H. Verbeek

Island divertor studies on W7-AS

Abstract Basic topological features of the island divertor concept for low shear stellarators are discussed with emphasis on the differences to tokamak divertors. Extensive measurements of the edge structures by two-dimensional plasma spectroscopy and by target calorimetry are in excellent agreement with predicted vacuum and equilibrium configurations, which are available up to central β values of ∼ 1%. For this β value the calculated field-line pitch inside the islands is twice that of the corresponding vacuum case. Video observations of the strike points indicate stability of the island structures for central β values up to ∼ 3.7%. The interpretation of the complex island divertor physics of W7-AS has become possible by the development of the three-dimensional plasma transport code EMC3 (Edge Monte Carlo 3D), which has been coupled self-consistently to the EIRENE neutral gas code. Analysis of high density NBI discharges gives strong indications of stable high recycling conditions for n e ≥ 10 20 m −3 . The observations are reproduced by the EMC3/EIRENE code and supported by calculations with the B2/EIRENE code adapted to W7-AS. Improvement of recycling, pumping and target load distribution is expected from the new optimized target plates and baffles to be installed in W7-AS.

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.

Ion temperature profiles from the plasma center to the edge of ASDEX combining high and low energy CX-diagnostics

The charge exchange (CX) neutral energy distributions from ASDEX measured with the conventional neutral particle analyzers (NPA) at energies >500 eV are combined with the low energy CX spectra from the low energy neutral analyzer (LENA). In the region of overlap their shapes fit each other very well. With the 3D EIRENE code the neutral gas was simulated and ion temperature ( T i ) profiles from the center to the edge are obtained. The T i values at the separatrix and the edge based on the LENA data are considerably lower than those suggested earlier from the NPA data. This is attributed to the different energy ranges—high energies for the NPA, low energies for LENA—that are used for the T i evaluation.

Characterizing the edge plasma of different Ohmic confinement regimes in ASDEX

To compare different Ohmic confinement regimes in ASDEX, the edge conditions are analyzed in detail. The results show that the improved Ohmic confinement coincides with a drop of the separatrix density. This drop allows the density profile to peak and seems to be the trigger of a change in the transport. A universal scaling between the electron temperature and the electron density at the separatrix prevails for all Ohmic scenarios. In addition, the total particle flux across the separatrix is evaluated and found to be strongly correlated to the separatrix density. Thus, the convective energy loss contributes less to the total energy losses when the confinement is improved. Since the correlations between the edge parameters do not change in different Ohmic confinement regimes of ASDEX, the edge physics appears to remain the same. Improved Ohmic confinement is characterized by an optimum separatrix density which provides a sufficiently high edge temperature together with low particle fluxes. These optimum conditions yield the maximum particle confinement.

The Particle Fluxes in the Edge Plasma During Discharges with Improved Ohmic Confinement in ASDEX

In the regime of Improved Ohmic Confinement (IOC) in ASDEX the energy confinement time tau E increases linearly with increasing line-averaged density ne up to the density limit. The establishment of the IOC is accompanied by a substantial reduction of the external gas feed, concomitant with large decreases of all plasma edge fluxes. However, the data do not supply conclusive evidence that the IOC is primarily connected with the recycling conditions. More recent observations with very clean machine conditions seem to indicate that the impurity radiation plays a significant role.

Experimental study on highly collisional edge plasmas in W7-AS island divertor configurations

Abstract Edge plasma scenarios in island divertor configurations (‘natural’ magnetic islands intersected by targets) are studied by comparing data from moderate to high density NBI discharges with 3D code (EMC3/EIRENE) results. The data strongly indicate that high recycling with significant particle flux enhancement was achieved in this geometry. But, plasma pressure losses towards the targets are relatively strong, and high recycling sets in only at n e > 10 20 m −3 . The respective density enhancement in front of the targets is moderate (up to a factor of about three relative to the upstream density). These scenarios are also in basic agreement with B2/EIRENE code predictions. At n e > 1.5 × 10 20 m −3 detachment seems to develop. Improvements are expected from additional coils controlling the field line pitch inside the islands, and from optimized targets which will better focus recycling neutrals into the islands. Both are in preparation.