Blob motion and control in simple magnetized plasmas

Intermittent convective transport caused by coherent structures, or blobs, are universally observed in the edge of laboratory plasmas. Besides being of fundamental physics interest, the dynamics of these structures in fusion reactors influence the density scale-length in the scrape-off layer, its impurity screening characteristics, wall-recycling and possibly the overall confinement properties. In TORPEX simple magnetized plasmas, blobs are generated from interchange turbulence and, driven by gradB and curvature-induced charge separation, propagate radially outwards. The magnitude of their velocity depends on the current paths to damp charge separation. Regimes dominated by either parallel or cross-field currents are achieved by varying the ion mass. An analytical expression for the blob velocity including cross-field ion polarization currents, cross-field ion currents due to neutral friction and parallel currents to the sheath is derived and shows good quantitative agreement with the experimental data. To confirm this interpretation, direct measurements of the 2D structure of the blob-induced parallel currents have been obtained using magnetic probes. Parallel blob dynamics are further studied with a Mach probe, revealing the convection of parallel momentum by blobs. Methods to influence and control blob motion are also being explored, such as the variation of the connection length or the use of poloidal arrays of biased electrodes. This study is part of a more general project of code validation on TORPEX. Methodology and results of a comparison of 2D and global 3D fluid simulations with experiments will be presented

Fast Imaging of Turbulent Plasmas in the TORPEX Device

This thesis addresses the issue of turbulence generated modes and intermittent structures using a fast visible imaging system on TORPEX. The plasma radiation mechanisms and experimental setup of the imaging system are discussed together with an optimized tomographic reconstruction method. In TORPEX typical plasmas, radiative ionization is the dominant radiation mechanism. The photon flux emitted by hydrogen plasmas is a linear function of √Te and ne for Te ≥ 4eV. A Photron Ultima APX-RS fast visible camera was used to acquire the light emission of TORPEX plasmas. The camera equipped with the image intensifier is able to image TORPEX plasmas up to 200 kframes/s of framing rate and down to 1µs of exposure time, which results in clear images of plasma structures. The time-resolved poloidal emissivity of TORPEX plasma emission is tomographically reconstructed from tangentially viewed images using a pixel method and singular value decomposition approach. The spatial resolution in the reconstructed emissivity profiles, is 2cm. The plasma emissivity and ion saturation current profiles are compared using the conditional average sampling (CAS) technique and statistical analysis of fluctuations. The CAS is performed to visualize the mode structures and to estimate their sizes. The resulting fast imaging of the plasma, non-perturbative and high spatio-temporal resolution diagnostics, can visualize small scale turbulent plasma structures, well beyond the typical spatial resolution of probe arrays. A two-dimensional gas puff (2D-GPI) imaging system, including a 2D movable gas puff nozzle and a fast framing camera, has been developed in the TORPEX device. Direct local measurements from a large number of probes, distributed across the plasma column, are used to establish a correlation between the plasma parameters and the locally measured visible light emission. GPI and ion saturation current signals are well correlated, with correlation coefficients higher than 0.75. Fourier analysis of the GPI and internal probes show that the GPI can detect the dominant interchange mode. Quasi-coherent interchange modes and intermittent, turbulence generated blobs can be detected and studied using fast visible imaging in TORPEX. For an increasing vertical magnetic field, the principal interchange mode is damped and a sharp transition occurs between low-k and high-k harmonics. The scaling of the radial width of different interchange harmonics with respect to the vertical wave-number (kz) shows good agreement between the experimental and theoretical radial-width-kz scaling in TORPEX plasmas. The gradient-removal mechanism, describing the level of turbulent fluctuations, was tested using experimental data from the visible camera. The saturation level of the interchange instabilities predicted by the gradient-removal mechanism is in reasonable agreement with the experimental measurements. Tomographically inverted data from the fast camera were used to study the propagation of intermittent blobs. The experimental speed-versus-size scaling of blobs in the camera data is in good agreement with theoretical predictions. This thesis confirms the possibility of using fast imaging to estimate saturation level of fluctuations associated with ideal interchange instability and to reconstruct the blob speed-size scaling law in tokamak plasmas non-perturbatively. This can be used to non-perturbatively estimate the transport levels in the SOL of fusion devices