W7-AS has recently been equipped with ten open divertor modules in order to experimentally evaluate the island divertor concept. First results are reported in this paper. The new divertors enable access to a new NBI-heated, very high density (up to ne = 3.5 × 10 20 m −3 ) operating regime with promising confinement properties. The energy confinement time increases steeply with density and then saturates. In contrast, the particle and impurity confinement times decrease with increasing density. This allows full density control and quasi-steady-state operation also under conditions of partial detachment from the divertor targets. Radiated power fractions are low to moderate in attached regimes and reach up to about 90% in detachment scenarios. The radiation always stays peaked at the edge. The extremely high densities necessitated the development of non-standard heating techniques for central heating. For the first time efficient heating of an NBI target plasma by electron Bernstein waves (140 GHz, second harmonic) is achieved. In addition, this heating scenario enables fine tuning of the upstream boundary conditions for divertor operation.
A promising new plasma operational regime on the Wendelstein stellarator W7-AS has been discovered. It is extant above a threshold density and characterized by flat density profiles, high energy and low impurity confinement times, and edge-localized radiation. Impurity accumulation is avoided. Quasistationary discharges with line-averaged densities n(e) to 4 x 10(20) m(-3), radiation levels to 90%, and partial plasma detachment at the divertor target plates can be simultaneously realized. Energy confinement is up to twice that of a standard scaling. At B(t) = 0.9 T, an average beta value of 3.1% is achieved. The high n(e) values allow demonstration of electron Bernstein wave heating using linear mode conversion.
This paper presents a detailed analysis of the transport behaviour of the detached plasmas in W7-AS based on an extended numerical study using the EMC3-EIRENE code, aimed at understanding the underlying physics responsible for the geometry-dependent detachment stability observed in W7-AS island divertor experiments. Here, a stable detachment can only be established when the control coils are switched on to generate sufficiently large islands with relatively short connection lengths. Special attention will be paid to a discussion of the carbon radiation, location and dynamics of the radiation layer, the neutral screening efficiency specific to the island divertor geometry and its impact on the detachment stability. Based on the three-dimensional simulation results, a linear stability model is presented in order to obtain some insight into the mechanisms driving the instability. The radiation behaviour and the location and evolution of the radiation zone in the island divertor will be discussed with respect to those of tokamak-MARFEs.
Experiments at DIII-D investigated the effects of magnetic error fields similar to those expected from proposed ITER test blanket modules (TBMs) containing ferromagnetic material. Studied were effects on: plasma rotation and locking, confinement, L-H transition, the H-mode pedestal, edge localized modes (ELMs) and ELM suppression by resonant magnetic perturbations, energetic particle losses, and more. The experiments used a purpose-built three-coil mock-up of two magnetized ITER TBMs in one ITER equatorial port. The largest effect was a reduction in plasma toroidal rotation velocity v across the entire radial profile by as much as v/v ∼ 60% via non-resonant braking. Changes to global n/n, β/β and H 98 /H 98 were ∼3 times smaller. These effects are stronger at higher β. Other effects were smaller. The TBM field increased sensitivity to locking by an applied known n = 1 test field in both L-and H-mode plasmas. Locked mode tolerance was completely restored in L-mode by re-adjusting the DIII-D n = 1 error field compensation system. Numerical modelling by IPEC reproduces the rotation braking and locking semi-quantitatively, and identifies plasma amplification of a few n = 1 Fourier harmonics as the main cause of braking. IPEC predicts that TBM braking in H-mode may be reduced by n = 1 control. Although extrapolation from DIII-D to ITER is still an open issue, these experiments suggest that a TBM-like error field will produce only a few potentially troublesome problems, and that they might be made acceptably small.
͑W7-AS͒. W7-AS ͓G. Grieger et al., Phys. Fluids B 4, 2081 ͑1992͔͒ has demonstrated the feasibility of modular coils and has pioneered the island divertor and the modeling of its three-dimensional characteristics with the EMC3/EIRENE code ͓Y. Feng, F. Sardei et al., Plasma Phys. Controlled Fusion 44, 611 ͑2002͔͒. It has extended the operational range to high density ͑4 ϫ 10 20 m −3 at 2.5 T͒ and high ͗͘ ͑3.4% at 0.9 T͒; it has demonstrated successfully the application of electron cyclotron resonance heating ͑ECRH͒ beyond cutoff via electron Bernstein wave heating, and it has utilized the toroidal variation of the magnetic field strength for ion cyclotron resonance frequency beach-wave heating. In preparation of W7-X ͓J. Nührenberg et al., Trans. Fusion Technol. 27, 71 ͑1995͔͒, aspects of the optimization concept of the magnetic design have been successfully tested. W7-AS has accessed the H-mode, the first time in a "non-tokamak" and has extended H-mode operation toward high density by the discovery of the high-density H-mode ͑HDH͒, characterized by H-mode energy and L-mode-level impurity confinement. In the HDH-mode quasisteady state operation is possible close to operational limits without noticeable degradation in the plasma properties. High- phases up to t pulse / E = 65 have been achieved, which can already be taken as an indication of the intrinsic stellarator capability of steady-state operation. Confinement issues will be discussed with emphasis on the similarities to tokamak confinement ͑general transport properties, H-mode transition physics͒ but also with respect to distinct differences ͑no confinement degradation toward operational boundaries, positive density scaling, lack of profile resilience, no distinct isotope effect, H-mode operational window͒. W7-AS turned out to be an important step in the development of the Wendelstein stellarator line towards an independent fusion power plant concept.
In the Wendelstein 7-AS stellarator (Renner et al 1989 Plasma Phys. Control. Fusion 31 1579, under particularly high plasma densities, a non-stationary radiation zone is observed to be formed on the inboard side of the torus. It causes a degradation of the diamagnetic energy of up to 50%. The configurational aspects of the magnetic field influence the development of the radiation zone as follows. The critical density is related to the connection length of the magnetic field, i.e. the observed degradation sets in at lower densities for magnetic configurations with large connection lengths. From camera observations, in conjunction with forward calculations, it is found that the radiation zone is located on closed field lines and forms a toroidal belt. Based on complementary observations, it is concluded that the radiation zone is caused by a radiative condensation instability (or multi-faceted asymmetric radiation from the edge). Fluctuations of the radiation zone were recorded using a fast framing camera with a time resolution of 25 µs. Temporal variations as well as spatial movements were observed. The fluctuations were found on various lines-of-sight around the torus with correlation and phase shifts compatible with a toroidal propagation.
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