Jan Weiland

Comparisons and physics basis of tokamak transport models and turbulence simulations

The predictions of gyrokinetic and gyrofluid simulations of ion-temperature-gradient (ITG) instability and turbulence in tokamak plasmas as well as some tokamak plasma thermal transport models, which have been widely used for predicting the performance of the proposed International Thermonuclear Experimental Reactor (ITER) tokamak [Plasma Physics and Controlled Nuclear Fusion Research, 1996 (International Atomic Energy Agency, Vienna, 1997), Vol. 1, p. 3], are compared. These comparisons provide information on effects of differences in the physics content of the various models and on the fusion-relevant figures of merit of plasma performance predicted by the models. Many of the comparisons are undertaken for a simplified plasma model and geometry which is an idealization of the plasma conditions and geometry in a Doublet III-D [Plasma Physics and Controlled Nuclear Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. 1, p. 159] high confinement (H-mode) experiment. Most of the mo...

Fully toroidal ion temperature gradient driven drift modes

A simple quadratic dispersion relation is derived for electrostatic ion temperature gradient driven modes without expansion in the inverse aspect ratio. It is also shown that these modes experience the local curvature only on the outside of the magnetic surface.

Simulation of toroidal drift mode turbulence driven by temperature gradients and electron trapping

Turbulence and transport due to fully toroidal ion temperature gradient driven drift waves and a collisionless trapped electron mode have been studied by mode coupling simulations and with the quasi-linear theory. Diffusion coefficients in good agreement with the simulations have been obtained. The observed tendency for equilibration of the temperature and density scale lengths leads to particle or heat pinch effects that are in agreement with experimental trends.

Diffusive particle and heat pinch effects in toroidal plasmas

A fully toroidal fluid description has been derived for electrostatic drift modes in plasmas with electron trapping. The system includes both a modified toroidal ion temperature gradient mode (ηi mode) and a new trapped electron mode. The transport, obtained both by quasi-linear theory and non-linear numerical simulations, shows several features that are in agreement with observations in tokamaks, such as particle or heat pinch effects. A general tendency for equilibration of the inhomogeneity scale lengths for the density and temperature is found.

Overview of toroidal momentum transport

Toroidal momentum transport mechanisms are reviewed and put in a broader perspective. The generation of a finite momentum flux is closely related to the breaking of symmetry (parity) along the field. The symmetry argument allows for the systematic identification of possible transport mechanisms. Those that appear to lowest order in the normalized Larmor radius (the diagonal part, Coriolis pinch, E x B shearing, particle flux, and up-down asymmetric equilibria) are reasonably well understood. At higher order, expected to be of importance in the plasma edge, the theory is still under development.

Transport due to toroidal ηi mode turbulence in tokamaks

The ion temperature gradient driven drift mode turbulence (ηi mode) has been studied with a fully toroidal fluid model. The saturation amplitudes and ion energy transport have been obtained by numerical simulations. An analytical model for mode coupling saturation in combination with a quasi-linear approximation has been used to derive a simple diffusion coefficient which is in good agreement with the simulations. The scaling of the diffusion coefficient χI with Ln/LB indicates a considerably improved agreement with the experimental radial variation, and the scaling with Te/Tj has been found to favour tokamak operation in the hot ion regime. The diffusion coefficient is consistent both with the improved confinement for pellet injection experiments and with the H-mode results for very large ηi.

Symmetry breaking effects of toroidicity on toroidal momentum transport

A derivation of symmetry breaking toroidicity effects on toroidal momentum transport has been made from the stress tensor. This effect is usually stronger than the symmetry breaking caused by the flowshear on the eigenfunction. The model obtained generalizes a recent derivation of diagonal transport elements from the stress tensor to convective elements of turbulent equipartition or thermoelectric types. This makes it possible to interpret the same type of effects previously obtained from a phase space conserving nonlinear gyrokinetic equation.

Fully toroidal fluid model for low‐frequency localized modes in magnetized plasmas

A fully toroidal fluid model for the low‐frequency ion density response has been compared with the gyrokinetic response in the electrostatic limit. In general a good agreement is obtained, particularly for unstable modes and modes propagating in the electron drift direction. In particular, good agreement for the stability threshold of ηi modes in the most interesting parameter regimes is obtained. The magnetic drift resonances are found to be destabilizing close to marginal stability.

Collisionality and Shear Dependences of Density Peaking in JET and Extrapolation to ITER

Results from an extensive database analysis of JET density profiles in stationary conditions show that the density peaking factor n(e0)/ in JET H modes increases from near 1.2 at high collisionality to around 1.5 as the plasma collisionality decreases towards the values expected for ITER. This result confirms an earlier observation on AUG. The density peaking behaviour of L modes is remarkably different from that of H modes, scaling with overall plasma shear as (n(e0)/ similar to 1.5l(i)), independently of collisionality. H-mode density profiles show no shear dependence, except at the lowest collisionalities. No evidence for L-Te, L-Ti, rho* or beta dependences has been obtained. Carbon impurity density profiles from charge exchange spectroscopy are always less peaked than electron density profiles and usually flat in H modes. The peaking of the electron density profiles, together with the flatness of the impurity density profiles, are favourable for fusion performance if they can be extrapolated to ignited conditions.

Zonal flow generation in ion temperature gradient mode turbulence

In the present work the zonal flow (ZF) growth rate in toroidal ion-temperature-gradient (ITG) mode turbulence including the effects of elongation is studied analytically. The scaling of the ZF growth with plasma parameters is examined for typical tokamak parameter values. The physical model used for the toroidal ITG driven mode is based on the ion continuity and ion temperature equations whereas the ZF evolution is described by the vorticity equation. The results indicate that a large ZF growth is found close to marginal stability and for peaked density profiles and these effects may be enhanced by elongation.