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A most persistent limitation of the geometrical-optics (GO) approximation is the difficulty in integrating the focusing/defocusing term in the equation for the wave amplitude when rays go through singularities (i.e., caustics and cutoffs), points where GO fails and wavelengths and other wave field-related quantities (e.g., the wave amplitude and energy density) become arbitrarily large. A new asymptotic matching (AM) technique is thus presented which allows one to recover the wave amplitude of rays crossing singularities and which improves on a previous approach [A. H. Glasser and A. Bravo-Ortega, Phys. Fluids 30, 797 (1987)]: it goes higher in the order of the asymptotic expansion about the singular point and eventually leads to a less critical violation of the GO ordering when a ray approaches and crosses a caustic or a cutoff. The implementation of this new AM technique is verified and validated against analytical solutions in slab geometry for the lower-hybrid (LH) cutoff and is illustrated with numerical examples of LH wave propagation in a tokamak plasma, using parameters characteristic of a LH current drive experiment. The new and previous approaches are compared regarding several computed ray quantities (e.g., the second derivatives of the eikonal phase, which contribute to the focusing/defocusing term, as well as the wave amplitude and energy density), the inclusion of more terms in the asymptotic expansion about the singularity making it possible to start, say, jumping the latter in a region where GO is less severely challenged. Practical criteria for AM implementation and for testing the validity of GO and of the asymptotic expansion are also provided and discussed. [ABSTRACT FROM AUTHOR]

Parametric decay instabilities (PDIs) occur for large-amplitude waves in quadratically nonlinear media, where they provide a limit of validity of linear theories and allow efficient coupling between different, well-defined wave modes. We investigate PDIs near the upper hybrid resonance in plasmas by injection of high-power electron cyclotron (EC) waves at the ASDEX Upgrade tokamak. Our measurements of PDIs have an unprecedented frequency resolution, far below the ion cyclotron frequency, allowing the first observations of secondary and tertiary PDIs during the saturation phase in a controlled laboratory setting. Furthermore, we are for the first time able to systematically compare theoretical predictions of the EC wave power thresholds, which must be exceeded to excite such PDIs, with experimental observations, validating the theory. Our findings are relevant for EC wave heating and current drive in tokamaks and stellarators, including future fusion power plants, as well as in low-temperature laboratory and industrial plasmas, inertial confinement fusion, and ionospheric modification experiments. [ABSTRACT FROM AUTHOR]

The spherical torus or spherical tokamak (ST) is a member of the tokamak family with its aspect ratio (A=R0/a) reduced to A~1.5, well below the normal tokamak operating range of A≥2.5. As the aspect ratio is reduced, the ideal tokamak beta β (radio of plasma to magnetic pressure) stability limit increases rapidly, approximately as β~1/A. The plasma current it can sustain for a given edge safety factor q-95 also increases rapidly. Because of the above, as well as the natural elongation κ, which makes its plasma shape appear spherical, the ST configuration can yield exceptionally high tokamak performance in a compact geometry. Due to its compactness and high performance, the ST configuration has various near term applications, including a compact fusion neutron source with low tritium consumption, in addition to its longer term goal of an attractive fusion energy power source. Since the start of the two mega-ampere class ST facilities in 2000, the National Spherical Torus Experiment in the United States and Mega Ampere Spherical Tokamak in UK, active ST research has been conducted worldwide. More than 16 ST research facilities operating during this period have achieved remarkable advances in all fusion science areas, involving fundamental fusion energy science as well as innovation. These results suggest exciting future prospects for ST research both near term and longer term. The present paper reviews the scientific progress made by the worldwide ST research community during this new mega-ampere-ST era. [ABSTRACT FROM AUTHOR]

On the Tore Supra tokamak, a far infrared polarimeter diagnostic has been routinely used for diagnosing the current density by measuring the Faraday rotation angle. A high precision of measurement is needed to correctly reconstruct the current profile. To reach this precision, electronics used to compute the phase and the amplitude of the detected signals must have a good resilience to the noise in the measurement. In this article, the analogue card's response to the noise coming from the detectors and their impact on the Faraday angle measurements are analyzed, and we present numerical methods to calculate the phase and the amplitude. These validations have been done using real signals acquired by Tore Supra and JET experiments. These methods have been developed to be used in real-time in the future numerical cards that will replace the Tore Supra present analogue ones., (© 2011 American Institute of Physics)

The present work is motivated by a long standing discrepancy between the electron temperature measurements of Thomson scattering (TS) and electron cyclotron emission (ECE) diagnostics for plasmas with strong auxiliary heating observed at both JET and TFTR above 6–7 keV, where in some cases the ECE electron temperature measurements can be 15%–20% higher than the TS measurements. Recent analysis based on ECE results at JET has shown evidence of distortions to the Maxwellian electron velocity distribution and a correlation with the TS and ECE discrepancies has been suggested. In this paper, a technique to determine the presence of non-Maxwellian behavior using TS diagnostics is outlined. The difficulties and limitations of modern TS system designs to determine the electron velocity distribution are also discussed. It is demonstrated that small deviations such as those suggested by previous ECE analysis could be potentially detected, depending on the spectral layout of the TS polychromators. The spectral layout of the JET high resolution Thomson scattering system is such that it could be used to determine these deviations between 1 and 6 keV, and the results presented here indicate that no evidence of non-Maxwellian behavior is observed in this range. In this paper, a modification to the current polychromator design is proposed, allowing non-Maxwellian distortions to be detected up to at least 10 keV.