Volker Naulin

Geodesic acoustic modes excited by finite beta drift waves

Presented in this paper is a mode-coupling analysis for the nonlinear excitation of the geodesic acoustic modes (GAMs) in tokamak plasmas by finite beta drift waves. The finite beta effects give rise to a strong stabilizing influence on the parametric excitation process. The dominant finite beta effect is the combination of the Maxwell stress, which has a tendency to cancel the primary drive from the Reynolds stress, and the finite beta modification of the drift waves. The zonal magnetic field is also excited at the GAM frequency. However, it does not contribute to the overall stability of the three-wave process for parameters of relevance to the edge region of tokamaks.

Current filaments in turbulent magnetized plasmas

Direct measurements of current density perturbations associated with non-linear phenomena in magnetized plasmas can be carried out using in situ magnetic measurements. In this paper we report such measurements for three different kinds of phenomena. Current density fluctuations in the edge density gradient region of a fusion plasma confined in reversed field pinch configuration and in a density gradient region in the Earth magnetosphere are measured and compared, showing that in both environments they can be attributed to drift-Alfv?n vortices. Current structures associated with reconnection events measured in a reversed field pinch plasma and in the magnetosheath are detected and compared. Evidence of current filaments occurring during ELMs in an H-mode tokamak plasma is displayed.

Turbulent Impurity Transport

Impurity transport is an important topic for ITER as impurity density in the core has to stay below a relative concentration of 5×10−5 [1] to reach ignition conditions. The reasons for that is mainly impurity radiation leading to cooling. In the edge region on the other hand site, higher impurity densities, going along with radiation energy losses are beneficial as the radiative cooling arranges for good power distribution onto the first wall.

13th EU-US Transport Task Force Workshop on transport in fusion plasmas

This report summarizes the contributions presented at the 13th EU-US Transport Task Force Workshop on transport in fusion plasmas, held in Copenhagen, Denmark, 1-4 September 2008. There were sessions on core heat and particle transport; core and edge momentum transport; edge and scrape-off-layer transport and MHD and fast particle interaction with transport.

Geodesic Acoustic Mode in Toroidal Plasma

Geodesic acoustic modes (GAMs) are low frequency modes that were first predicted theoretically to occur in magnetically confined toroidal plasmas and found later in simulations of tokamak edge plasmas. They have been observed in various toroidal devices. Thus, investigating the source of excitation of these modes has been a very active area of research. Recently, we have addressed the issue of nonlinear excitation of these modes by three‐wave resonant mode coupling to electron drift waves. We have extended this work to include finite beta effects. The finite beta effects have a strong stabilizing influence on the parametric excitation process. We have also investigated the role of plasma inhomogeneity in the edge plasma on the excitation of these modes. The two space scale analysis and eigenvalue studies shows that there is a short scale structure of the GAMs determined by the resonance condition and an envelope determined predominantly by the drift‐wave and ambient inhomogeneity of the edge plasmas. The ...