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Abstract: The Electron Cyclotron Emission diagnostic will be the main tool for the measurement of the electron temperature profile in the stellarator Wendelstein 7-X presently under construction in Greifswald, Germany. Design criteria and constraints for the ECE diagnostic are discussed. The resulting set-up is introduced with respect to the line of sight and the in-vessel optics. The 32-channel heterodyne radiometer system already used at the predecessor experiment Wendelstein 7-AS is adapted to the conditions at Wendelstein 7-X. [Copyright &y& Elsevier]

ECE correlation radiometry is a technique for measuring high frequency electron temperature fluctuations in the plasma core of fusion relevant devices. Two correlation methods, based on different physical principles, have been developed at W7-AS and TEXT-U. Depending on the required application, the two techniques can offer complementary advantages. At W7-AS a comparison of both techniques has been carried out. In all measurements both techniques yield identical results to within measurement error. The advantages of each method are discussed. [ABSTRACT FROM AUTHOR]

A new radiometer has been built up for the W7-AS stellarator which consists of a frontend with an extreme broadband mixer, two intermediate frequency (IF) parts with 16 channels each and a novel additional 16 filter section with variable IF. The system was designed with the future W7-X in mind. All settings can therefore be remotely changed. This “W7-X” system is integrated into the electron cyclotron emission (ECE) systems at W7-AS. EC radiation is collected using elliptical mirrors in Gaussian optics. W7-AS results will be shown. The “zoom” device is installed at DIII-D at the moment; first results are also shown. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Reports on the use of a broadband heterodyne reflectometer operating in the frequency range 75-110 GHz in extraordinary mode polarization, at the W7-AS stellarator for both fast density profile determination and density fluctuation studies. System description; Phase measurements obtained; Density profiles produced.

A broadband heterodyne reflectometer, operating in the frequency range 75-110 GHz in extraordinary mode polarization, has been installed and operated at the Wendelstein 7-AS stellarator for the study of density fluctuations and density profile determination. At 2.5 T it covers the density range between 5×10[sup 18] and 5×10[sup 19] m[sup -3]. The system differs from the usual setup as it combines the advantages of heterodyne detection with broadband capability avoiding the limitations of phase-locked sources and permits the unambiguous determination of the phase delay independent of amplitude fluctuations in the reflected beam. After a first downconversion to an intermediate frequency of about 6 GHz, two additional mixing steps lead to an intermediate frequency of 60 MHz, which carries the phase delay from the plasma as a phase modulation. The phase information is yielded numerically from sin/cos detection at 60 MHz. Due to the balanced detection scheme, the drift as well as the broadband noise of the signal and local oscillators are canceled. The system has a dynamic range of about 60 dB and is within this range almost insensitive to amplitude fluctuations of the reflected beam. Examples are given demonstrating the potential of the system installed. [ABSTRACT FROM AUTHOR]

An eight-channel radiometer for simultaneous measurement of electron temperatures at different radii via electron cyclotron emission is described. It is divided into two parts, the interchangeable front end and the filter section. By replacing the front end, both first and second-harmonic electron cyclotron emission measurements are possible in the range of 50–180 GHz. The instrument is absolutely calibrated with an accuracy estimated to be within ±10%. The spatial resolution ΔR/R is of the order 10-3, the temporal resolution 20 μs. [ABSTRACT FROM AUTHOR]

Presents an abstract of a paper describing the Wendelstein 7-X (W7-X), a concept test for properties of reactor relevant plasmas in advanced stellarators. Prominent features of the stellarator; Heating method for the W7-X; Diagnostics.

A broadband heterodyne reflectometer, operating in the frequency range 75-110 GHz, has been installed and operated at the W7-AS Stellarator in Garching for the study of density fluctuations and fast density profile determination. Both local (LO) and signal (rf) oscillators can be swept over the whole frequency range. Due to the balanced detection scheme, all drifts, as well as the broadband noise of the BWO (rf and LO) oscillators, are canceled at the different downconversion stages. After installation on W7-AS, first results reproduced the original findings of the former homodyne system [J. Sanchez, H. J. Hartfuss, E. Anabitarte, A. P. Navarro, et al. Nucl. Fusion 30, 2383 (1990)] as well as new results on the radial distribution of density fluctuations have been obtained. A phase drift due to asymmetries in the spectrum around the last IF has been observed; this phenomenon is strongly dependent on the plasma conditions and could be caused by plasma rotation. It is presently under study. The system offers broadband operation together with the possibility to decouple phase and amplitude oscillations of the reflected beam. The high dynamic range (up to 80 dB) is able to overcome the large amplitude oscillations of the reflected beam. The reflectometer described offers a good basis for further developments on large devices, which will require a very high sensitivity due to the long waveguide runs, together with the advantage of the operation in a continuous frequency range and fast sweeping capability. [ABSTRACT FROM AUTHOR]

