Search

The ablation of carbon pellets injected into ECR-heated plasmas of Wendelstein 7-AS (W7-AS) has been investigated using a photographic technique with the aim of studying features of the ablation process in stellarator plasmas. The ablation rate profiles were measured assuming a proportionality of the C II radiation and the ablation rate. This assumption was confirmed experimentally and by simulations performed by a quasi-three-dimensional code. The fraction of continuum radiation, which is detected together with the C II line radiation (when using the light filters with finite spectral width), did not affect these measurements.No significant drifts of pellet clouds in the toroidal, poloidal and radial directions were observed in the C II line (723 nm) emission. From fast electron temperature measurements (Δt = 10 µs) using ECE diagnostics, no indications of a pre-cooling wave were found.A difference between the ablation rates for discharges with low and high plasma density, which lies beyond the error bars, was observed. A specific zone of enhanced ablation was detected at the plasma periphery at high density. The appearance of such ablation enhancement, points to the existence of an additional local heat flux. However, the mechanism that could explain the enhanced ablation phenomena at high plasma densities remains unclear.

The lower hybrid (LH) plasma turbulence triggered by injection of quasi-transverse hydrogen beams at the W7-AS stellarator is investigated by the collective Thomson scattering (CTS) technique. Simultaneously, ion cyclotron emission (ICE) is measured with a broadband loop antenna. Good correlation of the ICE and CTS data is found at the LH frequency; the activity at lower ICE harmonics is also detected with injection of heating neutral beams. Theoretical interpretation of the observations is based on numerical simulations of fast ion distribution functions originating from different NBIs using a time-dependent bounce-averaged collisional Fokker–Planck code. With these distributions, the instabilities of electrostatic modes in the vicinity of ion cyclotron harmonics following from the solution of the local dispersion relation are investigated. Two cases are analysed: the hydrodynamic instability under the double-resonance condition when the LH frequency coincides with a high ion cyclotron harmonic (interpretation of the CTS results), and the kinetic instability of ion Bernstein waves at harmonics lower than the LH frequency (qualitative understanding of ICE data).

Electron density and electron temperature profiles are reconstructed from Thomson scattering data on the stellarator Wendelstein 7-AS by means of systematic statistical modelling employing the Bayesian probability theory (BPT). The BPT allows for systematic combination of all information entering the measurement descriptive model considering all uncertainties of the measured data, calibration measurements, physical model parameters and measurement nuisance parameters. The BPT results are consistent with the ratio-evaluation method (REM) which is used to determine the electron temperature from the ratios of scattering signals. If compared to the sequential REM, the Bayesian error analysis is much more informative because it yields probability density functions of the quantities of interest. Moreover, systematic consideration of all the obtainable raw data, in particular those data suffering from low signal levels, results in an improved evaluation for weakly informative data. Sensitivity analysis of model parameters allows for finding crucial uncertainties which has impact on both diagnostic improvement and design.

Plasma generation has been achieved in the stellarator W7-AS using neutral beam injection only. Plasma evolution is analysed using a zero-dimensional model. The model gives guidelines for this method to be successful. Both a low neutral gas starting pressure and a small fraction of wall recycling are necessary to get `ignition' by beams. The predictions of the model are compared to experiments on W7-AS and LHD.

ICRH has proved to be an efficient method of heating the plasma in toroidal devices. The high voltages needed at the coupling structure are, however, a severe handicap of this method. The possibility is investigated of having the highest voltages between the bars of the Faraday shield (FS), where they are both necessary and easier to maintain. For this purpose a resonant Faraday shield (RFS) antenna where the first and last bars of the FS are connected by an inductive strip is proposed. In front of this strip there is a second strip, fed, as in a conventional antenna, by an RF generator. It is shown that if the toroidal length of the FS is larger than λ/2 the strip connecting the bars of the FS acts as the secondary coil of a tuned transformer, the strip fed by the generator being the primary. It is therefore possible, by varying the frequency and the distance between the two strips, i.e. the coupling coefficient, to match the impedance of the primary to that of the generator.

