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This paper presents an overview of results from L–H transition experiments that were performed at ASDEX Upgrade (AUG) with the aim of identifying the underlying mechanisms leading to H-mode confinement. With a broad variety of experiments and new diagnostic techniques, as well as modeling efforts, AUG has contributed substantially to improving our understanding of the L–H transition over the past years. In this review, the important roles of the ion heat channel and the edge radial electric field (E r ) in the L–H transition physics are brought into context with known dependencies of the H-mode power threshold ( P L H ), such as the impact of wall material, magnetic perturbations, and the magnetic configuration. Furthermore, experimental and theoretical results obtained at AUG on the L-mode edge turbulence are connected to the mean-field E r and its related shear flow. This led to a deeper understanding of the I-phase plasma regime, has resolved the so-called isotope effect of P L H , and led to the development of a semi-analytical model that can describe AUG’s experimental observations of the L–H transition together with the L- and H-mode density limits.

The prediction of plasma rotation is of high interest for fusion research due to the effects of the rotation upon magnetohydrodynamic (MHD) instabilities, impurities, and turbulent transport in general. In this work, an analysis method was studied and validated to reliably extract momentum transport coefficients from neutral beam injection (NBI) modulation experiments. To this end, a set of discharges was created with similar background profiles for the ion and electron temperatures, the heat fluxes, the electron density, and the plasma rotation that, therefore, should exhibit similar momentum transport coefficients. In these discharges, a range of temporal perturbations were imposed by modulating and varying the power deposition of the NBI, electron-cyclotron-resonance heating (ECRH), and ion-cyclotron-resonance heating (ICRH). The transport model including diffusion, convection, and residual stress was implemented within the ASTRA code. The Prandtl number P r = χ φ / χ i was assessed via the GKW code. A convective Coriolis pinch was fitted and the intrinsic torque from the residual stress was estimated. The obtained transport coefficients agree within error bars for sufficiently small imposed temperature perturbations, as would be expected, from the similar background profiles. This successful validation of the methodology opens the door to study the parametric dependence of the diffusive and convective momentum transport of the main ions of the plasma as well as the turbulent intrinsic torque in a future work.

Future fusion reactors require a safe, steady state divertor operation. The required detached operation is, in tokamaks with metal walls, usually achieved by seeding of impurities, such as nitrogen. With strong seeding levels, the dominant radiation is emitted from a small, poloidally localized volume inside the confined region, in the vicinity of the X-point. The location of the radiating volume is observed to vary relative to the X-point depending on seeding and power levels, i.e. depending on the degree of detachment. At the ASDEX Upgrade tokamak, the position of the radiator relative to the X-point can be controlled in real time by a modulation of the nitrogen puff level. At a certain height of the radiator above the X-point, an ELM-suppressed regime is observed with minimal reduction of confinement. While the control of the X-point radiator already allows operation in full detachment at a dissipated power fraction of around 95 %, which is required for a future reactor and was previously never achieved in a controlled way, such an ELM-suppressed regime additionally eliminates the challenge of the transient, intolerably high heat fluxes by ELMs. Both requirements are met in the presented regime while maintaining a high energy confinement at high density.

Poloidal asymmetries of the propagation velocity of density fluctuations perpendicular to the magnetic field measured with Doppler reflectometry have been reported in several magnetic confinement plasma devices. Careful analysis of a large variety of different low confinement mode plasma scenarios performed at the ASDEX Upgrade tokamak does not reveal such an asymmetry outside the uncertainties of the evaluation process of the measurement data. The perpendicular velocity is investigated between mid-radius and the plasma edge and follows the poloidal dependence of the E × B drift velocity regardless of the probed turbulence structure size. Compared to measurements of a charge exchange recombination spectroscopy diagnostic this points towards a significantly smaller phase velocity than the E × B drift velocity. The analysis technique is presented in a representative discharge together with a sensitivity study of the impact of density, magneto hydrodynamic equilibrium and diagnostic alignment on the interpretation of the measured Doppler shift using ray tracing and thus on poloidal asymmetries. Three more highly different plasma scenarios with poloidally symmetric velocity profiles are shown.

Fast ion physics is an active field of research in the fusion community, but most studies focus on deuterium fast ions. The generation and investigation of energetic helium in present devices, however, provide significantly more insight on how the fast alpha particles produced from fusion reactions, will behave in future reactor plasmas. Fast helium ion populations can be measured with charge exchange recombination spectroscopy (CXRS) in the wings of the helium spectral line (He II n = 4–3, 468.6 nm) providing information on their distribution function. CXRS measurements of energetic 3He ions, a first for ASDEX Upgrade, are presented. The 3He ions are accelerated to high energies by a three-ion cyclotron resonance frequency (ICRF) heating scenario in a mixed hydrogen–deuterium plasma. The spectral signature of the energetic helium ions in the charge exchange spectra is presented and compared with the theoretical predictions obtained with the TORIC-SSFPQL code. The magnitude of the predicted charge exchange spectral radiance, obtained via forward-modelling of the spectrum utilising TORIC-SSFPQL distribution functions, and the expected energies of the ions agree well with the measurement, confirming that the spectral feature is due to ICRF-accelerated 3He ions. Comparisons between the experimental measurement and the modelling reveal discrepancies that illuminate details of the velocity distribution function of these ions.

Argon is being considered as a radiator for future fusion devices. To support this, experiments in present day devices are needed to assess its effectiveness as a radiator and to study its compatibility with key aspects of reactor operation. For these purposes, accurate measurement of the Ar density in the confined plasma region are required. Charge exchange recombination spectroscopy is capable of providing this information, but requires validated cross-sections to produce accurate density profiles. For typical ASDEX Upgrade plasma parameters Ar+16 is the charge state of the most interest and the ArXVI n = 15–14 was identified as the best target for these measurements. Due to the fine structure splitting, the Ar charge exchange (CX) emission lines are highly asymmetric and detailed modeling is required to extract accurate ion temperatures or rotations. For the evaluation of Ar+16 densities, there are two main sets of CX cross-sections available: the data calculated by Schultz et al (Whyte et al 2010 J. Phys. B At. Mol. Opt. Phys 43 144002, Schultz et al 2010 J. Phys. B At. Mol. Opt. Phys. 43 144002) (ORNL) and by Errea et al (2006 J. Phys. B At. Mol. Opt. Phys. 39 L91) (UAM). These cross-sections differ by over an order of magnitude and have very different energy dependencies. In this work, the validity of these datasets is tested experimentally. The Ar density profiles calculated using the ORNL cross-sections are 10–50× too large while the UAM densities are a factor of 2.73 too small. The UAM data, however, does a much better job at capturing the observed energy dependence. The authors conclude that the CTMC calculations of Errea, which use a hydrogenic distribution to describe the initial target and donor distributions better reproduce the experimental data. However, to produce accurate Ar density profiles, these cross-sections need to be corrected downward and the best profiles are produced with a small correction to the UAM energy dependence.

The electron cyclotron resonance heating (ECRH) system of the ASDEX Upgrade tokomak has been upgraded over the last 15 years from a 2 MW, 2 s, 140 GHz system to an 8 MW, 10 s, dual frequency system (105/140 GHz). The power exceeds the L/H power threshold by at least a factor of two, even for high densities, and roughly equals the installed ion cyclotron range of frequencies power. The power of both wave heating systems together (>10 MW in the plasma) is about half of the available neutral beam injection (NBI) power, allowing significant variations of torque input, of the shape of the heating profile and of Qe/Qi, even at high heating power. For applications at a low magnetic field an X3-heating scheme is routinely in use. Such a scenario is now also forseen for ITER to study the first H-modes at one third of the full field. This versatile system allows one to address important issues fundamental to a fusion reactor: H-mode operation with dominant electron heating, accessing low collisionalities in full metal devices (also related to suppression of edge localized modes with resonant magnetic perturbations), influence of Te/Ti and rotational shear on transport, and dependence of impurity accumulation on heating profiles. Experiments on all these subjects have been carried out over the last few years and will be presented in this contribution. The adjustable localized current drive capability of ECRH allows dedicated variations of the shape of the q-profile and the study of their influence on non-inductive tokamak operation (so far at q 95 > 5.3). The ultimate goal of these experiments is to use the experimental findings to refine theoretical models such that they allow a reliable design of operational schemes for reactor size devices. In this respect, recent studies comparing a quasi-linear approach (TGLF) with fully non-linear modeling (GENE) of non-inductive high-beta plasmas will be reported.

