Theory of a cylindrical Langmuir probe parallel to the magnetic field and its calibration with interferometry

International audienceA theory for data interpretation is presented for a cylindrical Langmuir probe in plasma parallel to the magnetic field direction. The theory is tested in a linear low-temperature plasma device Aline, in a capacitive radio-frequency (RF) discharge. The probe is placed on a 3D manipulator and a position scan is performed. To exclude strong RF perturbations the probe is RF compensated. Using the theory electron densities are obtained from the current at the plasma potential, where no sheath is present. Results are calibrated by line-integrated density measurements of a 26.5 GHz microwave interferometer. Reasonable agreement is observed for probe and interferometer measurements. Furthermore, preceding, more general probe theory is compared to the one developed in the current work and the application limits are discussed

Modelling of Backscattering off Filaments Using the Code IPF-FD3D for the Interpretation of Doppler Backscattering Data

Filaments or blobs are well known to strongly contribute to particle and energy losses both in L- and H-mode, making them an important plasma characteristic to investigate. They are plasma structures narrowly localized across a magnetic field and stretched along magnetic field lines. In toroidal devices, their development is observed to take place in the peripheral plasma. Filament characteristics have been studied extensively over the years using various diagnostic techniques. One such diagnostic is the Doppler backscattering (DBS) method employed at the spherical tokamak Globus-M/M2. It has been observed that the DBS signal reacts to the backscattering from filaments. However, the DBS data have proven difficult to analyze, which is why modelling was undertaken using the code IPF-FD3D to understand what kind of information can be extrapolated from the signals. A circular filament was thoroughly investigated in slab geometry with a variety of characteristics studied. Apart from that, the motion of the filaments in the poloidal and radial directions was analyzed. Additionally, other shapes of filaments were presented in this work. Modelling for the real geometry of the Globus-M/M2 tokamak was performed

Edge turbulence effect on ultra-fast swept reflectometry core measurements in tokamak plasmas

International audienceUltra-fast frequency-swept reflectometry (UFSR) enables one to provide information about the turbulence radial wave-number spectrum and perturbation amplitude with good spatial and temporal resolutions. However, a data interpretation of USFR is quiet tricky. An iterative algorithm to solve this inverse problem was used in past works, Gerbaud (2006 Rev. Sci. Instrum. 77 10E928). For a direct solution, a fast 1D Helmholtz solver was used. Two-dimensional effects are strong and should be taken into account during data interpretation. As 2D full-wave codes are still too time consuming for systematic application, fast 2D approaches based on the Born approximation are of prime interest. Such methods gives good results in the case of small turbulence levels. However in tokamak plasmas, edge turbulence is usually very strong and can distort and broaden the probing beam Sysoeva et al (2015 Nucl. Fusion 55 033016). It was shown that this can change reflectometer phase response from the plasma core. Comparison between 2D full wave computation and the simplified Born approximation was done. The approximated method can provide a right spectral shape, but it is unable to describe a change of the spectral amplitude with an edge turbulence level. Computation for the O-mode wave with the linear density profile in the slab geometry and for realistic Tore-Supra density profile, based on the experimental data turbulence amplitude and spectrum, were performed to investigate the role of strong edge turbulence. It is shown that the spectral peak in the signal amplitude variation spectrum which rises with edge turbulence can be a signature of strong edge turbulence. Moreover, computations for misaligned receiving and emitting antennas were performed. It was found that the signal amplitude variation peak changes its position with a receiving antenna poloidal displacement

Modelling of charge-exchange induced NBI losses in the COMPASS Upgrade tokamak

The COMPASS Upgrade tokamak [1] will be a tokamak of major radius R0 = 0.894m with the possibility to reach high field (Bt ~ 5 T) and high current (Ip ~ 2 MA). The machine should see its first plasma in 2023 and H-mode plasma will be obtained from 2025. The main auxiliary heating system used to access H-mode will be 4MW of Neutral Beam Injection (NBI) power. The NBI will have a nominal injection energy of 80 keV, a maximum injection radius Rtan = 0.65m and will create a population of well-confined energetic D ions. In this contribution, our modelling studies the NBI deposition and losses when a significant edge background density of neutrals is assumed. We follow the fast particles in the 3D field generated by the 16 toroidal field (TF) coils using the upgraded EBdyna orbit solver. We have implemented a Coulomb collision operator similar to that of NUBEAM and a charge-exchange operator that follows neutrals and allows for multiple re-ionizations. Detailed integrated modelling with the METIS code yields the pressure and current profiles for various sets of achievable engineering parameters. The FIESTA code calculates the equilibrium and a Biot-Savart solver is used to calculate the intensity of the perturbation induced by the TF coils. Initial distributions of the NBI born fast ions are obtained from the newly developed NUR code, based on [S. Suzuki et al. 1998 Plasma Phys. Control. Fusion 40 2097]. We evolve the NBI ions during the complete thermalization process and we calculate the amount of NBI ions loss in the edge region due to neutralizations. Results indicate the NBI losses for various injection geometries, various engineering parameters and various assumptions on the magnitude of the background neutral density. [1] R. Panek et al. Fusion Engineering and Design 123 (2017) 11–1

Modelling of charge-exchange induced NBI losses in the COMPASS Upgrade tokamak

The COMPASS Upgrade tokamak [1] will be a tokamak of major radius R0 = 0.894m with the possibility to reach high field (Bt ~ 5 T) and high current (Ip ~ 2 MA). The machine should see its first plasma in 2023 and H-mode plasma will be obtained from 2025. The main auxiliary heating system used to access H-mode will be 4MW of Neutral Beam Injection (NBI) power. The NBI will have a nominal injection energy of 80 keV, a maximum injection radius Rtan = 0.65m and will create a population of well-confined energetic D ions. In this contribution, our modelling studies the NBI deposition and losses when a significant edge background density of neutrals is assumed. We follow the fast particles in the 3D field generated by the 16 toroidal field (TF) coils using the upgraded EBdyna orbit solver. We have implemented a Coulomb collision operator similar to that of NUBEAM and a charge-exchange operator that follows neutrals and allows for multiple re-ionizations. Detailed integrated modelling with the METIS code yields the pressure and current profiles for various sets of achievable engineering parameters. The FIESTA code calculates the equilibrium and a Biot-Savart solver is used to calculate the intensity of the perturbation induced by the TF coils. Initial distributions of the NBI born fast ions are obtained from the newly developed NUR code, based on [S. Suzuki et al. 1998 Plasma Phys. Control. Fusion 40 2097]. We evolve the NBI ions during the complete thermalization process and we calculate the amount of NBI ions loss in the edge region due to neutralizations. Results indicate the NBI losses for various injection geometries, various engineering parameters and various assumptions on the magnitude of the background neutral density. [1] R. Panek et al. Fusion Engineering and Design 123 (2017) 11–1

Investigation of nonlinear effects in Doppler reflectometry using full-wave synthetic diagnostics

International audienceIn this work, Doppler reflectometry (DR) and radial correlation DR (RCDR) nonlinear scattering effects were studied using full-wave modeling and a set of representative FT-2 tokamak turbulences as inputs. Narrowing of the RDCR correlation function and widening of the DR poloidal wavenumber spectrum are demonstrated. An effect on the dependence of the DR signal frequency shift on the probing wavenumber is found, namely, this dependence "linearizing" in the nonlinear scattering regime. Nonlinear effects are shown to be weaker for O-mode probing than for X-mode probing, while a faster transition to nonlinear regime is demonstrated for RCDR compared to DR in both probing scenarios

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