G. Por

Low frequency sawtooth precursor activity in ASDEX Upgrade

This paper describes the precursor activity observed in the ASDEX Upgrade tokamak before sawtooth crashes in various neutral beam heated plasmas, utilizing the soft x-ray diagnostic. In addition to the well-known (m, n) = (1,1) internal kink mode and its harmonics, a lower frequency mode is studied in detail. Power modulation of this mode is found to correlate with the power modulation of the (1, 1) kink mode in the quasistationary intervals indicating possible nonlinear interaction. Throughout the studied sawtooth crashes, the power of the lower frequency mode rose by several orders of magnitude just before the crash. In addition to its temporal behaviour, its spatial structure was estimated and the most likely value was found to be (1, 1). A possible role of this mode in the mechanism of the sawtooth crash is discussed.

Cross-correlation based time delay estimation for turbulent flow velocity measurements: Statistical considerations

Time delay estimation methods (TDE) are well-known techniques to investigate poloidal flows in hot magnetized plasmas through the propagation properties of turbulent structures in the medium. One of these methods is based on the estimation of the time lag at which the cross-correlation function (CCF) estimation reaches its maximum value. The uncertainty of the peak location refers to the smallest determinable flow velocity modulation, and therefore the standard deviation of the time delay imposes important limitation to the measurements. In this article, the relative standard deviation of the CCF estimation and the standard deviation of its peak location are calculated analytically using a simple model of turbulent signals. This model assumes independent (non interacting) overlapping events (coherent structures) with randomly distributed spatio-temporal origins moving with background flow. The result of our calculations is the derivation of a general formula for the CCF variance, which is valid not exclus...

Deconvolution-based correction of alkali beam emission spectroscopy density profile measurements

A deconvolution-based correction method of the beam emission spectroscopy (BES) density profile measurement is demonstrated by its application to simulated measurements of the COMPASS and TEXTOR tokamaks. If the line of sight is far from tangential to the flux surfaces, and the beam width is comparable to the scale length on which the light profile varies, the observation may cause an undesired smoothing of the light profile, resulting in a non-negligible underestimation of the calculated density profile. This effect can be reduced significantly by the emission reconstruction method, which gives an estimate of the emissivity along the beam axis from the measured light profile, taking the finite beam width and the properties of the measurement into account in terms of the transfer function of the observation. Characteristics and magnitude of the mentioned systematic error and its reduction by the introduced method are studied by means of the comprehensive alkali BES simulation code RENATE.

Application of a bandpower correlation method to the statistical analysis of MHD bursts in quiescent Wendelstein-7 AS stellarator plasmas

This paper presents the analysis of Mirnov coil data from pure electron-cyclotron resonance heated Wendelstein-7 AS discharges with transient magnetohydrodynamical (MHD) mode activity in otherwise MHD inactive plasmas. Amplitude modulations were found on the 100 s time scale, expected to originate from transient changes of the plasma pressure proflle by anomalous transport events. The analysis was done using our custom developed bandpower correlation method, which combines short-time Fourier transform for the decomposition of the signal with a correlation analysis to quantitatively characterize the statistical connection between the modulation in the power of difierent MHD modes. The efiect of a conflnement transition (near a rational rotational transform) on the correlation between the power modulation of difierent frequency modes has been studied. As high as 50% correlation has been found between difierent frequencies in "bad conflnement" shots with enhanced transient transport, while for "good conflnement" shots no signiflcant correlation was observed.

Evaluation of the Boron Dilution Method for Moderator Temperature Coefficient Measurements

Abstract A measurement of the at-power moderator temperature coefficient (MTC) at the pressurized water reactor Unit 4 of the Ringhals Nuclear Power Plant (Sweden) during fuel cycle 16 is analyzed. The measurement was performed when the boron concentration decreased under 300 ppm in the reactor coolant system, by using the boron dilution method. Detailed calculations were made to estimate all reactivity effects taking place during such a measurement. These effects can only be accounted for through static core calculations that allow calculating contributions to the reactivity change induced by the moderator temperature change. All the calculations were performed with the Studsvik Scandpower SIMULATE-3 code. Analysis of the measurement showed that the contribution of the Doppler effect (in the fuel) was almost negligible, whereas the reactivity effects due to other than the Doppler fuel coefficient and the boron change were surprisingly significant. It was concluded that due to the experimental inaccuracies, the uncertainty associated with the boron dilution method could be much larger than previously expected. The MTC might then be close to –72 pcm/°C, whereas the main goal of the measurement is to verify that the MTC is larger (less negative) than this threshold. The usefulness of the boron dilution method for MTC measurements can therefore be questioned.

The Advanced Loose Parts Monitoring System (ALPS) and wavelet analysis

The Advanced Loose Parts monitoring System (ALPS), is installed in each Unit of Paks NPP. Its characteristics and some interesting results are presented. Wavelet analysis is being introduced to data evaluation techniques. The short-time Fourier transform and the continuous wavelet transform techniques have been used to present the time signal in a time-frequency and time-scale plane. Characteristic frequencies of the physical acoustic system and the growing frequencies of spectrum components during the start-up of the main coolant pumps are clearly seen on those pictures. The newly applied wavelet coherence promises to find new oscillation in the pair of signals, which remain hidden in time-dependent autospectra.