ECE imaging
Diagnostic systems for fluctuation measurements in plasmas have, of necessity, evolved from simple 1-D systems to multi-dimensional systems due to the complexity of turbulence physics of plasmas illustrated by advanced numerical simulations. Using the significant advancements in millimeter wave imaging technology, Microwave Imaging Reflectometry (MIR) and Electron Cyclotron Emission Imaging (ECEI), capable of measuring density and temperature fluctuations are developed. Both systems require large collection optics for the reflected waves from the "cut-off layer" in the MIR system and vertically (poloidally) extended emissions in ECEI system. Because both systems operate in the X-mode and in a similar microwave range (the MIR frequency range is ~89 GHz and ECEI ranges from 95 to 130 GHz for TEXTOR), it is feasible to combine the two systems. Reflectometry signals are separated from the ECE radiation by a beam splitter, to allow simultaneous temperature and density fluctuation measurements. Both systems utilize state-of-the-art millimeter-wave planar detector arrays positioned at the focal point of the optical system that forms an image of the plasma fluctuations on the detector plane (see Figure 1).
Figure 1: CAD drawing of the combined ECEI/MIR system at TEXTOR.
Electron Cyclotron Emission Imaging (ECEI)
In previous ECEI systems, including the first ECEI system that was used on TEXTOR, the measurements were essentially 1-D in nature in that the detected radiation from each array element was sampled at only a single frequency at a given time. This is in contrast to the conventional wideband ECE radiometer, in which multiple frequency elements are simultaneously detected from a single antenna resulting in a horizontally aligned sampling. A step forward is the 2D ECE Imaging system that has been recently installed on TEXTOR. This system combines the advantages of wideband radiometer and ‘classical’ ECEI systems to provide a true 2D image (with a total of 128 channels, arranged in a matrix of 8 (horizontal) × 16 (vertical) sample volumes), of Te fluctuations, corresponding to a total area of 8 by 6 cm of a poloidal cross-section. The radial position of the sample area can be shifted through the plasma by tuning the local oscillator frequency, to cover the plasma region of interest (frequency range 95 – 130 GHz). The system has already been used to make beautiful movies of the sawtooth precursor and crash (see Figure 2).
Figure 2: Movie of electron temperature fluctuations associated with a sawtooth precursor and crash in TEXTOR discharge #94569. The time instants of the various images are indicated by red lines in the electron temperature traces. The double curved line in the images indicates the position of the sawtooth inversion radius. Blue colours indicate areas that are colder than average, while yellow colours indicate regions that are hotter.(Click on the figure to see the movie - 1.4 MB)
Microwave Imaging Reflectometer (MIR)
MIR (89 GHz) carefully matches the probing beam of the reflectometer to the position and curvature of the reflecting surface, and collects the reflected radiation in as large as possible an acceptance angle. The detailed testing of the optical system for the MIR was performed with known targets (corrugated metal surfaces) and compared with the response of a conventional 1D-system. It was shown that detailed structures of the reflecting surface could be recovered by imaging the reflected signal on a detector array positioned at a location relatively far away from the reflecting surface (about 2 meter in our case). This was impossible to achieve with a conventional 1-D reflectometer, where correlation between measurement and surface structure decreased rapidly over distance between reflecting surface and receiver antenna. The MIR system was installed on TEXTOR in September 2003. It was not yet possible to quantitatively measure density fluctuations in the plasma (the ultimate aim of the diagnostic), but already detailed measurements of the rotation velocity of the turbulence could be made (see Figure 3).
Figure 3: ‘Movie’ of electron density fluctuations during a TEXTOR discharge in which NBI is switched off. Due to this the plasma rotation (proportional to the slope of the line in the figures) reverses sign at the position of the cutoff surface.