Understanding the dependence of the H-mode power threshold, PLH on the main ion composition is crucial for H-mode operation in ITER. In this work, H-mode power threshold studies conducted at ASDEX Upgrade are presented. These were performed in hydrogen plasmas with helium doping and in mixed hydrogen-deuterium plasmas. PLH stays constant on the level of pure hydrogen in hydrogen plasmas with helium doping up to cHe = 20%. Power balance calculations with the transport code ASTRA show that the edge ion heat flux at the L-H transition is independent of helium concentration and applied auxiliary heating, supporting that a critical edge ion heat flux is needed to enter H-mode. A non-linear dependence of PLH on the relative hydrogen content is found. PLH and the edge ion heat flux are constant at the level of pure deuterium for between 0 and 0.5 and constant on the level of pure hydrogen for between 0.8 and 1.

A new method was developed to model the neutral population produced by the gas puff based charge exchange recombination spectroscopy systems at ASDEX Upgrade (AUG). With this method, the edge impurity density on the high field side (HFS) and low field side (LFS) can be obtained without the need to apply a neutral beam injection system. The neutral penetration needed for the calculation of the impurity density is obtained with a new gas puff module implemented in the FIDASIM code. The LFS impurity density profile evaluated with the new gas puff module matches the impurity density calculated with standard beam-based charge exchange diagnostics. Impurity temperature, rotation and density profiles at the HFS and LFS of an AUG H-mode discharge are presented. Edge impurity toroidal and poloidal flows show asymmetric structures. The impurity density asymmetries obtained with the new gas puff module are consistent with the observed flow structure.

A detailed understanding of island seeding is crucial to avoid neoclassical tearing modes and their negative consequences like confinement degradation and disruptions. In the present work, we investigate the growth of 2/1 islands in response to magnetic perturbations. Although we use externally applied perturbations produced by resonant magnetic perturbation (RMP) coils for this study, the results are directly transferable to island seeding by other MHD instabilities creating a resonant magnetic field component at the rational surface. Experimental results for 2/1 island penetration from ASDEX Upgrade are presented extending previous studies. Simulations are based on an ASDEX Upgrade L-mode discharge with low collisionality and active RMP coils. Our numerical studies are performed with the 3D, two-fluid, nonlinear MHD code JOREK. All three phases of mode seeding observed in the experiment are also seen in the simulations: first, a weak response phase characterized by large perpendicular electron flow velocities followed by a fast growth of the magnetic island size accompanied by a reduction of the perpendicular electron velocity and finally the saturation to a fully formed island state with perpendicular electron velocity close to zero. Thresholds for mode penetration are observed in the plasma rotation as well as in the RMP coil current. A hysteresis of the island size and electron perpendicular velocity is observed between the ramping up and down of the RMP amplitude consistent with an analytically predicted bifurcation. The transition from dominant kink/bending to tearing parity during the penetration is investigated.

Impurity seeding will be mandatory for ITER to protect the divertor tiles from excessive heat loads. Nitrogen seems to be a possible candidate due to its good radiation properties. The N content in the divertor plasma is crucial for cooling by radiation but is difficult to characterize. To assess the nitrogen fluence from the divertor plasma in nitrogen seeded discharges of ASDEX Upgrade (AUG), the concentration of two nitrogen containing molecules, ammonia and molecular nitrogen (N2) was measured. The N2 measurements were strongly affected by local effects which indicates that a significant fraction of the puffed N2 did not enter the plasma. In contrast, the measured ammonia fluence is proportional to the integrated nitrogen core density. This leads to the possibility of gaining the nitrogen fluence of the plasma, with the help of the created ammonia, for nitrogen seeded discharges. In discharges where the wall inventory was the only source of nitrogen in the plasma, 50% of the nitrogen was converted to ammonia, in the inner divertor. This allowed for the assessment of the nitrogen atom fluence from the plasma based on the detected ammonia in N2-seeded discharges.

The research program of the TCV tokamak ranges from conventional to advanced-tokamak scenarios and alternative divertor configurations, to exploratory plasmas driven by theoretical insight, exploiting the device's unique shaping capabilities. Disruption avoidance by real-time locked mode prevention or unlocking with electron-cyclotron resonance heating (ECRH) was thoroughly documented, using magnetic and radiation triggers. Runaway generation with high-Z noble-gas injection and runaway dissipation by subsequent Ne or Ar injection were studied for model validation. The new 1 MW neutral beam injector has expanded the parameter range, now encompassing ELMy H-modes in an ITER-like shape and nearly non-inductive H-mode discharges sustained by electron cyclotron and neutral beam current drive. In the H-mode, the pedestal pressure increases modestly with nitrogen seeding while fueling moves the density pedestal outwards, but the plasma stored energy is largely uncorrelated to either seeding or fueling. High fueling at high triangularity is key to accessing the attractive small edge-localized mode (type-II) regime. Turbulence is reduced in the core at negative triangularity, consistent with increased confinement and in accord with global gyrokinetic simulations. The geodesic acoustic mode, possibly coupled with avalanche events, has been linked with particle flow to the wall in diverted plasmas. Detachment, scrape-off layer transport, and turbulence were studied in L- and H-modes in both standard and alternative configurations (snowflake, super-X, and beyond). The detachment process is caused by power 'starvation' reducing the ionization source, with volume recombination playing only a minor role. Partial detachment in the H-mode is obtained with impurity seeding and has shown little dependence on flux expansion in standard single-null geometry. In the attached L-mode phase, increasing the outer connection length reduces the in-out heat-flow asymmetry. A doublet plasma, featuring an internal X-point, was achieved successfully, and a transport barrier was observed in the mantle just outside the internal separatrix. In the near future variable-configuration baffles and possibly divertor pumping will be introduced to investigate the effect of divertor closure on exhaust and performance, and 3.5 MW ECRH and 1 MW neutral beam injection heating will be added., This work was supported in part by the Swiss National Science Foundation.

The design and optimization of a future tokamak fusion reactor, from the point of view of the plasma performance and output fusion power, requires the understanding of the underlying physics and the validation on present experiments of the theory-based tools used for the predictions. Present experimental efforts are devoted to approach reactor-relevant parameters (collisionality, , normalized heat fluxes and sources, temperatures ratio) as much as possible. In this work a series of discharges performed on ASDEX Upgrade are presented, where plasma density and auxiliary power levels are scanned to cover a parameter space that moves towards reactor relevant parameters. On this dedicated discharge dataset, a first-principle-based model is then applied for validation. The degree of agreement between code results and experimental measurements is shown and discussed. In cases where discrepancy is found, possible causes are identified. It is shown that the employed modeling tool can predict the overall trend, consistent with more fundamental theoretical considerations, although quantitative extrapolation still has to be performed with care. Key results of this work show that the density peaking is mainly sustained by turbulence, with a minor role for the fueling source, and how this experimental demonstration is important to predict future reactor density profiles. Moreover, the role of electromagnetic effects is also pointed out.

