[en] In this work, the influence of pellet injection on the energy confinement time in different types of discharges has been investigated. During the ablation of a pellet, the electron density is increased. At the same time the electron temperature is decreased because the energy of the plasma is used to ionize and heat the ablated particles of the pellet. After the pellet injection the plasma is reheated. In ohmic plasmas with high density particles are transported into the core during this phase. In this way the density and the temperature in the core are increased leading to a pressure increase and an increase in the energy content of the plasma and to an increase in the energy confinement time. The inward transport of particles can be stopped by reappearing of sawtooth activity which was stopped by the pellet injection; the plasma relaxes to the initial state. In strongly heated plasmas with low confinement (L-mode) and ohmic discharges with low density, the increased transport causes a rapid loss of the additional density coming from the pellet. Because of this the energy increase is much smaller or vanishes. (orig.)

[en] Simulations of motional Stark effect (MSE) spectra for Wendelstein 7-X (W7-X) are reported. While contributing to experimental equilibrium reconstruction in general, the measurements aim at the detection of small deviations of vacuum fields, such as the diamagnetic effect. For the diagnostic beam on W7-X, the expected plasma β implies a diamagnetic effect as low as 3%. A data model for the measurement has been developed for the application to W7-X. The model includes polarisation effects and the beam attenuation to estimate the required signal dynamics. The model has been applied to ASDEX Upgrade data (copyright 2010 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim) (orig.)

[en] An advanced high confinement mode scenario with normalized beta (β_N) up to 3.5 in stationary conditions (up to 20 energy confinement times) and β_N=3.8 transiently, has been obtained in ASDEX Upgrade. For transport analysis of high β_N discharges, two types of simulations have been performed with the automated system for transport analysis code. One is the simulation of the current density profile with experimental data assuming neoclassical electrical conductivity. The other is the simulation of the energy transport and the current density profile using the Weiland transport model (ion temperature gradient and trapped electron mode limit the temperature gradient length). The comparison of the simulations of the q-profiles and temperature profiles with the experimental profiles shows good agreement and demonstrates that for this plasmas regime the temperature profiles are stiff (no internal transport barrier) as in conventional H-mode discharges. The contribution of the Ohmic, bootstrap and neutral beam driven current to the total current density profiles is calculated. The sum of the non-inductively driven plasma current is in the range 50-57% of the total plasma current. Simulations with the Weiland model are performed to investigate the influence of neoclassical tearing modes, which limit confinement and achievable β_N in this scenario, by comparing with the measured kinetic data. In addition, the high β_N discharges are compared to the improved H-mode discharges in ASDEX Upgrade with respect to the profile stiffness, MHD activity and experimental conditions. (author)

[en] A report is given on ELM mitigation results in ASDEX Upgrade, where the scope is further restricted to those methods involving 3D, local non-axisymmetric perturbations. Perturbations applied were cryogenic pellet injection, localized supersonic pulsed injection of D gas and laser blow off from targets carrying C or Al micropellets. The corresponding perturbations differ strongly both in the space and time domain and provide an instrument to probe the local stability. Varying the parameters of the external perturbations by e.g. changing the amount of injected material or the injection speed, the ELM trigger threshold was mapped out with high spatial and temporal resolutions

[en] Interpretative analysis of experimental data from the ASDEX Upgrade tokamak current ramps-up and ramps-down is carried out to shed light on the properties of confinement and transport in these particular phases of the plasma discharge. It is found that the two ramps are similar in parameters evolution, but are not symmetric with respect to the current level, for several reasons that will be elucidated. Theory-based energy transport modeling allows to understand the underlying transport processes at play, although there are limitations in describing the edge part of the plasma at very low currents, in particular during ramp-up. Possible reasons for this discrepancy are discussed. Finally, the relevant turbulence (linear) regime is identified simultaneously with calculations of quasi-linear particle transport, showing that a broad range of modes, from Trapped Electron Modes (TEM) to Ion Temperature Gradient (ITG) modes, is explored during both the ramp-up and ramp-down.

[en] ASDEX Upgrade has a large variety of technical systems, heating and fuelling as well as digital control systems needed for control and optimisation of high performance plasmas. The neutral beam injection (NBI), electron cyclotron RF (ECRF) and ion cyclotron resonance frequency (ICRF) heating systems are able to operate in heating as well as in current drive (CD) mode. One of two NBI systems has recently been redirected to allow more tangential injection to provide larger CD efficiency. With improved control procedures significant success is achieved in the optimisation of the conventional H-mode (ITER reference scenario) as well as of 'advanced tokamak' scenarios. The established operational boundaries encounter or exceed those of the reference scenario. H-modes showing type II ELMs with clearly reduced peak power loads on the target plates are produced at densities close to the empirical Greenwald density limit. Discharges with nearly full non-inductively driven plasma current at Greenwald densities are performed. MHD instabilities like sawteeth and neoclassical tearing modes (NTM) can be tailored and/or completely be stabilised via electron cyclotron frequency CD at relatively low injection power levels. In addition, several non-standard feedback control circuits for plasma performance improvements and machine protection circuits are devised

No abstract available

[en] Simultaneous current ramping and application of lower hybrid heating and current drive (LHCD) have produced a region with zero current density within measurement errors in the core (r/a≤0.2) of JET tokamak optimized shear discharges. The reduction of core current density is consistent with a simple physical explanation and numerical simulations of radial current diffusion including the effects of LHCD. However, the core current density is clamped at zero, indicating the existence of a physical mechanism which prevents it from becoming negative

[en] In the ASDEX Upgrade tokamak, high poloidal beta up to β_pol=3 at the Greenwald density with H-mode confinement has been reached. Because of the high beta, the plasma current is driven almost fully noninductively, consisting of 51% bootstrap and 43% neutral beam driven current. To reach these conditions the discharge is operated at low plasma current (I_P=400 kA ) and high neutral beam heating power (P_NBI=10 MW ). The discharge combines an edge (H mode) and internal transport barrier at high densities without confinement-limiting MHD activities. The extrapolation to higher plasma currents may offer a promising way for an advanced scenario based fusion reactor

[en] A detailed analysis of experimental data from the ASDEX Upgrade tokamak is carried out to shed light on the properties of confinement and transport in the current ramp-up and ramp-down phases of the plasma discharge. The experimental database is used to identify the relevant ranges of parameters explored during the ramp-up and the ramp-down. The energy confinement time observed in the two ramps displays interesting evolution, in many cases attaining different values at the same current level between ramp-up and ramp-down. The possible reasons for this behaviour are investigated. Interpretative transport simulations are used as a tool to clarify the interplay between different parameters, which are coupled in a non-linear way. In addition, a theory-based transport model is used to understand the behaviour of confinement as observed in the experiment, evidencing the role of both turbulent and neoclassical transport. Linear gyrokinetic calculations are performed to identify the relevant turbulence regime, showing that a broad range of frequencies, in the trapped electron modes (TEMs) and in the ion temperature gradient modes (ITGs) regimes, is explored during both the ramp-up and ramp-down. In the same framework, a quasi-linear model is applied to calculate the value of the local logarithmic density gradient and compare it with the experimental value. Finally, first non-linear simulations of heat transport during the current ramps are presented.