No abstract

The transport of silicon has been investigated for various heating scenarios in ASDEX Upgrade H-mode discharges. Inside of r ≈ a/4, the diffusion coefficient D is either mainly neoclassical or anomalous depending on the heating method. For all investigated scenarios with NBI-heating and off-axis ECRH or off-axis ICRH, the diffusion coefficient is approximately neoclassical, and the effective heat diffusion coefficient χ eff is below the neoclassical ion heat diffusion χ i,neo in the plasma core. When central ECRH is added, χ eff is above χ i,neo , and D strongly increases by a factor of 3-10, i.e. becomes predominantly anomalous. For central ICRH, D is above the neoclassical level by a factor of 2.For radii outside of r ≈ a/4, D is always anomalous and increases towards the plasma edge. For r a/4, we find a clear scaling of D in terms of χ eff , where D is about equal or above χ eff . A strong inward drift parameter v/D is only observed in the core and only for cases, when the diffusion coefficient is neoclassical. With central wave heating, the drift parameter decreases to small values.

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.

The existence of an anomalous particle pinch in magnetized tokamak plasmas is still questioned. Contradictory observations have been collected so far in tokamaks. Clear experimental evidence that density peaking in tokamak plasmas drops with increasing collisionality is provided for the first time. This phenomenon is explained by means of existing theoretical models based on the fluid description of drift wave instabilities, provided that such models include the dissipative effects introduced by collisions on the mentioned instabilities. These results reconcile the apparent contradictions found so far in the experiments.

An asymptotic method for solving the wave equation in the short-wavelength limit is presented. This method, called beam tracing, takes into account the wave properties, i.e., diffraction and interference. It reduces the full wave equation to a set of ordinary differential equations. In this respect, it differs from all other asymptotic techniques describing diffraction which end up with much more complicated partial differential equations. The resulting system of beam tracing equations is expressed in terms of the same Hamiltonian function as in geometric optics (ray tracing) and, similar to the ray tracing, allows powerful numerical solving algorithms. Thus the beam tracing combines the simplicity of ray tracing with a description of the wave phenomena, which are not included in the ray tracing. The beam tracing technique provides an efficient tool for calculation of wave fields in all problems where the short-wave approximation is applicable such as rf heating, current drive and plasma diagnostics with microwave beams.