Within the EUROfusion MST1 work package, a series of experiments has been conducted on AUG and TCV devices to disentangle the role of plasma fueling and plasma shape for the onset of small ELM regimes. On both devices, small ELM regimes with high confinement are achieved if and only if two conditions are fulfilled at the same time. Firstly, the plasma density at the separatrix must be large enough (), leading to a pressure profile flattening at the separatrix, which stabilizes type-I ELMs. Secondly, the magnetic configuration has to be close to a double null (DN), leading to a reduction of the magnetic shear in the extreme vicinity of the separatrix. As a consequence, its stabilizing effect on ballooning modes is weakened. Peer Reviewed Article escrit per 365 autors/autores: Labit, B.; Eich, T.; Harrer, G. F.; Wolfrum, E.; Bernert, M.; Dunne, M. G.; Frassinetti, L.; Hennequin, P.; Maurizio, R.; Merle, A.; Meyer, H.; Saarelma, S.; Sheikh, U.; Adamek, J.; Agostini, M.; Aguiam, D.; Akers, R.; Albanese, R.; Albert, C.; Alessi, E.; Ambrosino, R.; Andr be, Y.; Angioni, C.; Apruzzese, G.; Aradi, M.; Arnichand, H.; Auriemma, F.; Avdeeva, G.; Ayllon-Guerola, J. M.; Bagnato, F.; Bandaru, V. K.; Barnes, M.; Barrera-Orte, L.; Bettini, P.; Bilato, R.; Biletskyi, O.; Bilkova, P.; Bin, W.; Blanchard, P.; Blanken, T.; Bobkov, V.; Bock, A.; Boeyaert, D.; Bogar, K.; Bogar, O.; Bohm, P.; Bolzonella, T.; Bombarda, F.; Boncagni, L.; Bouquey, F.; Bowman, C.; Brezinsek, S.; Brida, D.; Brunetti, D.; Bucalossi, J.; Buchanan, J.; Buermans, J.; Bufferand, H.; Buller, S.; Buratti, P.; Burckhart, A.; Calabr, G.; Calacci, L.; Camenen, Y.; Cannas, B.; Cano Megías, P.; Carnevale, D.; Carpanese, F.; Carr, M.; Carralero, D.; Carraro, L.; Casolari, A.; Cathey, A.; Causa, F.; Cavedon, M.; Cecconello, M.; Ceccuzzi, S.; Cerovsky, J.; Chapman, S.; Chmielewski, P.; Choi, D.; Cianfarani, C.; Ciraolo, G.; Coda, S.; Coelho, R.; Colas, L.; Colette, D.; Cordaro, L.; Cordella, F.; Costea, S.; Coster, D.; Cruz Zabala, D. J.; Cseh, G.; Czarnecka, A.; Cziegler, I.; D’Arcangelo, O.; Dal Molin, A.; David, P.; De Carolis, G.; De Oliveira, H.; Decker, J.; Dejarnac, R.; Delogu, R.; Den Harder, N.; Dimitrova, M.; Dolizy, F.; Domínguez-Palacios Durán, J. J.; Douai, D.; Drenik, A.; Dreval, M.; Dudson, B.; Dunai, D.; Duval, B. P.; Dux, R.; Elmore, S.; Embréus, O.; Erds, B.; Fable, E.; Faitsch, M.; Fanni, A.; Farnik, M.; Faust, I.; Faustin, J.; Fedorczak, N.; Felici, F.; Feng, S.; Feng, X.; Ferreira, J.; Ferr, G.; Février, O.; Ficker, O.; Figini, L.; Figueiredo, A.; Fil, A.; Fontana, M.; Francesco, M.; Fuchs, C.; Futatani, S.; Gabellieri, L.; Gadariya, D.; Gahle, D.; Galassi, D.; Gałązka, K.; Galdon-Quiroga, J.; Galeani, S.; Gallart, D.; Gallo, A.; Galperti, C.; Garavaglia, S.; Garcia, J.; Garcia-Lopez, J.; Garcia-Mu oz, M.; Garzotti, L.; Gath, J.; Geiger, B.; Giacomelli, L.; Giannone, L.; Gibson, S.; Gil, L.; Giovannozzi, E.; Giruzzi, G.; Gobbin, M.; Gonzalez-Martin, J.; Goodman, T. P.; Gorini, G.; Gospodarczyk, M.; Granucci, G.; Grekov, D. 1; Grenfell, G.; Griener, M.; Groth, M.; Grover, O.; Gruca, M.; Gude, A.; Guimarais, L.; Gyergyek, T.; Hacek, P.; Hakola, A.; Ham, C.; Happel, T.; Harrison, J.; Havranek, A.; Hawke, J.; Henderson, S.; Hesslow, L.; Hitzler, F.; Hnat, B.; Hobirk, J.; Hoelzl, M.; Hogeweij, D.; Hopf, C.; Hoppe, M.; Horacek, J.; Hron, M.; Huang, Z.; Iantchenko, A.; Iglesias, D.; Igochine, V.; Innocente, P.; Ionita-Schrittwieser, C.; Isliker, H.; Ivanova-Stanik, I.; Jacobsen, A.; Jakubowski, M.; Janky, F.; Jardin, A.; Jaulmes, F.; Jensen, T.; Jonsson, T.; Kallenbach, A.; Kappatou, A.; Karpushov, A.; Kasilov, S.; Kazakov, Y.; Kazantzidis, P. V.; Keeling, D.; Kelemen, M.; Kendl, A.; Kernbichler, W.; Kirk, A.; Kocsis, G.; Komm, M.; Kong, M.; Korovin, V.; Koubiti, M.; Kovacic, J.; Krawczyk, N.; Krieger, K.; Kripner, L.; Křivská, A.; Kudlacek, O.; Kulyk, Y.; Kurki-Suonio, T.; Kwiatkowski, R.; Laggner, F.; Laguardia, L.; Lahtinen, A.; Lang, P.; Likonen, J.; Lipschultz, B.; Liu, F.; Lombroni, R.; Lorenzini, R.; Loschiavo, V. P.; Lunt, T.; MacUsova, E.; Madsen, J.; Maggiora, R.; Maljaars, B.; Manas, P.; Mantica, P.; Mantsinen, M. J.; Manz, P.; Maraschek, M.; Marchenko, V.; Marchetto, C.; Mariani, A.; Marini, C.; Markovic, T.; Marrelli, L.; Martin, P.; Martín Solís, J. R.; Martitsch, A.; Mastrostefano, S.; Matos, F.; Matthews, G.; Mayoral, M.-L.; Mazon, D.; Mazzotta, C.; Mc Carthy, P.; McClements, K.; McDermott, R.; McMillan, B.; Meineri, C.; Menkovski, V.; Meshcheriakov, D.; Messmer, M.; Micheletti, D.; Milanesio, D.; Militello, F.; Miron, I. G.; Mlynar, J.; Moiseenko, V.; Molina Cabrera, P. A.; Morales, J.; Moret, J.-M.; Moro, A.; Moulton, D.; Nabais, F.; Naulin, V.; Naydenkova, D.; Nem, R. D.; Nespoli, F.; Newton, S.; Nielsen, A. H.; Nielsen, S. K.; Nikolaeva, V.; Nocente, M.; Nowak, S.; Oberkofler, M.; Ochoukov, R.; Ollus, P.; Olsen, J.; Omotani, J.; Ongena, J.; Orain, F.; Orsitto, F. P.; Paccagnella, R.; Palha, A.; Panaccione, L.; Panek, R.; Panjan, M.; Papp, G.; Paradela Perez, I.; Parra, F.; Passeri, M.; Pau, A.; Pautasso, G.; Pavlichenko, R.; Perek, A.; Pericoli Radolfini, V.; Pesamosca, F.; Peterka, M.; Petrzilka, V.; Piergotti, V.; Pigatto, L.; Piovesan, P.; Piron, C.; Piron, L.; Plyusnin, V.; Pokol, G.; Poli, E.; Pölöskei, P.; Popov, T.; Popovic, Z.; Pór, G.; Porte, L.; Pucella, G.; Puiatti, M. E.; Pütterich, T.; Rabinski, M.; Juul Rasmussen, J.; Rasmussen, J.; Rattá, G. A.; Ratynskaia, S.; Ravensbergen, T.; Réfy, D.; Reich, M.; Reimerdes, H.; Reimold, F.; Reiser, D.; Reux, C.; Reznik, S.; Ricci, D.; Rispoli, N.; Rivero-Rodriguez, J. 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T.; Walkden, N.; Wauters, T.; Weiland, M.; Weinzettl, V.; Wensing, M.; Wiesen, S.; Wiesenberger, M.; Wilkie, G.; Willensdorfer, M.; Wischmeier, M.; Wu, K.; Xiang, L.; Zagorski, R.; Zaloga, D.; Zanca, P.; Zaplotnik, R.; Zebrowski, J.; Zhang, W.; Zisis, A.; Zoletnik, S.; Zuin, M.

