Archived project

W7-AS research

Goal: Turbulence measurements

Date: 1 January 1999 - 1 July 2002

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Nils Basse
added a research item
At certain values of the edge rotational transform a, the confinement quality of plasmas in the Wendelstein 7-AS (W7-AS) stellarator is found to react very sensitively to small modifications of the edge rotational transform a. As a can be reproducibly changed either by external fields or by a small plasma current, these transitions offer a precise way to systematically analyse differences in plasma turbulence between bad and good confinement cases. This paper presents results of the study of electron density fluctuations associated with confinement changes. Wavenumber and frequency spectra and radial profiles are compared. A slow and reproducible transition is induced by a small plasma current and the sequence of events leading to bad confinement is investigated. The laser scattering core plasma density fluctuation measurements are complemented by edge beam emission spectroscopy results and magnetic fluctuation measurements with Mirnov coils. Clear correspondence between plasma fluctuations and confinement degradation is observed: the weight of larger structures increases, fluctuations increase in the plasma core, the poloidal flow velocity decreases and regions of mode-like activity move radially in the plasma. These changes occur gradually and controllably if the magnetic configuration is steered by a small plasma current.
Attila Bencze
added a research item
Diagnosing the density profile at the edge of high temperature fusion plasmas by an accelerated lithium beam is a known technique since decades. By knowledge of the relevant atomic physics rate coefficients, the plasma electron density profile can be calculated from the relatively calibrated light profile along the beam. Several additional possibilities have already been demonstrated: Charge Exchange Resonance Spectroscopy (CXRS) for ion temperature/flow and Zeeman polarimetry for edge plasma current; therefore the Li-beam diagnostic offers a wealth of information at the plasma edge. The weaknesses of the method are the relatively faint light signal, background light, and technical difficulties of the beam injector which usually seriously limit the applicability. In this talk, we present systematic developments in alkali-beam diagnostics (Li, Na) for the injector and the observation system and detectors which resulted in strongly increased capabilities. Advanced systems have been built, and microsecond scale density profile, turbulence, and zonal flow measurement have been demonstrated. A novel edge current measurement technique has also been designed, and components have been tested with potential microsecond-scale time resolution. Additional possibilities of these advanced systems for spectral measurements (CXRS and various Zeeman schemes) are also discussed.
Nils Basse
added a research item
This Ph.D. thesis contains theoretical and experimental work on plasma turbulence measurements using collective light scattering. The motivation for measuring turbulence in hot fusion plasmas is, along with the method used and results obtained, the subject of chapter 1. The theoretical part is divided into three chapters. Chapter 2 contains a full analytical derivation of the expected dependency of the detected signal on plasma parameters. Thereafter, spatial resolution of the measurements using different methods is treated in chapter 3. Finally, the spectral analysis tools used later in the thesis are described and illustrated in chapter 4. The experimental part is divided into four chapters. In chapter 5 transport concepts relevant to the thesis are outlined. Main parameters of the Wendelstein 7-AS (W7-AS) stellarator in which measurements were made are collected in chapter 6. The setup used to study fluctuations in the electron density of W7-AS plasmas is covered in chapter 7. This localised turbulence scattering (LOTUS) diagnostic is based on a CO2 laser radiating at a wavelength of 10.59 µm. Fast, heterodyne, dual volume detection at variable wavenumbers between 14 and 62 cm−1 is performed. The central chapter of the thesis, chapter 8, contains an analysis of the measured density fluctuations before, during and after several confinement transition types. The aim was to achieve a better understanding of the connection between turbulence and the confinement quality of the plasma. Conclusions and suggestions for further work are summarised in chapter 9.
Nils Basse
added 4 research items