Electron transport coefficients have been determined in the W7-AS stellarator. The diffusion and convection coefficients were obtained by modulating the gas feed into the plasma and by measuring the propagation of the density perturbation. The experiments were carried out at a variety of plasma densities, heating powers and magnetic fields. The results are summarized in the form of a scaling expression for the diffusion coefficient. Transient inward convection was found in the boundary plasma. This convection plays a minor role in the core plasma except with higher heating power, where outward convection was observed. The results are compared with earlier W7-AS transport results.

The island divertor concept has successfully been realized at W7-AS. The divertor gives access to a new NBI-heated high density regime with densities up to 4×10²⁰m-3 and energy confinement well above customary scalings. This regime appears promising with respect to the requirements of both confinement and exhaust. Many features are reminiscent of a quiescent H-mode, but plasma particle transport behaves differently. Beyond a threshold density energy confinement approximately doubles whereas particle confinement dramatically decreases. The density profile flattens and the inward impurity pinch is reduced. This eases density control, prevents impurity accumulation, and allows for quasi-stationary conditions with radiation profiles peaked at the edge. At the highest densities partial detachment occurs with a radiated power fraction up to 90% at a tolerable expense of plasma energy. The sub-divertor pressure is sufficient for pumping. Major experimental results, such as the lack of a high recycling phase preceding detachment, are also predicted by the EMC3/EIRENE code.

W7-AS operates with an island divertor, which utilises the natural edge islands of the low-shear stellarator configuration to divert the plasma. High densities (up to 4×1020m-3) and partly detached divertor conditions have been attained. The details of the divertor operation will be described along with the 3D-efforts to model the divertor observations. At a density beyond 1.5×1020m-3 another confinement bifurcation appears, which allows steady state operation at good energy and low impurity confinement. The transition is possible from a state of normal confinement or from ELMy or quiescent H-modes. Starting with normal confinement, bifurcation is initiated by a broadening of the density profile. This regime can easily be heated above cut-off by ECRH using electron-bernstein-wave mode conversion. The highest beta-values (>3%) are achieved at high density. The relevance of the W7-AS data for Wendelstein 7-X and the Helias-reactor will be discussed.

Investigations of the performance and stability of high-beta plasmas have been restarted in W7-AS utilizing higher heating powers and the new divertor system. The combination of beneficial effects resulted in a significant increase of the volume averaged β from 2 % up to 3.1 % and in MHD-quiescent, quasi-stationary discharges at low radiation levels. Experimental studies of equilibrium effects and of MHD mode activity have been performed with the X-ray tomography system. In addition results of computational MHD stability studies are presented, which show an increase of the stability with increasing β due to the pressure induced deepening of the magnetic well in combination with increasing magnetic shear, in qualitative agreement with the experimental data. The maximum achieved β is limited by the available heating power and not by stability effects. The equilibrium β-limit is approached in the case of low-iota configurations. Particularly with regard to current carrying stellarators and comparisons with tokamaks the modification of the stability of high-beta plasmas by significant OH-currents has been investigated.

The capability and versatility of ECRH is reviewed using W7-AS as an example experiment. A two frequency system with 70 GHz (0.4 MW) and 140 GHz (2.0 MW) is routinely operated at W7-AS. Basic heating scenarios such as 1st harmonic O-mode (70 GHz at 2.5 T), 2nd harmonic X-Mode (70 GHz at 1.25 T and 140 GHz at 2.5 T) as well as advanced scenarios like mode-conversion heating via the OXB process were investigated at W7-AS and selected results are reported. Recently the OXB mode conversion heating was extended to densities above the O2-mode cut-off at 2.4 . 1020m-3. The experiments cover a wide plasma parameter range from ne=0.1 to 3.5 . 1020m-3 and electron temperatures of up to 7 keV. ECRH related heat transport phenomena, which are attributed to the existence of strong radial electric fields are discussed. Experiments with combined NBI and ECRH at low and high density are reported. The absence of strong ohmic currents qualifies stellarators for Electron Cyclotron Current Drive investigations, because even small driven currents can be measured with high accuracy. Experimental results are compared to linear and nonlinear theory. First experiments with current drive by Bernstein-waves are reported.

