Littérature scientifique sur le sujet « O-X-B Mode Conversion »

Spherical tokamaks like the MAST Upgrade device are often operated in an overdense regime. As a consequence, conventional electron cyclotron resonance heating (ECRH) and current drive (ECCD) are typically not possible. MAST Upgrade is planned to investigate a mode coupling scheme known as O-X-B at high power, which may allow gyrotrons to heat and drive current in overdense plasmas by coupling electromagnetic waves to electrostatic electron Bernstein waves (EBWs) at the upper hybrid (UH) layer. However at the gyrotron beam intensities planned for MAST Upgrade, several nonlinear effects may degrade the linear mode coupling into EBWs. Using particle-in-cell simulations, parametric decay instabilities (PDIs) and stochastic electron heating (SEH) are investigated in the region near the UH layer. It is found that nonlinear effect could have a substantial impact on the O-X-B scheme in MAST Upgrade at high gyrotron intensities.

The conversion of thermally-born electrostatic waves in tokamak plasmas has the potential to be a powerful diagnostic for tokamak edge physics. Analytic theory and full wave modelling both conclude that analysis of emission in the microwave region carries with it information on the magnetic field in spatially localised areas which depend on density and frequency. Knowing these quantities and the 3D (2D + frequency) microwave emission pattern, it is in theory possible to calculate the current density which is vital to the understanding of the plasma pedestal. Motivated by a pilot experiment carried out on the Mega Ampere Spherical Tokamak (MAST), a novel microwave imaging device has been developed to obtain the first images of mode conversion in a Tokamak and to prove the principle of the synthetic aperture imaging technique on Tokamak devices. Here the design and calibration of the Synthetic Aperture Microwave Imaging (SAMI) radiometer is described, as well as the presentation and comparison of some of the first images of mode conversion with full wave simulations.

The work presented in this PhD Thesis has been performed in the framework of research on Thermonuclear Fusion in magnetically confined plasmas of tokamak-like devices. The effort is aimed at a feasibility study of low-field side Electron Cyclotron Resonance Heating (ECRH) in overdense plasmas. The main aim of this Thesis consists in the study of the applicability of the mode conversion scheme, known as ”O-X-B Double Mode Conversion”, to the Italian tokamak FTU (Frascati Tokamak Upgrade), with the use of millimeter-waves at the 140 GHz frequency. This overdense plasma heating technique, not yet demonstrated at electron density of 2.4 ·10^20 m^−3 and consequently at such a high frequency, exploits the conversion of an ordinary polarized wave (O) into the extraordinary (X) one, which can occur only for radiation propagating in a very narrow angular range at the cutoff region, followed by a subsequent conversion to Bernstein (B) waves, which are then absorbed by the plasma. Simulations have been performed, by using a ray tracing code, to find the optimal launching conditions for the O-X coupling in FTU. The assessment of conversion efficiency was carried out first with the use of one-dimensional models, that describe the density and the magnetic field gradients of the plasma. Moreover, the effects predicted by recent bi-dimensional theoretical models available in literature have been evaluated. The inhomogeneities of a toroidal plasma are thus accounted with a more realistic description. The experimental part of the work for the Thesis can be divided into two main activities. The first one has been carried out at the laboratories of the research center ENEA in Frascati (Roma), where the tokamak FTU is operating. In this phase, experiments have been performed, aimed at the detailed study of the density profiles and gradients, which characterize the overdense plasma regimes. Proper experimental procedures have been developed, to prepare with reliability the optimal plasma ’target’. The second experimental activity consists in the contribution given for the design of a new EC millimeter-waves launcher for FTU, whose installation is scheduled for the first months of 2011. The system has been designed to reach the launching angles requested for O-X-B mode conversion, which have been defined in the present work and that are not achievable with the present launching system. The results of the predictive work confirm that the requested precision in the injection of the wave into the plasma is very high. The simulations on conversion efficiency performed with single ray tracing, show that angular deviations of ±1 degree , in either vertical or horizontal direction, with respect to the optimal injection, implies a 50% drop in the couplingThe work presented in this PhD Thesis has been performed in the framework of research on Thermonuclear Fusion in magnetically confined plasmas of tokamak-like devices. The effort is aimed at a feasibility study of low-field side Electron Cyclotron Resonance Heating (ECRH) in overdense plasmas. The main aim of this Thesis consists in the study of the applicability of the mode conversion scheme, known as ”O-X-B Double Mode Conversion”, to the Italian tokamak FTU (Frascati Tokamak Upgrade), with the use of millimeter-waves at the 140 GHz frequency. This overdense plasma heating technique, not yet demonstrated at electron density of 2.4 ·10^20 m^−3 and consequently at such a high frequency, exploits the conversion of an ordinary polarized wave (O) into the extraordinary (X) one, which can occur only for radiation propagating in a very narrow angular range at the cutoff region, followed by a subsequent conversion to Bernstein (B) waves, which are then absorbed by the plasma. Simulations have been performed, by using a ray tracing code, to find the optimal launch- ing conditions for the O-X coupling in FTU. The assessment of conversion efficiency was carried out first with the use of one-dimensional models, that describe the density and the magnetic field gradients of the plasma. Moreover, the effects predicted by recent bi-dimensional theoretical models available in literature have been evaluated. The inho- mogeneities of a toroidal plasma are thus accounted with a more realistic description. The experimental part of the work for the Thesis can be divided into two main activities. The first one has been carried out at the laboratories of the research center ENEA in Frascati (Roma), where the tokamak FTU is operating. In this phase, experiments have been performed, aimed at the detailed study of the density profiles and gradients, which characterize the overdense plasma regimes. Proper experimental procedures have been developed, to prepare with reliability the optimal plasma ’target’. The second experimen- tal activity consists in the contribution given for the design of a new EC millimeter-waves launcher for FTU, whose installation is scheduled for the first months of 2011. The system has been designed to reach the launching angles requested for O-X-B mode conversion, which have been defined in the present work and that are not achievable with the present launching system. The results of the predictive work confirm that the requested precision in the injection of the wave into the plasma is very high. The simulations on conversion efficiency performed with single ray tracing, show that angular deviations of ±1◦ , in either vertical or horizon- tal direction, with respect to the optimal injection, implies a 50% drop in the coupling efficiency. Moreover, models which account for the shape of the incident beam, show that the maximum reachable efficiency under optimal wave injection does not exceed 40-45%, depending on the model considered. Thus, the development of a control system operating in real-time and in feedback on the plasma parameters, turns out to be important, in order to perform an overdense plasma heating with acceptable efficiency. The best reproducible plasma has been experimentally defined in FTU, with 5.2 T of central magnetic field and 500 kA of plasma current. The first tests on the operation of both the new launching system and its steering control show a good agreement with the design specifications, in particular with the ones needed to perform first experiments on mode conversion in FTU.