Abstract. A general feature of particle transport in the core of Tokamak plasmas is that, when core particle sources are small, a stationary peaked density profile is provided by a balance of outward diffusion and inward convection, driven by either neoclassical or turbulent mechanisms. The turbulent contribution to the off-diagonal elements of the transport matrix is very sensitive on the type of dominant instability of the background turbulence. We present here a detailed quasilinear gyrokinetic analysis of stationary turbulent particle transport by means of analytical and numerical calculations to show how the actual parametric dependence of the stationary normalized density gradient can strongly vary between an Ion Temperature Gradient (ITG) dominated turbulence and a Trapped Electron Mode (TEM) dominated turbulence regime. It is also shown how the maximal achievable normalized density gradient is reached when the turbulence regime is in a mixed state. This result is interpreted as the interplay of different physical mechanisms arising from (linear) wave-particle resonances. The results presented here are addressed to interpret some of the still unresolved issue in interpreting known experimental results.

The physical processes producing electron particle transport in the core of tokamak plasmas are described. Starting from the gyrokinetic equation, a simple analytical derivation is used as guidance to illustrate the main mechanisms driving turbulent particle convection. A review of the experimental observations on particle transport in tokamaks is presented and the consistency with the theoretical predictions is discussed.An overall qualitative agreement, and in some cases even a specific quantitative agreement, emerges between complex theoretical predictions and equally complex experimental observations, exhibiting different dependences on plasma parameters under different regimes. By these results, the direct connection between macroscopic transport properties and the character of microscopic turbulence is pointed out, and an important confirmation of the paradigm of microinstabilities and turbulence as the main cause of transport in the core of tokamaks is obtained. Finally, the impact of these results on the prediction of the peaking of the electron density profile in a fusion reactor is illustrated.

The physics base for the ITER Physics Design Guidelines is reviewed in view of application to DEMO and areas are pointed out in which improvement is needed to arrive at a consistent set of DEMO Physics Design Guidelines. Amongst the proposed improvements, the area of power exhaust plays a crucial role since predictive capability of present-day models is low and this area is expected to play a major role in limiting DEMO designs due to the much larger value of Ptot/R in DEMO than in present-day devices and even ITER.

Observations in the ASDEX Upgrade tokamak show a correlation between the gradient of the intrinsic toroidal rotation profile and the logarithmic gradient of the electron density profile. The intrinsic toroidal rotation in the center of the plasma reverses from co- to countercurrent when the logarithmic density gradients are large, and the turbulence is either dominated by trapped electron modes or is at the transition between ion temperature gradient and trapped electron modes. A study based on local gyrokinetic calculations suggests that the dominant trend in the observations can be explained by the combination of residual stresses produced by E × B and profile shearing mechanisms.

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

This contribution presents theoretical results on the transport of light and heavy impurities, as well as of energetic α particles, produced by the background electrostatic plasma turbulence. Linear and nonlinear simulations with three gyrokinetic codes, GS2, GYRO, and the recently developed GKW, are performed in concert with analytical derivations, in order to elucidate the basic transport mechanisms of impurities and energetic α particles. The relevance of these theoretical results in the transport modelling of the ITER standard scenario is assessed by means of ASTRA simulations, in which the transport of minority species like α particles and He ash is described by means of formulae which fit the gyrokinetic results.

Abstract. Operation of DEMO in comparison to ITER will be significantly more demanding, as various additional limitations of physical and technical nature have to be respected. In particular a set of extremely restrictive boundary conditions on divertor operation during and in between ELMs will have to be respected. It is of high importance to describe these limitations in order to consider them as early as possible in the ongoing development of the DEMO concept design. This paper extrapolates the existing physics basis on power and particle exhaust to DEMO.In phases between ELMs or with mitigated ELMs surface overheating and W sputtering pose challenging boundary conditions. For attached divertor conditions at 90% total radiation fraction a peak power density of about 15MW/m 2 convected or radiated to the outer divertor is estimated. As this clearly exceeds the tolerable limit, some degree of divertor detachment is regarded as essential for the operation of DEMO. A loss of detachment with a peak power density of more than 30MW/m 2 can not be tolerated for more than a second before the divertor would suffer from a destructive event. The combination of the limitations on the peak power flux density and W sputtering rate necessitates divertor temperatures less than 4eV.For uncontrolled ELMs sizes in the order of 100MJ are estimated. Results on ELM broadening from JET suggest that in DEMO an energy density limit of 0.5MJ/m 2 per ELM is exceeded by a factor of about 8 for a large range of relative ELM sizes. This highlights the necessity of a reactor-relevant ELM control technique for DEMO, which is capable of reducing the maximum size of the energy loss per ELM to the divertor by more than an order of magnitude without a strong reduction of confinement.

Local gyrokinetic calculations of the logarithmic gradients at mid-radius of both electron and boron densities in ASDEX Upgrade H-mode plasmas are presented and compared with the experimental observations. The experimental results show that both the electron and the boron density profiles increase their peaking in response to the addition of central electron cyclotron heating over a background of neutral beam injection (NBI) heating. The boron density profiles are always less peaked than the electron density profiles in the confinement region, and are flat or even slightly hollow in the presence of NBI heating only. The experimental behaviours are well reproduced by the theoretical predictions. The agreement allows the identification, through theoretical modelling, of the transport mechanisms responsible for the observed dependences. In particular, the observed increase in the logarithmic electron density gradient with increasing central electron heating is explained by a concurrent reduction of the outward pure convection and an increase in the inward thermodiffusion. In addition, it is found that the plasma toroidal rotation velocity and its radial gradient play a non-negligible role in the turbulent boron transport, and allow the prediction of a decrease in boron peaking with increasing rotation velocity, which is consistent with the experimental observations.