positive and negative rotation gradient. [ABSTRACT FROM AUTHOR]

Upgraded spectroscopic hardware and an improved impurity concentration calculation allow accurate determination of boron density in the ASDEX Upgrade tokamak. A database of boron measurements is compared to quasilinear and nonlinear gyrokinetic simulations including Coriolis and centrifugal rotational effects over a range of H-mode plasma regimes. The peaking of the measured boron profiles shows a strong anti-correlation with the plasma rotation gradient, via a relationship explained and reproduced by the theory. It is demonstrated that the rotodiffusive impurity flux driven by the rotation gradient is required for the modelling to reproduce the hollow boron profiles at higher rotation gradients. The nonlinear simulations validate the quasilinear approach, and, with the addition of perpendicular flow shear, demonstrate that each symmetry breaking mechanism that causes momentum transport also couples to rotodiffusion. At lower rotation gradients, the parallel compressive convection is required to match the most peaked boron profiles. The sensitivities of both datasets to possible errors is investigated, and quantitative agreement is found within the estimated uncertainties. The approach used can be considered a template for mitigating uncertainty in quantitative comparisons between simulation and experiment. [ABSTRACT FROM AUTHOR]

In the all-tungsten ASDEX Upgrade tokamak nitrogen seeding is a reliable method for radiative cooling at the plasma edge, regularly applied in high power scenarios. Interestingly, in the presence of nitrogen seeding the energy confinement is observed to improve significantly, compared with similar unseeded discharges, with an increase by 10-25% in plasma stored energy and HIPB98(y,2). In this paper, we document the improvement and we analyse the transport properties in the core plasma, by comparing similar discharges with and without nitrogen seeding. The increase in the suprathermal energy content is assessed as well. The impurity nitrogen is shown not to penetrate significantly into the core plasma. Non-linear gyro-kinetic simulations predict that the improvement observed in the pedestal confinement is transferred to the core via profile stiffness, in agreement with the experimental evidence, whereas the direct contribution from core confinement is small. [ABSTRACT FROM AUTHOR]

The development of the electron density profile and its dependences after the L-H transition have been investigated. Only electron cyclotron resonance heated H-modes have been analysed to exclude core particle fuelling. While the density gradient in the edge transport barrier increases significantly after the L-H transition, the pedestal top temperature rises continuously with the applied heating power and shows no pronounced change at the transition. The H-mode density saturates at a level which correlates with the neutral gas density in the divertor prior to the L-H transition. Although the density build-up varies with the available deuterium inventory, the initial increase in the edge density gradient is similar. This has been observed independent of the L-mode plasma collisionality (ν* ≈ 2.8-5.5). The analysis of electron density and temperature profiles reveals that L-H and H-L transitions occur at similar pedestal top pressures, but the pedestal top densities are always higher at the time of the H-L back transition. [ABSTRACT FROM AUTHOR]

In tokamaks, turbulent particle and toroidal momentum transport are both characterized by the presence of off-diagonal contributions which play an essential role in establishing the profile shapes of the density and the toroidal rotation under most conditions. In this paper similarities and differences between the two turbulent transport channels are pointed out and, thereby, interesting physical aspects which connect the two channels are identified. The main contributions to off-diagonal particle and toroidal momentum transport are reviewed by means of a rather simplified description, which aims at providing, when possible, a direct connection between theoretical, modelling and experimental research. [ABSTRACT FROM AUTHOR]

The parallel flows in the H-mode edge of ASDEX Upgrade are investigated. Beam-based charge-exchange recombination spectroscopy (CXRS) provides the toroidal and poloidal impurity flow velocities at the outboard midplane, while a deuterium-puff based CXRS measurement provides the toroidal impurity flow velocities at the inboard midplane. In order to more easily compare these measurements to fundamental boundary conditions, a basic overview of flows on a flux surface is presented. The boundary conditions are given by the continuity equation and mean that the flow velocities on a flux surface must have a specific structure in order to provide zero divergence. At first, poloidal impurity density asymmetries and radial transport are neglected. Inside of the pedestal-top of the electron density profile the measurements agree with the postulated flow structure, while they do not agree at the pedestal itself. Here, an extension of the theoretical scheme, which allows for a poloidal impurity density asymmetry, suggests that the measured flow velocities could be explained by an excess impurity density at the inboard midplane. In detail, the inboard impurity density is postulated to be at the separatrix up to a factor of 6.5 higher than impurity density at the outboard midplane. Near the pedestal-top of the electron density, this asymmetry disappears. Radial transport is considered as an explanation for that asymmetry. A conclusive disentanglement of the driving mechanisms requires further investigation. [ABSTRACT FROM AUTHOR]

Measurements of plasma core impurity ion temperature and toroidal rotation are obtained during the current rampup phases of plasma discharges in the ASDEX Upgrade tokamak, by means of charge exchange recombination spectroscopy utilizing short neutral beam injected blips. The phenomenology of these profiles as the current is varied is presented. The observations are reported for both ohmically heated and electron cyclotron heating assisted ramps, with auxiliary heating applied both on-axis and off-axis. The evolution of global parameters is also reported. A comparison between ion and electron kinetic profiles evolution is made. Power balance analysis is performed to establish the local confinement properties and their evolution during the current ramp. Transport modelling of several cases with a theory-based model is carried out and a recipe, which involves edge-relevant parameters, is proposed to account for the transport in the outer plasma region. Finally, linear gyrokinetic calculations of dominant microinstabilities allow us to correlate the observed transport with the background turbulence regime. [ABSTRACT FROM AUTHOR]

High-resolution charge-exchange recombination spectroscopic measurements of B5+ ions have enabled the first spatially resolved calculations of the radial electric field (Er) in the Alcator C-Mod pedestal region [E. S. Marmar, Fusion Sci. Technol. 51, 261 (2006)]. These observations offer new challenges for theory and simulation and provide for important comparisons with other devices. Qualitatively, the field structure observed on C-Mod is similar to that on other tokamaks. However, the narrow high-confinement mode (H-mode) Er well widths (5 mm) observed on C-Mod suggest a scaling with machine size, while the observed depths (up to 300 kV/m) are unprecedented. Due to the strong ion-electron thermal coupling in the C-Mod pedestal, it is possible to infer information about the main ion population in this region. The results indicate that in H-mode the main ion pressure gradient is the dominant contributor to the Er well and that the main ions have significant edge flow. C-Mod H-mode data show a clear correlation between deeper Er wells, higher confinement plasmas, and higher electron temperature pedestal heights. However, improved L-mode (I-mode) plasmas exhibit energy confinement equivalent to that observed in similar H-mode discharges, but with significantly shallower Er wells. I-mode plasmas are characterized by H-mode-like energy barriers, but with L-mode-like particle barriers. The decoupling of energy and particle barrier formation makes the I-mode an interesting regime for fusion research and provides for a low collisionality pedestal without edge localized modes. [ABSTRACT FROM AUTHOR]

The Charge Exchange Recombination Spectroscopy (CXRS) diagnostic has become a routine diagnostic on almost all major high temperature fusion experimental devices. For the optimized stellarator Wendelstein 7-X (W7-X), a highly flexible and extensive CXRS diagnostic has been built to provide high-resolution local measurements of several important plasma parameters using the recently commissioned neutral beam heating. This paper outlines the design specifics of the W7-X CXRS system and gives examples of the initial results obtained, including typical ion temperature profiles for several common heating scenarios, toroidal flow and radial electric field derived from velocity measurements, beam attenuation via beam emission spectra, and normalized impurity density profiles under some typical plasma conditions.

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