We describe and demonstrate a two-volume collective scattering system for localized measurements of plasma turbulence. The finite crossfield correlation length of plasma turbulence combined with spatial variations in the magnetic field direction are used to obtain spatially localized turbulence measurements at the Wendelstein 7-AS fusion experiment. By comparing with localized measurements obtained using the single wide beam technique Truc et al., Rev. Sci. Instrum. 63, 3716 1992 we provide a proof-of-principle demonstration of the two-beam method. An optimized configuration with large toroidal separation of the measurement volumes is predicted to be capable of providing spatial resolution better than 5 cm. © 2001 American Institute of Physics.
In this paper theory on collective scattering measurements of electron density fluctuations in fusion plasmas is revisited. We present the first full derivation of the expression for the photocurrent beginning at the basic scattering concepts. Thereafter we derive detailed expressions for the auto-and crosspower spectra obtained from measurements. These are discussed and simple simulations made to elucidate the physical meaning of the findings. In this context, the known methods of obtaining spatial localization are discussed and appraised. Where actual numbers are applied, we utilize quantities from two collective scattering instruments: The ALTAIR diagnostic on the Tore Supra tokamak [A. Truc et al., "ALTAIR: An infrared laser scattering diagnostic on the Tore Supra tokamak," Rev. Sci. Instrum. 63, 3716–3724 (1992)]. and the LOTUS diagnostic on the Wendelstein 7-AS stellarator [M. Saffman et al., "CO 2 laser based two-volume collective scattering instrument for spatially localized turbulence measurements," Rev. Sci. Instrum. 72, 2579–2592 (2001)].
Nils Basse
added 2 research items
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 10 20 m −3 at 2.5 T and high 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-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 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. © 2005 American Institute of Physics.
Nils Basse
added 6 research items
In this article measurements of small scale electron density fluctuations in dithering high confinement (H)-mode plasmas obtained by collective scattering of infrared light are presented. A scan of the fluctuation wavenumber was made in a series of similar discharges in the Wendelstein 7-AS (W7-AS) stellarator [H. Renner , Plasma Phys. Control. Fusion 31, 1579 (1989)]. The experimental setup and discharge properties are described. H-alpha-light observing an inner limiter was used to separate low confinement (L)- and H-mode phases of the plasma; the separated density fluctuations are characterized. It was found that L- (H-) mode fluctuations dominate at high (low) frequencies, respectively, and that they possess well-defined and distinguishable scaling properties. Wavenumber spectra for L- and H-mode measurements are calculated and fitted by power- laws and exponential functions. The separated measurements can be fitted with the same exponents in L- and H-mode. Correlations between the density fluctuations, the H-alpha- signal and magnetic fluctuations as measured by Mirnov coils were analyzed. Correlation calculations using 50 ms time windows (several dithering periods) with time lag steps of 100 mus showed that all the fluctuating quantities are highly correlated and that the maximum correlation occurs for high frequency density fluctuations. Performing separate L- and H- mode correlations on a 20 mus time scale between magnetic and density fluctuations leads to the result that the minimum correlation time scale in L-mode is of order 100 mus, while no correlation exists for H-mode. (C) 2002 American Institute of Physics.
Nils Basse
added a research item
Confinement transitions in the Wendelstein 7-AS stellarator [H. Renner et al., Plasma Phys. Controlled Fusion 31, 1579 (1989)] can be induced by varying either the internal plasma current or the external magnetic field. In this paper we report on experiments where closely matched confinement states (good and bad) were constructed using the latter method. Analysis using the former scheme has been reported upon previously [S. Zoletnik et al., Plasma Phys. Controlled Fusion 44, 1581 (2002)]. The electron temperature, along with the major spectral characteristics of magnetic and small-scale electron density fluctuations, changes dramatically at the transition from good to bad confinement. The fluctuation power is intermittent, and core bursts traveling in the electron diamagnetic drift (DD) direction are correlated between the bottom and top of the plasma, especially during degraded confinement. A corresponding top-bottom correlation for the edge ion DD direction turbulence feature was not found. Strong correlations are observed both between the two density fluctuation signals and between magnetic and density fluctuations in bad compared to good confinement. The correlation time of the bursts is of order 100 mus, similar to the lifetime observed during edge localized modes.
Nils Basse
added a project goal
Turbulence measurements