Parametrization of reflectometry fluctuation frequency spectra for systematic study of tokamak fusion plasma turbulence

We describe a way to parameterize power spectra extracted from fixed-frequency reflectometry data, with a view to systematic studies of turbulence properties in tokamak plasmas. Analysis of typical frequency spectra obtained from a new database suggests decomposition in a set of four key components: the direct current component, low-frequency fluctuations, broadband (BB) turbulence, and the noise level. For the decomposition in the identified components, different kinds of functions are tested and their fitting performance is analyzed to determine the optimal spectrum parametrization. In particular, for the BB turbulence, three models are compared qualitatively based on a number of representative spectrum test cases, notably the generalized Gaussian, the Voigt, and the Taylor model. In addition, quantitative performance testing is accomplished using the weighted residual sum of squares and the Bayesian information criterion in a large database including 350 000 spectra obtained in Tore Supra. Next, parametrization by the Taylor model is applied to Ohmically heated plasmas, and a BB energy basin is systematically observed in the core plasma region, which shrinks with decreasing radial position of the q = 1 surface. This basin might be explained by a drop of the density fluctuation level inside the q = 1 surface

Microwave imaging of magnetohydrodynamic instabilities in fusion plasma

Microwave imaging diagnostics are extremely useful for observing magnetohydrodynamic (MHD) instabilities in magnetic fusion plasmas. Two imaging diagnostics will be available on the WEST tokamak. A method was developed to reconstruct electron density maps from electron density profiles measured by ultrafast reflectometry, a technique based on FM-CW radar principle. It relies on plasma rotation to perform 2D reconstruction. An Electron Cyclotron Emission Imaging (ECEI) diagnostic will image directly the temperature fluctuations. It will be equivalent to 24 stacked vertically radiometers, each probing a spot of few centimetres. These two complementary techniques will contribute to the validation of MHD models.ope

The economic benefits of malaria elimination: do they include increases in tourism?

BACKGROUND: Policy makers have speculated that one of the economic benefits of malaria elimination includes increases in foreign direct investment, particularly tourism. METHODS: This study examines the empirical relationship between the demand for travel and malaria cases in two countries with large tourism industries around the time in which they carried out malaria-elimination campaigns. In Mauritius, this analysis examines historical, yearly tourist arrivals and malaria cases from 1978–1999, accounting for the background secular trend of increasing international travel. In Dominican Republic, a country embarking upon malaria elimination, it employs a time-series analysis of the monthly, international tourist arrivals from 1998–2010 to determine whether the timing of significant deviations in tourist arrivals coincides with malaria outbreaks. RESULTS: While naïve relationships exist in both cases, the results show that the relationships between tourist arrivals and malaria cases are relatively weak and statistically insignificant once secular confounders are accounted for. CONCLUSIONS: This suggests that any economic benefits from tourism that may be derived from actively pursuing elimination in countries that have high tourism potential are likely to be small when measured at a national level. Rather, tourism benefits are likely to be experienced with greater impact in more concentrated tourist areas within countries, and future studies should seek to assess these relationships at a regional or local level

Turbulence correlation properties measured with ultrafast sweeping reflectometry on Tore Supra

The ultrafast sweep reflectometry technique is used to investigate plasma turbulence on Tore Supra. Thanks to the X-mode polarization, the plasma density and fluctuations over the whole tokamak radius, from the far scrape-off layer (SOL) to the core can be measured. Cross-correlation analysis is applied to all the signal components to estimate the fluctuation correlation length radial profile. The correlation length decreases from the plasma centre to the last closed flux surface and grows again into the far SOL

The reconstruction of hollow areas in density profiles from frequency swept reflectometry

International audienceAll density profile reconstruction techniques for both O-mode and X-mode are based on the assumption that the cutoff frequency profile is monotonic. However, experimentally, there are many perturbations to the plasma that generate hollow areas in the cutoff frequency profile. They cause a significant immediate reconstruction error that is not rapidly damped. It is demonstrated that even though no reflections occur inside the hollow region, the higher probing frequencies that propagate through this area carry information about it that can be used to estimate its properties. In addition, the reconstruction algorithm is based on the WKB approximation of the phase, which ignores all full-wave features found in experimental signals. These effects were investigated with the use of full-wave simulations in 1D, with special attention paid to the frequency band where they are dominant. A database of perturbation signals is simulated on 5 dimensions of parameters an application is demonstrated for a magnetic island in a Tore Supra discharge. The new adapted reconstruction scheme improved the description of the density profile inside the hollow region and also along 10 cm after it

Extraction of quasi-coherent modes based on reflectometry data

International audienceAbstract The identification of turbulence sources would drive to a deeper understanding of confinement dynamics in tokamak plasmas. Turbulence results from a mixture of instabilities corresponding to sources at different timescales and spatial scales. Using poloidal correlation reflectometry and multi-pin Langmuir probe, it was shown in the T-10 and the Tokamak Experiment for Technology Oriented Research (TEXTOR) tokamaks that the reflectometry frequency spectrum is the superposition of several components: broadband component, quasi-coherent (QC) modes and low-frequency components. The relevance of QC modes is associated with their link with the trapped electron mode instability. This link was exhibited in the transition from the linear ohmic confinement (LOC) to the saturated ohmic confinement (SOC) regime. A method is presented in this paper to extract the QC mode component from the reflectometry data, enabling its separation from the broadband component and the study of its time evolution. It is a first step toward the discrimination of turbulence sources. The central idea explores a way to combine the approach of signal processing and machine learning. The continuous wavelet transform on the basis of complex Morlet wavelet has proved to be efficient in providing a decomposition of a signal at different scales over time for fluctuation tackling; clustering techniques, such as the mini-batch K -means, are able to tackle clusters at different scales. The method was applied to Tore Supra and TEXTOR reflectometry data. In Tore Supra, the amplitude of the extracted QC mode component decreases during the LOC–SOC transition. In TEXTOR, the amplitude of the coherent spectra of the extracted QC mode component is similar to the experimental coherent spectra obtained through correlation reflectometry. The developed method permits the extraction of components, preserving their physical and statistical properties

Particle Flow during Sawtooth Reconnection: Numerical Simulations of Experimental Observations

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