R. Wunderlich

B2-EIRENE simulation of ASDEX and ASDEX-Upgrade scrape-off layer plasmas

The 2D multifluid edge code B2 coupled with the 3D neutral gas Monte Carlo code EIRENE is being used for edge interpretation and model validation on the axisymmetric poloidal divertor experiments ASDEX and ASDEX-Upgrade. For this purpose B2 was significantly improved especially by a fully implicit treatment of the topological cuts appearing in X-point configurations, and a reasonably accurate handling of inclined target plates. A fast, automatic grid generator has been developed, which allows direct implementation of experimental MHD equilibria into B2-EIRENE. Typical ASDEX and ASDEX-Upgrade simulations are presented and discussed.

Modelling of Impurity Flow in the Tokamak Scrape-off Layer

The impurity flow parallel to the magnetic field lines in a collisional tokamak scrape-off layer is numerically investigated. The rate equations are solved treating each ionization state as a test fluid which interacts with the given hydrogen background plasma via collisions and the ambipolar electric field. Results for typical impurities (O, Fe, He, etc.) show that collisional friction usually forces the impurities to flow nearly at hydrogen speed. Thermal forces, however, can become dominant locally for small Mach number and large temperature gradient (e.g. strong target recycling), causing impurity flow reversal and subsequent accumulation outside the recycling region. The criterion for flow reversal is roughly M <λi/λT where λi is the hydrogen ion mean free path and λT is the temperature gradient length. Self-sputtering at the target plates is calculated, showing the importance of fractional impurity acceleration in addition to the charge-statedependent electrostatic energy gain.

Modelling of the Impurity Pumping by a Tokamak Scrape-Off Layer

Abstract The impurity flow along magnetic field lines in a collisional tokamak scrape-off layer is numerically investigated using a testfluid approach. Transport perpendicular to the magnetic field is approximately included by a local loss time constant τ⊥. Results for various impurities and hydrogen background plasma parameters show the significance of thermal forces in the high recycling, subsonic flow regime. In this case, impurity flow reversal on “hot” field lines may cause impurity circulation in the scrape-off layer and, as a consequence, a stronger coupling between the main plasma and a separate pumping chamber than previously assumed.

B2-Eirene modelling of ASDEX Upgrade

Abstract The extension of the computational region of the coupled fluid plasma, Monte-Carlo neutrals code, B2-Eirene, to the plasma center is discussed. The simulation of completely detached H-mode plasma is presented, as is the modelling of He and Ne compression.

A radially continuous two-chamber model for the high-recycling divertor edge layer

Abstract A radially continuous two-chamber model for the edge layer of a high recycling divertor tokamak is presented. Diffusive radial transport is chosen for the plasma in the main chamber as well as for the neutral gas in the divertor. Both regions are coupled by parallel conductive energy flow and subsonic particle flow. Standard sheath conditions at the target plate are used. Radial profiles of all relevant quantities are obtained including the self-consistent parallel flow velocity, which can be locally positive or negative. The existence of stationary solutions, their stability and dependence on boundary conditions are investigated. The applicability as an advanced edge model for large radial transport codes is tested.

Confinement regime transitions in ASDEX

The authors give an overview of the different confinement regimes observed on ASDEX and compare the changes during the transition phases with qualitative tendencies suggested by theoretical models. The transitions discussed are those between purely Ohmic heating and additional heating in the L-regime between the L- and the H-regime and between discharges with flat and peaked electron density profiles.

Auxiliary Heated Multipellet-Fuelled Discharges in ASDEX and Influence of Density Profile Shape on Confinement

Strongly peaked electron density profiles have been obtained in ASDEX by different refuelling methods: pellet fuelling (ohmic and co-injection heating), NBI counter-injection and recently by reduced gas puff fuelling scenarios. These discharges show in common increased density limits, a canonical electron temperature profile independent of the density profile and an improvement of the particle and energy confinement. Whereas the changes in particle transport are not fully understood, transport analyses point out that the improved energy transport can be explained by reduced ion conduction losses coming close to the neoclassical ones. The different results for the ion transport with flat and peaked density profiles are quantitatively consistent with that expected from eta i-driven modes. The analyses cannot yet explain the anomalous electron energy transport, apart from identified continuous trends such as inverse scaling with the isotope mass and enhancement with heating power.

Simulation of the oxygen transport at the periphery in a tokamak

The radial transport of oxygen at the edge of a tokamak plasma is considered. The resonant charge exchange between oxygen and hydrogen (O+ + H O + H+) as well as the ionization and disintegration of oxygen molecules are taken into account. Approximate analytic expressions for these effects as obtained from a kinetic model are added to the one-dimensional oxygen rate equations and solved numerically using a fast algorithm developed recently. A significantly enhanced penetration of oxygen into the central plasma as well as an increased oxygen recycling at the edge are obtained, in particular in the vicinity of a limiter, with high local hydrogen and oxygen fluxes. The non-resonant charge exchange of higher oxygen charge states with hydrogen neutrals changes their distribution and radiation in the central plasma but has little effect on the oxygen transport at the periphery. A simple analytical model has been developed to describe oxygen impurity transport in the scrape-off layer and the results are compared with numerical simulations.

HEATING SCENARIOS FOR THE ZEPHYR IGNITION EXPERIMENT

Two heating scenarios relevant for high density, high field ignition experiments like ZEPHYR are studied by ID transport codes: Compression boosted neutral injection and - in a simplified manner - high frequency heating without compression. Ignition is obtained for a wide range of plasma and heating parameters and different transport models. Impurities are included in a second step. The existence of a self-regulated radiation cold plasma layer is demonstrated and its compatibility with both heating methods is discussed.