Abstract
Tore Supra ECE diagnostic has been recently upgraded to study MHD modes driven by energetic particles up to 400 kHz. To improve the measurement sensitivity, the ECE signals of the 32 channels radiometer were amplified just below the saturation limit and sources of noise were investigated in order to keep it as low as possible. With such an improvement, fast particle driven modes with frequencies up to 200 kHz were detected. A 4-channel correlation ECE system using YIG filters with tuneable frequency was also installed. It allows fine radial scans of MHD modes and correlation length measurements. For the two kinds of YIG filter in use, the minimum frequency separation between two ECE channels that could be achieved was established measuring the correlation coefficient between the respective radiation noises. Finally, by modelling the ECE radiometer taking into account the antenna radiation pattern and the vertical position of the ECE beam relative to the plasma centre we improved the data analysis tools, thus giving a better determination of the phase radial structure of ECE oscillations. The poloidal structure of MHD modes can then be identified from ECE data and, for off axis ECE lines of sight, the direction of the plasma rotation can also be determined. This method allows identifying the occurrence of an inverse cascade of electron fishbone modes ranging from m/n=4/4 to 1/1 (m and n are the poloidal and toroidal mode numbers, respectively) which appears in lower hybrid current drive plasmas.
Highlights
Tore Supra ECE diagnostic has been recently upgraded to study MHD modes driven by energetic particles up to 400 kHz
To improve the measurement sensitivity, the ECE signals of the 32 channels radiometer were amplified just below the saturation limit and sources of noise were investigated in order to keep it as low as possible
By modelling the ECE radiometer taking into account the antenna radiation pattern and the vertical position of the ECE beam relative to the plasma centre we improved the data analysis tools, giving a better determination of the phase radial structure of ECE oscillations
Summary
At Tore Supra, the radial profile of electron temperature Te is routinely measured by an ECE radiometer with 32 equatorial line of sight channels and 1 GHz sampling frequency. 3. A four-channel system with adjustable frequencies provides Te fluctuation measurement with improved S/N ratio using correlation of two adjacent channels probing the same plasma region with two different frequencies [2]. Fast ECE- In 2009, the fast acquisition system of the ECE radiometer was improved to detect high frequency MHD modes up to 400 kHz during up to 7 seconds in all discharges. The observation time is long (up to 7 seconds), which is suitable to track physical phenomena from signal spectrograms Over such a long time: plasma equilibrium usually evolves, some perturbations of the additional heating can occur or periodic relaxation phenomena, such as sawtooth or giant oscillations, can lead to modifications of the mode frequencies. The 32 ECE channels are useful to locate MHD-induced oscillations in the radial direction and determine their radial structure by measuring the relative phase between channels
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