In ASDEX Upgrade with full-tungsten wall, boronization is an important tool to control tungsten sources and allow reliable operation at low collisionality. The duration of the beneficial effects is limited by the erosion of the boron layer, in particular on the tungsten ion-cyclotron antenna limiters. We report results from experiments exploring injection boron-rich powders in tokamak plasmas as a way to replenish boron coating and extend the lifetime of boronization effects. Pure boron and boron nitride powders were introduced gravitationally in plasma discharges at rates up to 60 mg/s for durations up to 3 s. Boron injection with cumulative amounts >100 mg appeared to improve wall conditions similarly to boronization, with indications of reduced influx of oxygen and tungsten from the antenna limiter. Moreover, cumulative injection of 340 mg of B appeared sufficient to enable successful execution of low collisionality scenarios critically relying on freshly boronized walls.

Integrated edge-localized-mode (ELM)-free or small ELM scenarios for the demonstration fusion power plant (DEMO) are investigated in ASDEX Upgrade using argon seeding for radiative power removal mainly in the pedestal region. An important aspect is the modification of the electron pressure in the pedestal by the additional radiative power losses. Full ELM suppression could be achieved in a no-ELM H-mode scenario featuring an edge electromagnetic quasicoherent mode up to a heating power of 5 MW, where argon radiation allowed the extension of the heating power operational space. At higher powers up to 12 MW (reaching the beta limit), ELMs of reduced size prevail and detachment is obtained by the argon seeding. Control of the position of a radiating zone localized inside the X-point was found to be favorable compared to the control of the separatrix power for low P s e p / P h e a t . Integration of pedestal cooling with Ar and ELM suppression by resonant magnetic perturbations (RMP) allowed an increase of the core radiation and a partial recovery of normalized confinement to H98 = 1. This favorable behavior is finally limited by the loss of the RMP density pump-out effect, followed by an ELM-free H-mode phase and re-occurrence of ELMs. For an active tailoring of the pedestal pressure profile, precise knowledge of the radiation profile is required. Modeling of the argon radiation profile with the STRAHL code showed good agreement with bolometry only if charge exchange of neutral deuterium with argon ions was taken into account, highlighting the importance of the neutral density in the pedestal region. In view of best integration of a no-ELM scenario with divertor detachment and high heating power, the divertor compression and enrichment of argon and neon were compared using dynamic gas puff experiments. Argon shows more than a factor of 3 higher divertor enrichment compared to neon, but the absolute values decrease with higher neutral deuterium pressure.

Non-linear, radially global, turbulence simulations of ASDEX Upgrade (AUG) plasmas are performed and the nonlinear generated intrinsic flow shows agreement with the intrinsic flow gradients measured in the core of Ohmic L-mode plasmas at nominal parameters. Simulations utilising the kinetic electron model show hollow intrinsic flow profiles as seen in a predominant number of experiments performed at similar plasma parameters. In addition, significantly larger flow gradients are seen than in a previous flux-tube analysis (Hornsby et al {\it Nucl. Fusion} (2017)). Adiabatic electron model simulations can show a flow profile with opposing sign in the gradient with respect to a kinetic electron simulation, implying a reversal in the sign of the residual stress due to kinetic electrons. The shaping of the intrinsic flow is strongly determined by the density gradient profile. The sensitivity of the residual stress to variations in density profile curvature is calculated and seen to be significantly stronger than to neoclassical flows (Hornsby et al {\it Nucl. Fusion} (2017)). This variation is strong enough on its own to explain the large variations in the intrinsic flow gradients seen in some AUG experiments. Analysis of the symmetry breaking properties of the turbulence shows that profile shearing is the dominant mechanism in producing a finite parallel wave-number, with turbulence gradient effects contributing a smaller portion of the parallel wave-vector.

One main programme topic at the ASDEX Upgrade all-metal-wall tokamak is development of a high-density regime with central densities at reactor grade level while retaining high-confinement properties. This required development of appropriate control techniques capable of coping with the pellet tool, a powerful means of fuelling but one which presented challenges to the control system for handling of related perturbations. Real-time density profile control was demonstrated, raising the core density well above the Greenwald density while retaining the edge density in order to avoid confinement losses. Recently, a new model-based approach was implemented that allows direct control of the central density. Investigations focussed first on the N-seeding scenario owing to its proven potential to yield confinement enhancements. Combining pellets and N seeding was found to improve the divertor buffering further and enhance the operational range accessible. For core densities up to about the Greenwald density, a clear improvement with respect to the non-seeding reference was achieved; however, at higher densities this benefit is reduced. This behaviour is attributed to recurrence of an outward shift of the edge density profile, resulting in a reduced peeling-ballooning stability. This is similar to the shift seen during strong gas puffing, which is required to prevent impurity influx in ASDEX Upgrade. First tests indicate that highly-shaped plasma configurations like the ITER base-line scenario, respond very well to pellet injection, showing efficient fuelling with no measurable impact on the edge density profile.

A newly installed core charge exchange recombination spectroscopy (CXRS) diagnostic at ASDEX Upgrade (AUG) enables the evaluation of the core poloidal rotation (upol) through the inboard-outboard asymmetry of the toroidal rotation with an accuracy of 0.5 to 1 km s-1. Using this technique, the total plasma flow has been measured in Ohmic L-mode plasmas across the transition from the linear to saturated ohmic confinement (LOC-SOC) regimes. The core poloidal rotation of the plasma around mid-radius is found to be always in the ion diamagnetic direction, in disagreement with neoclassical (NC) predictions. The edge rotation is found to be electron-directed and consistent with NC codes. This measurement provides as well the missing ingredient to evaluate the core E × B velocity (uE×B) from data only, which can then be compared to measurements of the perpendicular velocity of the turbulent fluctuations (u) to gain information on the turbulent phase velocity (vph). The non neoclassical upol from CXRS leads to good agreement between uE×B and u indicating that vph is small and at similar values as found with gyrokinetic simulations. Moreover, the data shows a shift of vph in the ion-diamagnetic direction at the edge after the transition from LOC to SOC consistent with a change in the dominant turbulence regime. The upgrade of the core CXRS system provides as well a deeper insight into the intrinsic rotation. This paper shows that the reversal of the core toroidal rotation occurs clearly after the LOC-SOC transition and concomitant with the peaking of the electron density.

A new ion cyclotron resonance frequency (ICRF) modulation technique has been developed and exploited at ASDEX Upgrade for obtaining time perturbed boron density signals. Square wave modulation of the ICRF heating power results in a periodic modulation of the boron density in the edge, which propagates inward. From this time perturbed boron density signal, the boron diffusivity and convection in the radial transport equation are individually determined. This task is done by solving an inverse problem by a quasi-Newton method. This implemented framework has been verified with the Method of Manufactured Solutions and benchmarked with the transport code STRAHL. The experimental transport coefficients are compared with neoclassical calculations with the code NEO and gyrokinetic simulations with the code GKW. The neoclassical diffusivity is well below the experimental one. The comparison to gyrokinetic theory shows a good agreement in the diffusivity, but the theoretical convection predicts a more peaked boron density profile than measured in the experiment.

