Bestimmung von 2-dim. Elektronendichtefluktuationen im Plasmarand von TEXTOR-94 mit zwei thermischen Li-Atomstrahlen
Bestimmung von 2-dim. Elektronendichtefluktuationen im Plasmarand von TEXTOR-94 mit zwei thermischen Li-Atomstrahlen
The turbulent fluctuations is the most probable origin of the anomalous transport of energy and particles in tokamaks. But the instability mechanism is not yet understood. The improved knowledge of the temporal-spatial structure of fluctuations is a prerequisite for deep comprehension of their natur...
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Personal Name(s): | Huber, Alexander (Corresponding author) |
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Contributing Institute: |
Publikationen vor 2000; PRE-2000; Retrocat |
Imprint: |
Jülich
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
1997
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Physical Description: |
128 p. |
Document Type: |
Report Book |
Research Program: |
Addenda |
Series Title: |
Berichte des Forschungszentrums Jülich
3422 |
Link: |
OpenAccess OpenAccess |
Publikationsportal JuSER |
The turbulent fluctuations is the most probable origin of the anomalous transport of energy and particles in tokamaks. But the instability mechanism is not yet understood. The improved knowledge of the temporal-spatial structure of fluctuations is a prerequisite for deep comprehension of their nature. None of the existent diagnostics up to beginning of this work provided the simultaneous measurements of the radial and poloidal fluctuations structure with sufficient spatial and temporal resolution. In this work a new technique using spectroscopical observations of atomic beams for the measurement of electron density fluctuation in the scrape-off layer was developed, tested and applied on TEXTOR-94. $\textbf{This new diagnostic allows simultaneous studies of radial}$ $\textbf{and poloidal fluctuation parameters}$, $\textbf{as the correlation lengths, as the wavenumber and frequency spectra}$ $\textbf{and as the propagation velocities and lifetimes}$. The advantage of this method is, as compared with traditional measurementswith Langmuir probes, that the good spatial and temporal resolution can be achieved without disturbance of the plasma. Moreover, the application of continuousbeams, permits the measurements over the whole discharge. The simultaneous determination of the radial and poloidal components of fluctuation parameters was achieved by operation of two narrow Li-beams (FWHM $\approx$ 10 mm at a distance of 100mrn), which were separated by 50mm. The beams of thermal lithium atoms have been generated by a newly developed oven, which provides a long operationtime and a high reliability. The atoms could be injected in any desired direction because the operation of the oven was insensitive to the positioning. The lithium atoms witha mean velocity of $\approx$ 1,7 · 10$^{3}$m/s were injected radially into the plasma edge, where they were excited and ionized by the plasma. The line radiation (resonance transition2s-2p, $\lambda$ = 670, 8nm) of the excited atoms was observed, allowing the determination of electron density profiles and its fluctuations. The measurements were performed withhigh temporal ($\Delta$t $\simeq$ 2 $\mu$s) and spatial ($\Delta_{r} \simeq$ 1 mm in radial and $\Delta_{p} \simeq$ 10mm in poloidal direction) resolution.A broad frequency band of $\nu \le$ 100kHz and a wavenumber spectra of k$_{r}$, k$_{p} \le $ (0,5- 1) mm$^{-1}$ have been observed in the scrape-off layer of TEXTOR-94. Relative fluctuationamplitudes ne/ne were found with levels of 5%-10% inside the last closed flux surface (LCFS). Outside LCFS within a radial range of 40mm they increased by a factor of3-4. Due to the poloidal separation of the two beams, the 2-dimensional correlation functions and the corresponding radial and poloidal correlation lengths could be simultaneouslyinvestigated. The radial correlation lengths in the scrape-off layer of TEXTOR-94 varied between 3mm and 6mm in plasmas with ohmic heating, respectively 7mmand 13mm with additional neutral beam heating. The measurements at all conditions indicate that the poloidal correlation length was a factor of 3 to 4 higher than the radialcorrelation length. The typical lifetimes of electron density fluctuations, as defined by a cross-correlation analysis, was 10 $\mu$s to 50 $\mu$s. Outside the LCFS in TEXTOR-94 thefluctuations propagated poloidally in the direction of the ion diamagnetic drift with a velocity of $\approx$ 10$^{3}$m/s, and their radial velocity was about $\approx$ 1,4 · 10$^{2}$m/s inwards. Insidethe LCFS, the fluctuations moved in the other direction. The propagation direction of the fluctuations were opposite with reversed magnetic field. |