TJ-II stellarator results on modelling and validation of plasma flow asymmetries due to on-surface potential variations, plasma fuelling physics, Alfvén eigenmodes (AEs) control and stability, the interplay between turbulence and neoclassical (NC) mechanisms and liquid metals are reported. Regarding the validation of the neoclassically predicted potential asymmetries, its impact on the radial electric field along the flux surface has been successfully validated against Doppler reflectometry measurements. Research on the physics and modelling of plasma core fuelling with pellets and tracer encapsulated solid pellet injection has shown that, although post-injection particle radial redistributions can be understood qualitatively from NC mechanisms, turbulence and fluctuations are strongly affected during the ablation process. Advanced analysis tools based on transfer entropy have shown that radial electric fields do not only affect the radial turbulence correlation length but are also capable of reducing the propagation of turbulence from the edge into the scrape-off layer. Direct experimental observation of long range correlated structures show that zonal flow structures are ubiquitous in the whole plasma cross-section in the TJ-II stellarator. Alfvénic activity control strategies using ECRH and ECCD as well as the relation between zonal structures and AEs are reported. Finally, the behaviour of liquid metals exposed to hot and cold plasmas in a capillary porous system container was investigated.
In this work, we study spontaneous electron to ion root transitions in TJ-II using Langmuir probes. By scanning the probe position on a shot to shot basis, we reconstruct a spatiotemporal map of the evolution of important turbulent quantities in the plasma edge region. We pay particular attention to the evolution of the cross phase between transport-relevant variables, showing the spatiotemporal evolution of this quantity for the first time, revealing the outward propagation of the changes associated with the transition. We also compute the intermittence parameter, which allows us to conclude that the turbulence, although its amplitude increases, condenses in a reduced number of dominant modes and becomes less bursty. The causal relationship between variables is studied using the transfer entropy, clarifying the interactions between the main variables and offering a rather complete picture of the complex evolution of the plasma across the confinement transition.
Lithium (Li) coating of the inner wall of tokamaks has been considered a practical technique to reduce fuel recycling. Li-coating also improves the tokamak wall condition and reduces high-Z material release from the tokamak wall during discharges. In a recent campaign in the STOR-M tokamak (R/a = 0.46/0.12 m, Bt = 0.7 T, Ip = 25 kA), 100 mg of Li has been coated on the tokamak chamber using a physical vacuum evaporation applicator developed by General Fusion Inc. A reduction in the plasma impurity contents and increase in both the peak plasma current and duration have been observed after Li-coating. The line averaged electron density in the plasma reduced after Li-coating. An increase in hard x-ray radiation has also been observed, suggesting an enhanced production of suprathermal run-away electrons because of the reduced electron density. In addition the Li atom emission line has been used to measure the plasma flow velocity based on the Doppler shift of the emission.
This work explores the impact of an imposed radial electric field on the intermittence parameter in magnetically confined plasmas. The intermittence is sensitive to both the magnetic configuration (dominant helical modes or low order rational surfaces) and to poloidal flows or radial electric fields. This behaviour was verified both in numerical turbulence calculations using a resistive MHD model and using Langmuir probe data obtained in experiments at the TJ-II stellarator. It is shown that the intermittence parameter can be used to detect when the local plasma rotation velocity with respect to the laboratory frame of reference is minimum.
In this work, we perform a rotational transform scan in the TJ-II stellarator, while measuring several turbulent quantities well inside the plasma edge by means of a Langmuir probe and applying a radial electric field to the edge plasma using a biasing probe. By calculating the intermittence parameter from floating potential measurements, we are able to identify a major low order rational surface and hence relate the probe measurements to the local value of the rotational transform. Based on the former, we are able to show that the poloidal plasma velocity (and hence radial electric field) has a significant radial structure that is clearly related to the rotational transform profile and in particular the lowest order rational surfaces in the range studied. The poloidal velocity is also affected by the edge biasing. The particle flux Γ was also found to exhibit a radial pattern, as did the flow shear suppression term ωE×B, but their relation to the low-order rational surfaces was less clear. We surmise that this lack of direct correspondence is due to an unknown term in the turbulence evolution equation: the instability growth rate, γ. We make use of a reduced Magnetohydrodynamic turbulence model to interpret the results. Overall, a picture is obtained in which the plasma self-organizes towards a state with a clear radial pattern of the radial electric field, in line with expectations from some numerical studies describing the spontaneous formation of an ‘E × B staircase’, consistence of alternating layers with fast and slow radial transport. In this state, profiles of various quantities (density, temperature, pressure) will not be smooth.
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