Highlights Plasma Diagnostics group
Multi-Pulse Thomson Scattering
A burst-mode operated 10 kHz TV Thomson scattering system is installed on TEXTOR. The system has a high spatial resolution and can operate with a repetition rate of up to 10 kHz during three bursts of about 5 ms long. Each burst consists of a train of up to 50 pulses with 15 J each. Operation of the laser during one burst has been achieved; operation in a three burst mode is in preparation. Ultra-fast CMOS cameras are used in a Littrow polychromator to detect the Thomson scattered light. One camera is used to record the Thomson scattering light, while the other one is required to sample the plasma light between subsequent laser pulses. Temperature and density profiles along the full plasma diameter of 900 mm can be sampled at 120 points with a spatial resolution of 7.5 mm. A separate system for edge observations between z = 340 and 500 mm with a resolution of 1.7 mm has been recently installed.
ECE Imaging and MIR
A combined Microwave Imaging Reflectometry (MIR) and Electron Cyclotron Emission Imaging (ECEI) system, capable of measuring density and temperature fluctuations has been developped at the TEXTOR tokamak in combination with teams from UC Davis and PPPL Princeton. The ECEI 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 16 cm2 of a poloidal cross-section. The system produced beautiful movies of the sawtooth precursor and crash (see Fig. 1). In the MIR the probing beam (89 GHz) is carefully matched to the position and curvature of the density cutoff surface. The reflected radiation is collected in as large as possible an acceptance angle. In the beginning of 2009 the ECE imaging system has been transferred to the ASDEX-UPgrade tokamak in Garching, Germany.
Figure 1: 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)
CXRS and MSE
The Motional Stark Effect (MSE) diagnostic that is installed on TEXTOR features lines of sight that are tangential to the magnetic flux surfaces for the best radial resolution. The light is collected by three rows of 20 channels (the middle one crosses the equatorial plane) covering 0.45 m of the low field side of the tokamak major radius (R0 = 1.75 m, a = 0.46 m) and crosses the magnetic axis. The spatial resolution of the measurement is about 3 cm with a channel-to-channel separation of the order of 1.5 - 2 cm. The current density profile is deduced from an analysis of the full polarization spectrum. The MSE system is combined with a Charge Exchange Recombination Spectroscopy (CXRS) system. This system is in routine use now and is used to yield detailed information on the plasma rotation and the ion temperature profiles.