Access conditions for full suppression of Edge Localised Modes (ELMs) by Magnetic Perturbations (MP) in low density high confinement mode (H-mode) plasmas are studied in the ASDEX Upgrade tokamak. The main empirical requirements for full ELM suppression in our experiments are: 1. The poloidal spectrum of the MP must be aligned for best plasma response from weakly stable kink-modes, which amplify the perturbation, 2. The plasma edge density must be below a critical value, $3.3 \times 10^{19}$~m$^{-3}$. The edge collisionality is in the range $\nu^*_i = 0.15-0.42$ (ions) and $\nu^*_e = 0.15-0.25$ (electrons). However, our data does not show that the edge collisionality is the critical parameter that governs access to ELM suppression. 3. The pedestal pressure must be kept sufficiently low to avoid destabilisation of small ELMs. This requirement implies a systematic reduction of pedestal pressure of typically 30\% compared to unmitigated ELMy H-mode in otherwise similar plasmas. 4. The edge safety factor $q_{95}$ lies within a certain window. Within the range probed so far, $q_{95}=3.5-4.2$, one such window, $q_{95}=3.57-3.95$ has been identified. Within the range of plasma rotation encountered so far, no apparent threshold of plasma rotation for ELM suppression is found. This includes cases with large cross field electron flow in the entire pedestal region, for which two-fluid MHD models predict that the resistive plasma response to the applied MP is shielded.

The ASDEX Upgrade (AUG) programme, jointly run with the EUROfusion MST1 task force, continues to significantly enhance the physics base of ITER and DEMO. Here, the full tungsten wall is a key asset for extrapolating to future devices. The high overall heating power, flexible heating mix and comprehensive diagnostic set allows studies ranging from mimicking the scrape-off-layer and divertor conditions of ITER and DEMO at high density to fully non-inductive operation (q95 = 5.5, ) at low density. Higher installed electron cyclotron resonance heating power 6 MW, new diagnostics and improved analysis techniques have further enhanced the capabilities of AUG. Stable high-density H-modes with MW m−1 with fully detached strike-points have been demonstrated. The ballooning instability close to the separatrix has been identified as a potential cause leading to the H-mode density limit and is also found to play an important role for the access to small edge-localized modes (ELMs). Density limit disruptions have been successfully avoided using a path-oriented approach to disruption handling and progress has been made in understanding the dissipation and avoidance of runaway electron beams. ELM suppression with resonant magnetic perturbations is now routinely achieved reaching transiently . This gives new insight into the field penetration physics, in particular with respect to plasma flows. Modelling agrees well with plasma response measurements and a helically localised ballooning structure observed prior to the ELM is evidence for the changed edge stability due to the magnetic perturbations. The impact of 3D perturbations on heat load patterns and fast-ion losses have been further elaborated. Progress has also been made in understanding the ELM cycle itself. Here, new fast measurements of and Er allow for inter ELM transport analysis confirming that Er is dominated by the diamagnetic term even for fast timescales. New analysis techniques allow detailed comparison of the ELM crash and are in good agreement with nonlinear MHD modelling. The observation of accelerated ions during the ELM crash can be seen as evidence for the reconnection during the ELM. As type-I ELMs (even mitigated) are likely not a viable operational regime in DEMO studies of 'natural' no ELM regimes have been extended. Stable I-modes up to have been characterised using -feedback. Core physics has been advanced by more detailed characterisation of the turbulence with new measurements such as the eddy tilt angle—measured for the first time—or the cross-phase angle of and fluctuations. These new data put strong constraints on gyro-kinetic turbulence modelling. In addition, carefully executed studies in different main species (H, D and He) and with different heating mixes highlight the importance of the collisional energy exchange for interpreting energy confinement. A new regime with a hollow profile now gives access to regimes mimicking aspects of burning plasma conditions and lead to nonlinear interactions of energetic particle modes despite the sub-Alfvénic beam energy. This will help to validate the fast-ion codes for predicting ITER and DEMO. This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014–2018 and 2019–2020 under grant agreement No. 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission. "Article signat per més de 100 autors/es: H. Meyer, for the AUG Team: D. Aguiam, C. Angioni, C.G. Albert, N. Arden, R. Arredondo Parra, O. Asunta, M. de Baar, M. Balden, V. Bandaru, K. Behler, A. Bergmann, J. Bernardo, M. Bernert, A. Biancalani, R. Bilato, G. Birkenmeier, T.C. Blanken, V. Bobkov, A. Bock, T. Bolzonella, A. Bortolon, B. Böswirth, C. Bottereau, A. Bottino, H. van den Brand, S. Brezinsek, D. Brida, F. Brochard, C. Bruhn, J. Buchanan, A. Buhler, A. Burckhart, Y. Camenen, D. Carlton, M. Carr, D. Carralero, C. Castaldo, M. Cavedon, C. Cazzaniga, S. Ceccuzzi, C. Challis, A. Chankin, S. Chapman, C. Cianfarani, F. Clairet, S. Coda, R. Coelho, J.W. Coenen, L. Colas, G.D. Conway, S. Costea, D.P. Coster, T.B. Cote, A. Creely, G. Croci, G. Cseh, A. Czarnecka, I. Cziegler, O. D'Arcangelo, P. David, C. Day, R. Delogu, P. de Marné, S.S. Denk, P. Denner, M. Dibon, A. Di Siena, D. Douai, A. Drenik, R. Drube, M. Dunne, B.P. Duval, R. Dux, T. Eich, S. Elgeti, K. Engelhardt, B. Erdös, I. Erofeev, B. Esposito, E. Fable, M. Faitsch, U. Fantz, H. Faugel, I. Faust, F. Felici, J. Ferreira, S. Fietz, A. Figuereido, R. Fischer, O. Ford, L. Frassinetti, S. Freethy, M. Fröschle, G. Fuchert, J.C. Fuchs, H. Fünfgelder, K. Galazka, J. Galdon-Quiroga, A. Gallo, Y. Gao, S. Garavaglia, A. Garcia-Carrasco, M. Garcia-Muñoz, B. Geiger, L. Giannone, L. Gil, E. Giovannozzi, C. Gleason-González, S. Glöggler, M. Gobbin, T. Görler, I. Gomez Ortiz, J. Gonzalez Martin, T. Goodman, G. Gorini, D. Gradic, A. Gräter, G. Granucci, H. Greuner, M. Griener, M. Groth, A. Gude, S. Günter, L. Guimarais, G. Haas, A.H. Hakola, C. Ham, T. Happel, N. den Harder, G.F. Harrer, J. Harrison, V. Hauer, T. Hayward-Schneider, C.C. Hegna, B. Heinemann, S. Heinzel, T. Hellsten, S. Henderson, P. Hennequin, A. Herrmann, M.F. Heyn, E. Heyn, F. Hitzler, J. Hobirk, K. Höfler, M. Hölzl, T. Höschen, J.H. Holm, C. Hopf, W.A. Hornsby, L. Horvath, A. Houben, A. Huber, V. Igochine, T. Ilkei, I. Ivanova-Stanik, W. Jacob, A.S. Jacobsen, F. Janky, A. Jansen van Vuuren, A. Jardin, F. Jaulmes, F. Jenko, T. Jensen, E. Joffrin, C.-P. Käsemann, A. Kallenbach, S. Kálvin, M. Kantor, A. Kappatou, O. Kardaun, J. Karhunen, S. Kasilov,, Y. Kazakov, W. Kernbichler, A. Kirk, S. Kjer Hansen, V. Klevarova, G. Kocsis, A. Köhn, M. Koubiti, K. Krieger, A. Krivska, A. Krämer-Flecken, O. Kudlacek, T. Kurki-Suonio, B. Kurzan, B. Labit, K. Lackner, F. Laggner, P.T. Lang, P. Lauber, A. Lebschy, N. Leuthold, M. Li, O. Linder, B. Lipschultz, F. Liu, Y. Liu, A. Lohs, Z. Lu, T. Luda di Cortemiglia, N.C. Luhmann, R. Lunsford, T. Lunt, A. Lyssoivan, T. Maceina, J. Madsen, R. Maggiora, H. Maier, O. Maj, J. Mailloux, R. Maingi, E. Maljaars, P. Manas, A. Mancini, A. Manhard, M.-E. Manso, P. Mantica, M. Mantsinen, P. Manz, M. Maraschek, C. Martens, P. Martin, L. Marrelli, A. Martitsch, M. Mayer, D. Mazon, P.J. McCarthy, R. McDermott, H. Meister, A. Medvedeva, R. Merkel, A. Merle, V. Mertens, D. Meshcheriakov, O. Meyer, J. Miettunen, D. Milanesio, F. Mink, A. Mlynek, F. Monaco, C. Moon, F. Nabais, A. Nemes-Czopf, G. Neu, R. Neu, A.H. Nielsen, S.K. Nielsen, V. Nikolaeva, M. Nocente, J.-M. Noterdaeme, I. Novikau, S. Nowak, M. Oberkofler, M. Oberparleiter, R. Ochoukov, T. Odstrcil, J. Olsen, F. Orain, F. Palermo, O. Pan, G. Papp, I. Paradela Perez, A. Pau, G. Pautasso, F. Penzel, P. Petersson, J. Pinzón Acosta, P. Piovesan, C. Piron, R. Pitts, U. Plank, B. Plaum, B. Ploeckl, V. Plyusnin, G. Pokol, E. Poli, L. Porte, S. Potzel, D. Prisiazhniuk, T. Pütterich, M. Ramisch, J. Rasmussen, G.A. Rattá, S. Ratynskaia, G. Raupp, G.L. Ravera, D. Réfy, M. Reich, F. Reimold, D. Reiser, T. Ribeiro, J. Riesch, R. Riedl, D. Rittich, J.F. Rivero-Rodriguez, G. Rocchi, M. Rodriguez-Ramos, V. Rohde, A. Ross1, M. Rott, M. Rubel, D. Ryan, F. Ryter, S. Saarelma, M. Salewski, A. Salmi, L. Sanchis-Sanchez, J. Santos, O. Sauter, A. Scarabosio, G. Schall, K. Schmid, O. Schmitz, P.A. Schneider, R. Schrittwieser, M. Schubert, T. Schwarz-Selinger, J. Schweinzer, B. Scott, T. Semer, E. Seliunin, M. Sertoli, A. Shabbir, A. Shalpegin, L. Shao, S. Sharapov, G. Sias, M. Siccinio, B. Sieglin, A. Sigalov, A. Silva, C. Silva, D. Silvagni, P. Simon, J. Simpson, E. Smigelskis, A. Snicker, C. Sommariva, C. Sozzi, M. Spolaore, A. Stegmeir, M. Stejner, J. Stober, U. Stroth, E. Strumberger, G. Suarez, H.-J. Sun, W. Suttrop, E. Sytova, T. Szepesi, B. Tál, T. Tala, G. Tardini, M. Tardocchi, M. Teschke, D. Terranova, W. Tierens, E. Thorén, D. Told, P. Tolias, O. Tudisco, W. Treutterer, E. Trier, M. Tripský, M. Valisa, M. Valovic, B. Vanovac, D. van Vugt, S. Varoutis, G. Verdoolaege, N. Vianello, J. Vicente, T. Vierle, E. Viezzer, U. von Toussaint, D. Wagner, N. Wang, X. Wang, M. Weiland, A.E. White, S. Wiesen, M. Willensdorfer, B. Wiringer, M. Wischmeier, R. Wolf, E. Wolfrum, L. Xiang, Q. Yang, Z. Yang, Q. Yu, R. Zagórski, I. Zammuto, W. Zhang, M. van Zeeland, T. Zehetbauer, M. Zilker, S. Zoletnik, H. Zohm and the EUROfusion MST1 Team55

