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For the first time, the predictive capabilities of the mixed Bohm/GyroBohm, Weiland and 'retuned' GLF23 transport models are investigated with ITB discharges from the ITPA ITB database with fully predictive, time-dependent transport simulations. A range of plasma conditions is examined for JET, JT-60U and DIII-D discharges with internal transport barriers (ITBs). The simulations show that the Bohm/GyroBohm model is able to follow the time evolution of the discharge from the preheating phase without an ITB through the ITB onset phase until the high performance phase with fair accuracy in most cases in JET and JT-60U. This indicates the importance of the interplay between the magnetic shear and ?E×Bflow shear in ITB formation since these are the mechanisms that govern the ITB physics in the model. In order to achieve good agreement in DIII-D, the ?-stabilization had to be included in the model, emphasizing the role played by the ?-stabilization in the physics of the ITBs. The Weiland and GLF23 transport models show limited agreement between the model predictions and experimental time evolution of the ITB and kinetic plasma profiles. The Weiland model does not form a clear ITB in any of the three tokamaks despite varying plasma profiles, such as the q-profile. On the other hand, the average temperatures and density are often in fair agreement with experimental values. The GLF23 model often predicts an ITB, but its radial location is often too far inside the plasma, or shrinks as the simulations proceed in time. Consequently, the central temperatures at the end of the simulations during the high performance phase are usually underestimated. It is worth noting that GLF23 features in general better predictions of the Teand Tiprofiles outside the ITB than the other models studied. Achieving the quantitative capability to predict the multi-channel ITB dynamics with fully predictive, time-dependent transport simulations has turned out to be extremely challenging.

The peculiarities of the electron temperature profile under dominant on- and off-axis heating in the Rijnhuizen Tokamak Project (RTP) were successfully described by a `shell model'. This model features electron transport barriers that are associated with simple rational values of the safety factor. In the present work, this model is tested against JET data. To this end, the model is incorporated into the JETTO code, which is used to simulate JET optimized shear discharges with pronounced electron transport barriers. It is found that the model is successful in reproducing the time and location of barrier formation, as well as the subsequent evolution of barrier location and strength.

High power combined NBI + ICRF heating experiments have been carried out in the JET Mark IIa divertor configuration in the hot ion ELM-free H mode regime, both in deuterium (DD) and in deuterium-tritium (DT) plasmas. Results are presented from a wide range of additional heating power levels, ICRF up to 9.5 MW tuned to the fundamental hydrogen minority, NBI up to 22 MW, and for plasma currents up to 4.2 MA and toroidal fields up to 3.6 T. Discharges with combined NBI + ICRF heating show a clear improvement in electron temperature, DD neutron yield and stored energy with respect to NBI only discharges. High energy neutral particle analyser data show that acceleration of the NBI deuterons takes place due to absorption of ICRF power at the second harmonic deuterium resonance. This is confirmed by numerical simulations with the PION code, indicating that up to 40% of the ICRF power is absorbed by bulk and NBI ions. ICRF heating has been an essential ingredient in the DT experiments in the ELM-free hot ion regime, contributing to the achievement of a record fusion power of 16.1 MW and a record stored energy of 17 MJ.

A numerical parameter study has been performed in order to find MHD stable operating regimes for advanced tokamak experiments. In this study we have concentrated on internal modes. Ballooning stability and stability with respect to infernal modes are considered. The calculations confirm that pressure gradients are the main driving force for infernal modes. It is possible to stabilize infernal modes by elimination of pressure gradients in the region of small shear. In the type of equilibrium considered a flat or slightly non-monotonic q profile is most unstable against infernal modes. Higher shear around the minimum q surface stabilizes these modes. Resistivity does not lead to a significantly more unstable situation. In the region of positive shear pressure, gradients are limited by the destabilization of ballooning modes. The results of the study have been used to construct an equilibrium that is stable up to beta _p=3.59, beta _N=5.44

