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An overview of the H-mode threshold power in ASDEX Upgrade which addresses the impact of the tungsten versus graphite wall, the dependences upon plasma current and density, as well as the influence of the plasma ion mass is given. Results on the H–L back transition are also presented. Dedicated L–H transition studies with electron heating at low density, which enable a complete separation of the electron and ion channels, reveal that the ion heat flux is a key parameter in the L–H transition physics mechanism through the main ion pressure gradient which is itself the main contribution to the radial electric field and the induced flow shearing at the edge. The electron channel does not play any role. The 3D magnetic field perturbations used to mitigate the edge-localized modes are found to also influence the L–H transition and to increase the power threshold. This effect is caused by a flattening of the edge pressure gradient in the presence of the 3D fields such that the L–H transitions with and without perturbations occur at the same value of the radial electric field well, but at different heating powers.

We present a method to obtain reliable edge profiles of the electron temperature by forward modelling of the electron cyclotron radiation transport. While for the core of ASDEX Upgrade plasmas, straightforward analysis of electron cyclotron intensity measurements based on the optically thick plasma approximation is usually justified, reasonable analysis of the steep and optically thin plasma edge needs to consider broadened emission and absorption profiles and radiation transport processes. This is carried out in the framework of integrated data analysis which applies Bayesian probability theory for joint analysis of the electron density and temperature with data of different interdependent and complementary diagnostics. By this means, electron cyclotron radiation intensity delivers highly spatially resolved electron temperature data for the plasma edge. In H-mode, the edge gradient of the electron temperature can be several times higher than the one of the radiation temperature. Furthermore, we are able to reproduce the ‘shine-through’ peak—the observation of increased radiation temperatures at frequencies resonant in the optically thin scrape-off layer. This phenomenon is caused by strongly down-shifted radiation of Maxwellian tail electrons located in the H-mode edge region and, therefore, contains valuable information about the electron temperature edge gradient.

The results from ASDEX Upgrade discharges dedicated specifically to the investigation of low-density L-to-H transitions are presented. The plasmas were heated by electron cyclotron resonance heating to achieve a separation of electron and ion heat channels. Under such conditions, the ratio of electron to ion temperature at the plasma edge increases with decreasing density at the L–H transition and can be as high as 3.5. Our results strongly support the essential role of the ion channel in the L–H transition, via the diamagnetic E r provided by the ion pressure gradient.

The L–H transition and the H-mode behaviour in the presence of non-axisymmetric n = 2 magnetic perturbations have been investigated. At low density no effect on the L–H transition is observed. Within a rather narrow density window around 50% of the Greenwald density limit, a transition to H-mode with small ELMs only and good confinement can be achieved. However, a strong density dependence of the L–H threshold power in the presence of magnetic perturbations forces the plasmas to remain in L-mode when the density is above 60% of the Greenwald value. The H-mode confinement time is not affected by the presence of the magnetic perturbations. All of these H-modes, with and without ELM mitigation, exhibit a common confinement degradation with increasing recycling.

In ASDEX Upgrade the compatibility of improved H-modes with an all-W wall has been demonstrated. Under boronized conditions light impurities and the radiated power fraction in the divertor were reduced, requiring N seeding to cool the divertor plasma. The impurity seeding does not only protect the divertor tiles but also considerably improves the performance of improved H-mode discharges by up to 25%. The energy confinement increases to H 98-factors up to 1.3 and thereby exceeds the best values in the carbon-dominated AUG at the same density and collisionality. This improvement is due to higher edge temperatures rather than to peaking of the electron density profile. Higher temperatures are reached at the pedestal top leading, via profile stiffness, to an increase in the total plasma pressure. There is no change to in the plasma core. The dilution at the plasma edge by nitrogen seems to play an important role since it allows higher ion temperatures at the same edge ion pressure as in the unseeded case. The dilution of the core plasma remains moderate.

The ASDEX Upgrade tokamak is currently being enhanced with a set of in-vessel saddle coils for non-axisymmetric perturbations aiming at mitigation or suppression of edge localized modes (ELMs). Results obtained during the first experimental campaign are reported. With n = 2 magnetic perturbations, it is observed that type-I ELMs can be replaced by benign small ELM activity with strongly reduced energy loss from the confined plasma and power load to the divertor. During these phases with ELM mitigation, no density reduction (density 'pump-out') is observed. ELM mitigation has, so far, been observed in plasmas with different shape and different heating mixes and, therefore, different momentum input. The ELM mitigation regime can be accessed with resonant and non-resonant perturbation field configurations. The main threshold requirement appears to be a critical minimum plasma edge density which depends on plasma current. So far it is not possible to distinguish whether this is an edge collisionality threshold or a critical fraction of the Greenwald density limit.

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.

The knowledge of the effective ion charge Zeff which indicates the degree of plasma pollution is of high interest, since the tolerable impurity concentration to achieve successful ignition of a fusion plasma is limited. The quantification of Zeff at ASDEX Upgrade has been carried out by means of Integrated Data Analysis in the framework of Bayesian probability theory which allows the combined analysis of different bremsstrahlung and charge exchange diagnostics to obtain one joint Zeff profile. With this tool it is possible to assess the quality of the data acquired by the individual diagnostics and to find a reasonable selection of data for the routine Zeff analysis. The Zeff results are validated by the examination of different discharge scenarios and successfully applied to simulations of the neutron rate and the loop voltage. The investigation of Zeff for nitrogen seeded discharges reveals a correlation between the confinement improvement and the increase in Zeff.