The miniaturization of receiver arrays permits the arrangement of numerous poloidally staggered radial sightlines of an electron cyclotron emission (ECE) diagnostic for the measurement of the electron temperature and its fluctuations, making two-dimensional (2D) imaging of the electron temperature and its fluctuations caused by plasma turbulence possible. For the stellarator W7-AS, the development of a fully monolithic microwave integrated circuit 150 GHz subharmonic mixer array is under development. As a first step, a 2D ECE system for the measurement of electron temperature fluctuations using four individual horn-reflector arrangements in conjunction with multichannel heterodyne radiometers was installed and set into operation. With Gaussian beam optics and four poloidally staggered sightlines, electron temperature fluctuations could be characterized in radial and poloidal directions simultaneously. First observations in purely electron cyclotron resonance heated stellarator plasmas reveal a broadband drift-wave feature. Earlier experiments showing a decrease of the electron temperature fluctuation level with increasing heating power were confirmed. Additionally, it was revealed, using the 2D ECE correlation radiometer, that an increased velocity shear might account for the decrease of the coherence length and thus for the reduction of the electron temperature fluctuation level. [ABSTRACT FROM AUTHOR]

Describes a ten-channel microwave interferometer for line integrated electron density measurements at the experiment W7-AS stellarator. Generation of the probing signals in the frequency range of 160-162 GigaHertz using Gunn oscillators; Value of the dynamic range of the millimeter wave part of the interferometer.

The status of the diagnostic developments for the quasistationary operable stellarator Wendelstein 7-X (maximum pulse length of 30 min at 10 MW ECRH heating at 140 GHz) will be reported on. Significant emphasis is being given to the issue of ECRH stray radiation shielding of in-vessel diagnostic components, which will be critical at high density operation requiring O2 and OXB heating. [ABSTRACT FROM AUTHOR]

This paper is a summary of some of the major results from the Wendelstein 7-AS stellarator (W7-AS). W7-AS [G. Grieger et al., Phys. Fluids B 4, 2081 (1992)] 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 (2002)]. It has extended the operational range to high density (4×1020 m-3 at 2.5 T) and high 〈β〉 (3.4% at 0.9 T); it has demonstrated successfully the application of electron cyclotron resonance heating (ECRH) 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 (1995)], 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 (HDH), 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-β phases up to tpulse/τE=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 physics) 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 window). W7-AS turned out to be an important step in the development of the Wendelstein stellarator line towards an independent fusion power plant concept. [ABSTRACT FROM AUTHOR]

A multichannel CO2 laser interferometer is planned for electron density profile measurements in the Wendelstein 7-X stellarator under construction. This article describes the mathematical methods used to find the optimum orientation of the various sightlines. The problem of reconstructing local densities from line integrated density measurements can be formulated as a matrix inversion. The quality of the reconstruction is largely determined by the condition number of the matrix describing the problem, which has to be minimized, i.e., the sightlines have to be chosen such that the information content of different channels is maximally distinct. For the W7-X interferometer four- and eight-sightline arrangements are investigated and their orientations are optimized for standard conditions of magnetic configuration and density profile. The optimized arrangements are tested by simulating the inversion of hypothetical reference density profiles in a number of different magnetic configurations. In the ideal case the error of a reconstruction using four sightlines is typically a few times larger than that with eight sightlines. The robustness of the optimization is demonstrated by a variation of the position of the whole interferometer, and the influence of noisy phase data on the reconstructed profiles is investigated. These factors significantly narrow the difference between the four- and eight-beam setups. Finally, the use of regularization methods for the analysis of experimental data is briefly discussed. [ABSTRACT FROM AUTHOR]

The theoretical and experimental development of stellarators has removed some of the specific deficiencies of this configuration, viz., the limitations in β, the high neoclassical transport, and the low collisionless confinement of a particles. These optimized stellarators can best be realized with a modular coil system. The W7-AS experiment [Plasma Phys. Controlled Fusion 31, 1579 (1989)] has successfully demonstrated two aspects of advanced stellarators, the improved equilibrium and the modular coil concept. Stellarator optimization will much more viably be demonstrated by W7-X [Plasma Physics and Controlled Fusion Research, Proceedings of the 12th International Conference, Nice, 1988 (IAEA, Vienna, 1989), Vol. 2, p. 369], the successor experiment presently under design. Optimized stellarators seem to offer an independent reactor option. In addition, they supplement, in a unique form, the toroidal confinement fusion program, e.g., energy transport is anomalous in stellarators too, but possibly more easily understandable in the frame of existing theoretical concepts than in tokamaks. [ABSTRACT FROM AUTHOR]

An eight channel interferometer is developed for density feedback control and the continuous measurement of electron density profiles in the stellarator W7-X. An additional sightline is launched in the geometry of the Thomson scattering for cross calibration. Due to the W7-X coil geometry access is strongly restricted. This motivates the optimization of the sightline geometry and design studies for supplementary chords. In-vessel retroreflectors will be used and inserted in the first wall elements. To cope with associated mechanical vibrations and thermal drifts during the discharges with envisaged duration of 30 min either two-color or second harmonic interferometry techniques must be applied. Optimum wavelengths are found to be about 10 and 5 μm. A CO2/CO interferometer (10 μm/5 μm) will be tested and compared with an existing CO2/HeNe test interferometer. A special difficulty of remotely operated diagnostics is the need of long transmission lines with a path length of about 60 m required from the diagnostics location to the torus hall and back. Different arrangements will be compared. [ABSTRACT FROM AUTHOR]

The critical issues in the development of diagnostics, which need to work robust and reliable under quasi-steady state conditions for the discharge durations of 30 min and which cannot be maintained throughout the one week duration of each operation phase of the Wendelstein 7-X stellarator, are being discussed. [ABSTRACT FROM AUTHOR]