This paper presents the analysis of Mirnov coil data from pure electron-cyclotron resonance heated Wendelstein-7 AS discharges with transient magnetohydrodynamical (MHD) mode activity in otherwise MHD inactive plasmas. Amplitude modulations were found on the 100 "s time scale, expected to originate from transient changes of the plasma pressure proflle by anomalous transport events. The analysis was done using our custom developed bandpower correlation method, which combines short-time Fourier transform for the decomposition of the signal with a correlation analysis to quantitatively characterize the statistical connection between the modulation in the power of difierent MHD modes. The efiect of a conflnement transition (near a rational rotational transform) on the correlation between the power modulation of difierent frequency modes has been studied. As high as 50% correlation has been found between difierent frequencies in 'bad conflnement' shots with enhanced transient transport, while for 'good conflnement' shots no signiflcant correlation was observed.

In the island divertor of the W7-AS stellarator a high-density zone (HDZ) near the divertor plates was discovered some years ago (Ramasubramanian et al 2004 Nucl. Fusion 44 992–8) with electron densities up to 7 × 1020 m−3. We shed further light on this phenomenon by determining the poloidal and radial location of this zone and discussing potential implications of these findings. The HDZ is in the vicinity of, but clearly separated from the nearest X-point line. The carbon emission is clearly spatially separated, residing near or at the X-point lines. The HDZ shows many similarities with the hydrogen or wall MARFE in Textor-94 (Samm et al 1999 J. Nucl. Mater. 266–269 666). The structure is associated with a strongly increased neutral pressure, thus enabling efficient pumping. This offers the possibility for a very efficient exhaust regime in a stellarator with island divertor such as W7-X, simultaneously with significantly reduced convective heat loads onto the divertor itself. The spatial separation of the HDZ and the carbon radiation region may imply that such a state can be reached even in a non-carbon machine, and might therefore be DEMO-relevant.

Transient MHD modes present in pure ECRH W7-AS plasmas have been shown to be in correlation with transient transport events (ELM-like modes). Here the spatial structure of the individual transients is analyzed using short-time Fourier transform and continuous analytical wavelet transform based techniques. Processing of Mirnov coil data partly confirms the properties derived from earlier, simpler analyses. Theoretical explanation of the properties of these modes (spatial structure and rapid damping) is attempted by models based involving drift-Alfven turbulence or shear Alfven waves.

Stationary Langmuir probe measurements of ion saturation current and floating potential in a plasma cannot give direct information on density and plasma potential fluctuations in the presence of temperature fluctuations. This problem can be avoided if the probe bias voltage is continuously swept faster than the fluctuation time scale, recording the current-voltage characteristic. This article reports the development of a spatiotemporal highly resolving Langmuir probe array with 15 fast swept tips, operating in the strongly magnetized, collisionless edge plasma of the Wendelstein 7-AS stellarator [Plasma Phys. Controlled Fusion 31, 1579 (1989)]. The probe tips are aligned in the poloidal direction, the tip spacing is 2 mm, and the sweeping frequency is 1.4 MHz. Current and voltage data are sampled with 50 MHz. The high bandwidth of the measurement is achieved by placing miniaturized differential amplifiers close to the probe tips in order to do an impedance transform. The surface-mounting technology and an additional inverse feedback module are utilized, allowing for an input voltage range of +/-100 V, and a common mode rejection rate of 55 dB at 4 MHz, which is sufficient to resolve the nonlinear probe characteristic. For the evaluation of the data, a fit model for stationary probes is employed and found adequate. Changes of the plasma parameters during one voltage sweep are taken into account by a linear interpolation of the fit parameters. Spatio-temporal fluctuation data gained by a fast swept Langmuir probe array, which can be relevant for the turbulent radial transport of particles and energy, are presented.