A new core charge exchange recombination spectroscopy diagnostic has been installed in the ASDEX Upgrade tokamak that is capable of measuring the impurity ion temperature, toroidal rotation, and density on both the low field side (LFS) and high field side (HFS) of the plasma. The new system features 48 lines-of-sight (LOS) with a radial resolution that varies from ± 2 cm on the LFS down to ± 0.75 cm on the HFS and has sufficient signal to run routinely at 10 ms and for special circum- stances down to 2.5 ms integration time. The LFS-HFS ion temperature profiles provide an additional constraint on the magnetic equilibrium reconstruction, and the toroidal rotation frequency profiles are of sufficiently high quality that information on the poloidal velocity can be extracted from the LFS-HFS asymmetry. The diagnostic LOS are coupled to two flexible-wavelength spectrometers such that complete LFS-HFS profiles from two separate impurities can be imaged simultaneously, albeit with reduced radial coverage. More frequently, the systems measure the same impurity providing very detailed information on the chosen species. Care has been taken to calibrate the systems as accurately as possible and to include in the data analysis any effects that could lead to spurious temperatures or rotations. European Commission (EUROfusion 633053)

The high frequency B-dot (HFB) probe diagnostic on the ASDEX Upgrade tokamak has undergone a considerable upgrade during the 2016 opening of the torus. The probe coverage is now greatly expanded toroidally, as well as radially with the addition of probes on the high field side and the removable manipulator head. A new 2-channel fast digitizer now allows to examine and record radio frequency (RF) wave emissions emanating from the plasma in the ion cyclotron range of frequencies (ICRF). Possible studies that can be achieved now include: a study of core ICRF power absorption efficiency; a study of ion cyclotron emissions from the plasma generated by energetic ions; and study of ICRF wave/plasma turbulence interactions in the scrape-off layer region.

The impact of lithium (Li) on plasma performance was investigated at the ASDEX Upgrade tokamak, which features a full tungsten wall. Li pellets containing 1.6 × 1020 Li atoms were launched with a speed of 600 m s−1 to achieve deep penetration into the plasma and minimize the impact on the first wall. Homogeneous transient Li concentrations in the plasma of up to 15% were established. The Li sustainment time in the plasma decreased with an increasing heating power from 150 to 40 ms. Due to the pellet rate being restricted to 2 Hz, no Li pile-up could take place. No significant positive impact on plasma properties, as reported from other tokamak devices, could be found; the Li pellets rather caused a small reduction in plasma energy, mainly due to enhanced radiation. Due to pellet injection, a short-lived Li layer was formed on the plasma-facing components, which lasted a few discharges and led to moderately beneficial effects during plasma start-up. Most pellets were found to trigger type-I ELMs, either by their direct local perturbation or indirectly by the altered edge conditions; however, reliability was less than 100%.

ELM mitigation by magnetic perturbations is studied at low pedestal collisionalities down to ITER-like values (V∗ e PED = 0.1) in ASDEX Upgrade. A comprehensive database of ELM energy losses for varying plasma density, heating power, edge safety factor and magnetic perturbation structure has been assembled to investigate parameter dependencies of ELM mitigation. It is found that magnetic perturbations with a toroidal mode number n = 2 can reduce the ELM energy loss normalized to the energy stored in the plasma pedestal from about 30% to less than 5%, i.e. by a factor of six, below an electron pedestal collisionality of V∗ e PED = 0.4. At this level of ELM mitigation a significant reduction of the pedestal pressure and, therefore, global plasma confinement occurs. This pedestal pressure reduction is mostly due to a reduction of plasma density, the so-called pump-out effect. Refueling by neutral beams and in particular by pellet injection is possible and can re-establish confinement, however, the ELM energy loss increases as well with increasing density. © 2017 Published by Elsevier B.V.