The results of numerical simulation of JET L, H and VH modes are presented. It is shown that L-H transitions are accompanied by a fast and large modification of the transport coefficients not only near the plasma edge but also across a large fraction of the plasma cross-section. Both experiments and numerical simulations show that giant ELMs in JET also have a global character and their penetration length increases with the amplitude of the D_alpha signal. Transport coefficients are of the order of the L mode values during these ELMs. Analysis shows also that within experimental accuracy the transport coefficients are the same for VH and ELM-free H mode discharges. The variation in the global energy confinement is mainly due to the differences in the impurity radiation, power deposition profiles and plasma recycling and related convection

A transport analysis of the different kinds of heat pulses in JET L and H mode discharges has been made. These discharges include L-H transitions, sawtooth crashes, cold pulses initiated by the impurity ablation and giant edge localized modes (ELMs). Analysis of experiments shows that all cold pulses initiated near the separatrix propagate inward on a time-scale that is much shorter than the characteristic energy confinement time. On the other hand, outward propagation of the sawtooth heat pulse, if it is not influenced by the short lived ballistic effect, can be easily described by the usual diffusive model. Two mechanisms of very fast propagation of the heat pulses have been tested. One is based on the assumption that plasma turbulence due to either toroidal or non-linear effects generates very long correlated structures across the magnetic field. In this case, the information about a sudden change of the turbulence induced transport coefficients propagates along the radius with the group velocity of the unstable oscillations. The second mechanism is based on the idea of critical marginality, which implies the existence of a finite threshold in T* for the excitation of the turbulence. If the thermal conductivity is sufficiently large above the threshold, the temperature profile will adjust in such a way as to be close to marginal stability almost everywhere. Numerical analysis of the above mentioned heat pulses leads to the conclusion that although the critical marginality models can reproduce some of the features of the global and local transport they, unlike the global model, fail to reproduce the asymmetry in the time-scales observed during transient phenomena

The current drive by HF waves in a tokamak with longitudinal electric field is studied on the basis of the two-dimensional kinetic equation for electrons. Analytical formulas for current and current drive efficiency are obtained for a low level of absorbed power. Numerical methods are used to solve the kinetic equation at high HF power level. It is shown that the dependence of the efficiency on the effective charge of the plasma changes with increasing power absorption. The existence of trapped particles reduces current drive efficiency. An analysis of the calculation results leads to simple approximate expressions for current and efficiency, convenient for comparisons with the experiments.

It is shown that the presence of a sufficient number of accelerated electrons in a plasma leads to anomalous diffusion of the electrons across the magnetic field. The diffusion of the particles is associated with the presence of waves driven unstable by the runaway electrons via anomalous and Doppler effect. Expressions are obtained for the diffusion coefficient, the oscillation spectrum and the distribution function of the accelerated electron beam.

Processes of resonance interaction of lower hybrid waves with low-frequency oscillations excited in the plasma by longitudinal current and inhomogeneity are discussed. It is shown that the existence of a low-frequency turbulence in a tokamak plasma may appreciably reduce the attenuation length of lower hybrid waves owing to decay processes. The existence of drift oscillations in the plasma must lead to blocking and subsequent accumulation of lower hybrid waves on the periphery of the plasma column.

The paper considers the effects associated with the development of accelerated electron beam instability in a tokamak. The authors have investigated the microscopic stage of instability development in greater detail than in their earlier work. The instability threshold has been found by taking the actual distribution function into account. It is shown that the development of instability comprises two stages - isotropization of electrons due to the anomalous Doppler effect and subsequent formation of a plateau on the electron distribution function. It is at the second stage that oscillations of noticeable energy occur. It is demonstrated that the result of quasi-linear relaxation depends substantially on whether or not the oscillations pass out of the resonance region during instability development. The detailed picture which emerged made it possible to evaluate the basic macroscopic effects - the number of trapped electrons and their energy, the magnitude of ion heating and the sign and magnitude of spikes on the voltage curve. The evaluations obtained give a correct qualitative and, in some cases, quantitative description of the dependences measured experimentally.