Usually different classes of reflectometric measurements are carried out using different specialized instruments which can be mainly grouped in swept frequency modulation for electron density profile reconstruction and fixed/slow varying frequency for correlation, Doppler and density fluctuation measurements. The upgrade of RFX]mod reflectometer is aimed to obtain the simultaneous characterization of both the density gradients and measurement of turbulence level and correlations, using a single instrument, by means of an extremely fast bi]directional frequency sweeping. Fast swept reflectometry is commonly used to overcome the effect of spurious fringes produced by density fluctuations in the received signal. The main source of disturbance is due the interference pattern produced by the microwave scattering due to high wavenumber density fluctuations dragged by the plasma flow. It can be shown that even when the simple phase delay .(t) at fixed frequency fails to give a correct estimation of the average phase shift, the group delay d./d?Ö from a fast swept measurement is nevertheless correct. The effect of low wave]number fluctuations instead cause an oscillation of the reflection which is seen as Doppler shift on the group delay which canft be eliminated by the simple increase of microwave sweep rate. The possibility to smooth out this effect by averaging different measurements, can be prevented by the gphase run]awayh effect generated by the plasma flow. A fast bi]directional up and down frequency sweep allows the correct evaluation and thus to cancel out these effects. Moreover being the frequency swept at in very short time, it become also possible to use portions of the received the signal for radial correlation measurements. In order to achieve these goals the duration of the modulation frequency of the sweep has to be gNyquisth higher characteristic frequency scales of the processes in play. In the case of RFX]mod typical fluctuations are found to have a frequency up to some hundredth kHz. Thus the upgrade of RFX]mod reflectometric system has been designed to provide a modulation rate in of 4 GHz/200 ns, with a repetition of 2 bidirectional Msweeps/s on three simultaneous bands (Ka, U, V); the signals are to be acquired at 200 Msamples/s. This should finally allow the association of fluctuations properties such as radial correlation and frequency spectra with density gradient and/or position, providing a new tool for the study of turbulence in plasmas.
Upgrade of the RFX-mod reflectometer
Capobianco R;Cervaro V;Ghiraldelli R;
2011
Abstract
Usually different classes of reflectometric measurements are carried out using different specialized instruments which can be mainly grouped in swept frequency modulation for electron density profile reconstruction and fixed/slow varying frequency for correlation, Doppler and density fluctuation measurements. The upgrade of RFX]mod reflectometer is aimed to obtain the simultaneous characterization of both the density gradients and measurement of turbulence level and correlations, using a single instrument, by means of an extremely fast bi]directional frequency sweeping. Fast swept reflectometry is commonly used to overcome the effect of spurious fringes produced by density fluctuations in the received signal. The main source of disturbance is due the interference pattern produced by the microwave scattering due to high wavenumber density fluctuations dragged by the plasma flow. It can be shown that even when the simple phase delay .(t) at fixed frequency fails to give a correct estimation of the average phase shift, the group delay d./d?Ö from a fast swept measurement is nevertheless correct. The effect of low wave]number fluctuations instead cause an oscillation of the reflection which is seen as Doppler shift on the group delay which canft be eliminated by the simple increase of microwave sweep rate. The possibility to smooth out this effect by averaging different measurements, can be prevented by the gphase run]awayh effect generated by the plasma flow. A fast bi]directional up and down frequency sweep allows the correct evaluation and thus to cancel out these effects. Moreover being the frequency swept at in very short time, it become also possible to use portions of the received the signal for radial correlation measurements. In order to achieve these goals the duration of the modulation frequency of the sweep has to be gNyquisth higher characteristic frequency scales of the processes in play. In the case of RFX]mod typical fluctuations are found to have a frequency up to some hundredth kHz. Thus the upgrade of RFX]mod reflectometric system has been designed to provide a modulation rate in of 4 GHz/200 ns, with a repetition of 2 bidirectional Msweeps/s on three simultaneous bands (Ka, U, V); the signals are to be acquired at 200 Msamples/s. This should finally allow the association of fluctuations properties such as radial correlation and frequency spectra with density gradient and/or position, providing a new tool for the study of turbulence in plasmas.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
