Study of fast-ion-driven toroidal Alfvén eigenmodes impacting on the global confinement in TCV L-mode plasmas

Following recent observations of unstable Toroidal Alfvén Eigenmodes (TAEs) in a counter-current Neutral Beam Injection (NBI) scenario developed in TCV, an in-depth analysis of the impact of such modes on the global confinement and performance is carried out. The study shows experimental evidence of non-degradation of ion thermal confinement despite the increasing of auxiliary power. During such an improved confinement period, Toroidal Alfvén Eigenmodes (TAEs) driven by fast ions generated through Neutral Beam Injection (NBI) are found unstable. Together with the TAEs, various instabilities associated with the injection of the fast neutrals are observed by multiple diagnostics, and a first characterization is given. Nonlinear wave-wave couplings are also detected through multi-mode analysis, revealing a complex picture of the stability dynamics of the TCV scenario at hand. The measurements provided by a short-pulse reflectometer corroborate the identification and radial localization of the instabilities. A preliminary, but not conclusive, analysis of the impact of TAEs on the amplitude of the electron density fluctuations is carried out. Local flux-tube gyrokinetic simulations show that the dominant underlying instabilities in the absence of fast ions are Trapped Electron Modes (TEM), and that these modes are effectively suppressed by zonal flows. Attempts to simulate the simultaneous presence of fast-ion driven TAEs and TEM turbulence show that elongated streamers develop up to the full radial extent of the flux-tube domain, thereby invalidating the local assumption and indicating that a global approach is mandatory in these TCV plasmas

Overview of the TCV tokamak experimental programme

The tokamak a configuration variable (TCV) continues to leverage its unique shaping capabilities, flexible heating systems and modern control system to address critical issues in preparation for ITER and a fusion power plant. For the 2019-20 campaign its configurational flexibility has been enhanced with the installation of removable divertor gas baffles, its diagnostic capabilities with an extensive set of upgrades and its heating systems with new dual frequency gyrotrons. The gas baffles reduce coupling between the divertor and the main chamber and allow for detailed investigations on the role of fuelling in general and, together with upgraded boundary diagnostics, test divertor and edge models in particular. The increased heating capabilities broaden the operational regime to include T (e)/T (i) similar to 1 and have stimulated refocussing studies from L-mode to H-mode across a range of research topics. ITER baseline parameters were reached in type-I ELMy H-modes and alternative regimes with \u27small\u27 (or no) ELMs explored. Most prominently, negative triangularity was investigated in detail and confirmed as an attractive scenario with H-mode level core confinement but an L-mode edge. Emphasis was also placed on control, where an increased number of observers, actuators and control solutions became available and are now integrated into a generic control framework as will be needed in future devices. The quantity and quality of results of the 2019-20 TCV campaign are a testament to its successful integration within the European research effort alongside a vibrant domestic programme and international collaborations

A theoretical investigation of the turbulent structures tilting measurements with radial correlation Doppler reflectometry

International audienceA new technique for measuring the tilting of the turbulent structures is studied analytically within the framework of Born approximation. Probing with O-mode, linear density profile and slab geometry are considered. Analytical expressions are derived for scattering signal and cross-correlation function of scattering signals valid for different probing frequencies. Comparison with previously used "intuitive model" is made, discrepancies pointed out and a more general expression for narrow probing beam is derived. General conclusions about the technique are made and a possible alternative is suggested. Analytical results are validated with linear numerical simulations employing reciprocity theorem, importance of poorly localized small-angle scattering is confirmed

Nonlinear Doppler reflectometry power response. Analytical predictions and full--wave modelling

International audienceThe transition of Doppler reflectometry diagnostic to nonlinear regime of scattering is studied analytically with the use of the perturbation theory applied to Helmholtz equation. Density fluctuations with large radial correlation length, O-polarization of the probing beam, linear background density profile and slab geometry are considered. Formulae for onset of nonlinearity are derived for different parameter ranges. Obtained results are generalized for the arbitrary turbulence radial correlation length. The results of physical optics model are reproduced, while new formulae are obtained for different parameter ranges and their experimental relevance is demonstrated. Qualitative explanation for scattered signal nonlinear power dependence is proposed. Analytical results are validated with IPF-FD3D code full-wave numerical modelling for radially uniform and fully 2D turbulence.

Validation of full-f global gyrokinetic modeling results against the FT-2 tokamak Doppler reflectometry data using synthetic diagnostics

International audienceTwo versions of the X-mode Doppler reflectometry (DR) synthetic diagnostics are developed within the framework of the ELMFIRE global gyrokinetic modeling of the FT-2 tokamak ohmic discharge. In the 'fast' version the DR signal is computed in the linear theory approximation using the reciprocity theorem, utilizing the probing wave field pattern provided by computation and taking into account the 2D plasma inhomogeneity effects; whereas the alternative 'slow' version DR synthetic diagnostic is based on the full-wave code IPF-FD3D describing the probing and scattered wave propagation in turbulent plasma. The DR signal frequency spectra and the dependence of their frequency shift, width and shape on the probing antenna position are computed and shown to be similar to those measured in the high-field side probing DR experiment at the FT-2 tokamak. The geodesic acoustic mode characteristics provided by the measurements and by the synthetic DR are close within a 12% accuracy. However, a substantial difference was found in the decay of the DR signal cross-correlation functions with growing frequency shift in the probing wave channels. The quick decrease in the radial correlation DR coherence observed in the experiment and full-wave synthetic diagnostic, compared to the fast synthetic DR, is attributed to the nonlinear effect of the probing wave phase modulation by the turbulence in the former two cases. The variation in the DR signal at a growing incidence angle in the experiment is also shown to be slower than predicted by both of the synthetic diagnostics, presumably due to underestimation of the probing wave phase modulation and consequent nonlinear saturation of the DR signal at lower incidence angles in modeling