In this work, a new type of high through-put Czerny-Turner spectrometer has been developed which allows us to acquire multiple channels simultaneously with a repetition time on the order of 10 μ s at different wavelengths. The spectrometer has been coupled to the edge charge exchange recom- bination system at ASDEX Upgrade which has been recently refurbished with new lines of sight. Construction features, calibration methods, and initial measurements obtained with the new setup will be presented. European Commission (EUROfusion 633053)

International audience; The progress made in understanding spontaneous toroidal rotation reversals in tokamaks is reviewed and current ideas to solve this ten-year-old puzzle are explored. The paper includes a summarial synthesis of the experimental observations in AUG, C-Mod, KSTAR, MAST and TCV tokamaks, reasons why turbulent momentum transport is thought to be responsible for the reversals, a review of the theory of turbulent momentum transport and suggestions for future investigations.

The RF properties of the four ion cyclotron range of frequencies (ICRF) antennas in the ASDEX Upgrade tokamak are characterized in H-mode magnetically perturbed 3D discharges. An n = 2 magnetic perturbation (MP) field is applied and rigidly rotated, which allows diagnosing the separatrix displacement and consequent coupling change. We find the antenna loading resistance to be coherently modified by the resulting non-axisymmetric plasma equilibria, thus becoming a function of the applied MP field poloidal mode spectra. We perform a detailed statistical analysis, which correlates the change in loading resistance to the fast wave R-cutoff layer movements. From it, a 1D scaling is derived that differs from previous studies evaluated in pure axisymmetric plasma conditions. This experimentally derived scaling is used to predict the average loading resistance change of the ITER ICRF antenna under applied MPs. ICRF coupling simulations using measured 1D density profiles are performed with the RAPLICASOL code, in order to investigate the predictive capabilities of numerical state of the art tools. We find that both 1D conventional scaling laws and 1D numerical simulations fail to capture the 3D physics, and can substantially overestimate the measured loading resistance change up to a factor of ∼3.

The injection of boron (B) and boron nitride (BN) powders into ASDEX Upgrade H-mode discharges have demonstrated effective control of tungsten influx in low density/collisionality operational regimes, similar to conventional boronization methods. Sub-mm powder particles are gravitationally accelerated into the upper edge of a lower single null H-mode plasma with a boundary shape roughly conforming to the shape of the poloidal midplane limiters. Visible spectroscopy measurements at one of the outer limiter showed increases in both B and N signal levels, as well as elevated B levels in the divertor, and an increase in total radiated power by greater than a factor of two during BN injection. Globally the BN injection improved energy confinement by 10%–20%, associated with improvements in pedestal performance similar to gaseous N injection. Following conditioning discharges with B powder injection, three low gas-fueling discharges with magnetic perturbations for ELM suppression were successfully conducted. These first results suggest that the application of B containing powders can be used to both improve plasma performance in real-time, and to improve overall wall conditions for subsequent discharges.

This paper describes joint ITPA studies of the I-mode regime, which features an edge thermal barrier together with L-mode-like particle and impurity transport and no edge localized modes (ELMs). The regime has been demonstrated on the Alcator C-Mod, ASDEX Upgrade and DIII-D tokamaks, over a wide range of device parameters and pedestal conditions. Dimensionless parameters at the pedestal show overlap across devices and extend to low collisionality. When they are matched, pedestal temperature profiles are also similar. Pedestals are stable to peeling–ballooning modes, consistent with lack of ELMs. Access to I-mode is independent of heating method (neutral beam injection, ion cyclotron and/or electron cyclotron resonance heating). Normalized energy confinement H 98,y2 ≥ 1 has been achieved for a range of 3 ≤ q 95 ≤ 4.9 and scales favourably with power. Changes in turbulence in the pedestal region accompany the transition from L-mode to I-mode. The L–I threshold increases with plasma density and current, and with device size, but has a weak dependence on toroidal magnetic field B T. The upper limit of power for I-modes, which is set by I–H transitions, increases with B T and the power range is largest on Alcator C-Mod at B > 5 T. Issues for extrapolation to ITER and other future fusion devices are discussed., United States. Department of Energy (DE-FC02-99ER54512-CMOD), United States. Department of Energy (DE-SC0012469), United States. Department of Energy (DE-FC02-04ER54698), United States. Department of Energy (DE-FG02-94ER54235), United States. Department of Energy (DE-AC52-07NA27344), United States. Department of Energy (DE-AC02-09CH11466), United States. Department of Energy (DE-FG02-89ER53296), United States. Department of Energy (DE-FG02-08ER54999), United States. Department of Energy (DE-FG02-08ER54984)

This paper presents an overview of results from the Imaging Motional Stark Effect (IMSE) diagnostic obtained during its first measurement campaign at ASDEX Upgrade since installation as a permanent diagnostic. A brief overview of the IMSE technique is given, followed by measurements of a standard H-mode discharge, which are compared to equilibrium reconstructions showing good agreement where expected. The development of special discharges for the calibration of pitch angle is reported and safety factor profile changes during sawteeth crashes are shown, which can be resolved to a few percent due to the high sensitivity at good time resolution of the new IMSE system.

Carbon (CD4) and nitrogen (N2) have been seeded in ASDEX Upgrade (AUG) with a tungsten wall and have both led to a 20–30% confinement improvement. The reference plasma is a standard target plasma with I p /B T = 1 MA/2.5 T, total input power P tot ~ 12 MW and normalized pressure of β N ~ 1.8. Carbon and nitrogen are almost perfectly exchangeable for the core, pedestal and divertor plasma in this experiment where impurity concentrations of C and N of 2% are achieved and Z eff only mildly increases from ~1.3 to ~1.7. As the radiation potentials of C and N are similar and peak well below 100 eV, both impurities act as divertor radiators and radiate well outside the pedestal region. The outer divertor is purposely kept in an attached state when C and N are seeded to avoid confinement degradation by detachment. As reported in earlier publications for nitrogen, carbon is also seen to reduce the high field side high density (the so-called HFSHD) in the scrape off layer above the inner divertor strike point by about 50%. This is accompanied by a confinement improvement for both low (δ ~ 0.25) and high (δ ~ 0.4) triangularity configurations for both seeding gases, due to an increase of pedestal temperature and stiff core temperature profiles. The electron density profiles show no apparent change due to the seeding. As an orthogonal effect, increasing the triangularity leads to an additionally increased pedestal density, independent of the impurity seeding. This experiment further closes the gap in understanding the confinement differences observed in carbon and metal wall devices; the absence of carbon can be substituted by nitrogen which leads to a similar confinement benefit. So far, no definite physics explanation for the confinement enhancement has been obtained, but the experimental observations in this paper provide input for further model development.

We demonstrate that collective Thomson scattering of millimeter wave electron cyclotron resonance heating radiation can be used for measurements of the main-ion temperature in the ASDEX Upgrade tokamak.

In order to test the peeling–ballooning ELM model, ECRH heating was applied to the edge of ASDEX Upgrade type-I ELMy H-mode plasmas to alter the pedestal pressure and current density profiles. The discharges were analysed with respect to ideal MHD stability. While the ELM frequency increased and the pedestal gradients relaxed with edge ECRH, the MHD stability boundary did not change. The results indicate that the peeling–ballooning model is insufficient to fully explain the triggering of ELM instabilities in the presence of edge ECRH heating.

The edge ion heat transport is analyzed in ASDEX Upgrade (AUG) by combining a comprehensive set of pedestal measurements with both interpretive and predictive modelling. The experimentally determined ion heat diffusivities, $\chi_{\rm i}$ , are compared with neoclassical theory and the impact of edge localized modes (ELMs) on the edge ion heat transport level is studied in detail. Pedestal matching experiments in deuterium and hydrogen plasmas show that the inter-ELM pedestal $\chi_{\rm i}$ remains close to the neoclassical value. The additional power needed in hydrogen to get similar pedestal temperatures as in deuterium plasmas mostly affects the electron heat channel, i.e. the electron heat diffusivity increases while the ion heat diffusivity stays at the same level within the uncertainties. Sub-ms measurements of the edge ion temperature allows us to extend the analysis to the entire ELM cycle. During the ELM crash, the ion heat transport is increased by an order of magnitude. The perturbed heat flux increases first at the separatrix, i.e. first the separatrix ion temperature increases, leading to a flatter ion temperature gradient, followed by a decrease of the whole pedestal profile. The ion heat transport returns to its pre-ELM neoclassical level 3–4 ms after the ELM crash.

The poloidal and toroidal impurity flows in the edge transport barrier of H-mode plasmas have been studied over a wide range of pedestal top ion collisionalities. A comparison of the edge poloidal rotation measurements to neoclassical predictions shows good agreement in all cases. The measured edge impurity toroidal rotation is observed to change sign from co-current to counter-current with decreasing collisionality. The switch occurs at the same collisionality at which neoclassical theory predicts the main ion poloidal rotation to change from the electron to the ion diamagnetic direction. The behaviour of these two species, when used to calculate the main ion toroidal rotation via the radial force balance equation, leads to fairly constant co-current main ion toroidal rotation. Hence, at low collisionality, due to a reduced frictional coupling, the main ion-impurity differential rotation can be quite large. The behaviour of impurity ion flows on a flux surface has also been investigated in detail and it was found that the measurements are consistent with the continuity equation only if the poloidally asymmetric impurity density distribution is taken into account. The asymmetry is found to be the result of the interplay of all forces in the parallel momentum balance, with the friction force providing the dominant drive. Close to the separatrix the poloidal centrifugal force, which is usually neglected, also gives an additional contribution to the impurity density asymmetry. Within the experimental uncertainties the ion temperature and the electrostatic potential are simultaneous flux functions, despite the presence of a poloidally asymmetric impurity density profile.

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 large degree 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...

Recent improvements to the heating and diagnostic systems on the ASDEX Upgrade tokamak allow renewed investigations into non-inductive operation scenarios with improved confinement in a full-metal device. Motivated by this, a scenario with , and a high non-inductive current fraction has been developed. The scenario offers good confinement with and normalised ion temperature gradients . Moreover, it is robust against resistive magnetohydrodynamic (MHD) instabilities, but does suffer from ideal MHD instability when . To verify the understanding of the plasma transport processes, the heat transport was modelled using TGLF. This revealed that electromagnetic effects at high β and/or from fast ions appear to be missing from TGLF's physics model. As accurate reconstruction of the plasma equilibrium is crucial for studies of advanced scenarios, this publication also documents the presence of polarised background light that can contaminate motional stark effect measurements and thus interfere with equilibrium reconstruction.

A gyro-kinetic analysis of intrinsic rotation is presented for the ASDEX Upgrade tokamak. The gyro-kinetic turbulence code, GKW and the neoclassical transport code, NEO are coupled so that the neoclassical equilibrium distribution function is included in the background distribution function in the gyro-kinetic turbulence simulation. This implementation is benchmarked against a similar implementation in the gyro-kinetic code, GS2 (Dorland et al 2000 Phys. Rev. Lett. 85 5579) and against analytical predictions. A quasi-linear and non-linear gyro-kinetic turbulence analysis is performed on Ohmic L-mode ASDEX Upgrade plasmas showing that the symmetry breaking effects due to neoclassical background flows can produce significant toroidal momentum transport. While its magnitude is of the order of other symmetry breaking mechanisms, such as the Coriolis pinch, up–down asymmetry in the magnetic flux surfaces and flow shear, the flow gradients it can sustain are appreciably smaller than the maximum gradients measured at the mid-radius of the ASDEX Upgrade tokamak core, which can be up to an order of magnitude larger. It is found that the gradient of the diamagnetic flow, and therefore the second derivatives of the density and temperature gradients are critical to the production of residual toroidal momentum flux. A quasi-linear estimate indicated that the second derivatives required to match the experimental flow gradient are up to an order of magnitude higher than the measured second derivatives. This analysis suggests that turbulent transport driven by neoclassical flows is not sufficient to explain the maximum flow gradients observed in ASDEX Upgrade.

Future fusion reactors, foreseen in the “European road map” such as DEMO, will operate under more demanding conditions compared to present devices. They will require high divertor and core radiation by impurity seeding to reduce heat loads on divertor target plates. In addition, DEMO will have to work at high core densities to reach adequate fusion performance. The performance of fusion reactors depends on three essential parameters: temperature, density and energy confinement time. The latter characterizes the loss rate due to both radiation and transport processes. The DEMO foreseen scenarios described above were not investigated so far, but are now addressed at the ASDEX Upgrade tokamak. In this work we present the transport analysis of such scenarios. Plasma with high radiation by impurity seeding: transport analysis taking into account the radiation distribution shows no change in transport during impurity seeding. The observed confinement improvement is an effect of higher pedestal temperatures which extend to the core via stiffness. A non coronal radiation model was developed and compared to the bolometric measurements in order to provide a reliable radiation profile for transport calculations. High density plasmas with pellets: the analysis of kinetic profiles reveals a transient phase at the start of the pellet fuelling due to a slower density build up compared to the temperature decrease. The low particle diffusion can explain the confinement behaviour.

At ASDEX Upgrade the ECRH system has been upgraded to provide up to 4 MW of heating power at 140 GHz (or 2.2 MW at 105 GHz). The power at 140 GHz exceeds the minimum H-mode power threshold for typical high Ip, Bt conditions by approximately a factor of two. The upgrade allows H-modes with dominant electron heating and significant electron-ion heat exchange to be studied, i.e. the situation expected in ITER. This paper reports on systematic studies varying the heating mix with NBI, ICRF and ECRH and its effect on pedestal parameters and core transport. The H-mode pedestal is hardly affected by the choice of heating mix, but the ion temperature in the plasma center is found to vary significantly. The ion channel dominates heat transport and ion temperature gradient modes (ITG) are found to be the most unstable microinstability in all the scenarios considered. R/LTi at half radius reduces by a factor of two when Te/Ti increases from 0.9 to 1.5. TGLF modelling of the electron and ion temperature and electron ...

The transport of beam-generated fast ions has been investigated experimentally at the ASDEX Upgrade tokamak in the presence of sawtooth crashes. After sawtooth crashes, phase space resolved fast-ion D-alpha measurements show a significant reduction of the central fast-ion density-more than 50%-together with an increase at larger radii. The corresponding loss of fast particles, detected by a fast-ion loss detector outside the plasma, is marginal during the crash phase, which proves that the sawteeth internally redistribute fast ions. This is also well reproduced by modelling results in which the fast-ion redistribution is assumed to be in line with the sawtooth-induced change of the magnetic field topology. The simulation, however, underestimates the total amount of redistributed fast ions which can be explained by additional drift effects. In the time intervals between the crashes, a neoclassical fast-ion behaviour is observed. Radial profiles and measured decay rates of the redistributed fast ions show that any anomalous fast-ion diffusion in between sawtooth crashes is well below 0.5m(2) s(-1).

The characteristics of the edge radial electric field (Er) are studied in deuterium, hydrogen and helium plasmas at ASDEX Upgrade. The minimum of the Er well is analyzed as a function of pedestal parameters and the best correlation is found between the depth of the Er well and the ion pressure at the pedestal top. This result is consistent with Er being balanced by the main ion pressure gradient term. Studying the radial position of the Er minimum reveals that the Er well moves closer to the last closed flux surface the deeper the Er well is. This suggests that for deeper Er wells the distance between the steepest gradients in the ion temperature and ion density profile is reduced. The width of the Er well shows no significant variations despite changing the ion temperature, magnetic field and plasma particle species. At AUG, the Er well is on average 1.2 cm wide. A multi-machine comparison supports a machine size scaling of Er and indicates that the Er shear layer covers the outer 2% of the plasma minor radius independent of the size of the machine. Based on this scaling, the width of the Er well in ITER is estimated to ~4 cm.