Agostini, Matteo (2008) Study of edge turbulence in magnetically confined fusion plasmas. [Ph.D. thesis]
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Understanding turbulent transport in magnetised plasmas is a subject of great importance for comprehending and optimising experiments and for designing a future fusion rector. In fact turbulence has since long been found to be the dominant reason for the observed limited confinement especially in the edge of fusion devices.
The edge of all fusion experiments, reversed field pinches, tokamaks and stellerators, is characterised by the flow of plasma toward material surfaces and by the outflow of heat and particles from the plasma core through transport. In particular this transport cannot be explained in the framework of classical theory, and understanding its underlying physics remains the outstanding critical issue of thermonuclear fusion physics. Nowadays it is commonly recognised that the main cause of this anomalous transport in the plasma edge is due to turbulence, both electrostatic and magnetic, and many progresses in its understanding have been done experimentally using electrostatic Langmuir probes and optical diagnostics like the gas puff imaging, and theoretically by the development of simulation codes.
Experimental measurements have pointed out that most of the outward transport in the edge is due to coherent structures such as blobs ejected from the edge.
In this thesis the characterisation of the edge turbulence in fusion experiments is carried out mainly by means of the optical diagnostic called gas puff imaging (GPI). Also Langmuir probes and fast magnetic pick-up coils are used. I studied the spectral and statistical properties of the turbulent signals, showing their universal behaviour independently from the magnetic configuration, indicating that the responsible mechanism should be common. The edge fluctuations are not self similar, i.e. their statistical properties depend on the time scale of the fluctuations themselves.
By comparing the data from the gas puff imaging optical diagnostic and the ones from Langmuir probes, the structure of density-potential blobs is characterised. Then, using the new gas puff imaging diagnostic, through the tomographic algorithm I have developed, I have directly shown the presence in the edge of RFX-mod device of coherent structures propagating along the toroidal direction and responsible for the greatest part of the anomalous transport. The link between these blobs and high frequency magnetic fluctuations can be explained if the edge structures are related to current filaments extending along the magnetic field lines.
Beyond the interaction with the magnetic field, also the link between the edge turbulence and the kinetic pressure is studied, as its gradient is normally considered one of the sources of free energy for the development of the turbulence. I made this studies in particular on the NSTX tokamak, and the results are compatible with the picture of blobs born near the separatrix (in the region of maximum radial gradient of electron pressure) and moving outward while becoming larger and larger.
In NSTX tokamak, also the relation between the edge turbulence and the transition from L-mode to the better confinement mode (H-mode) has been studied. This transition is always observed to be accompained by the development of a transport barrier in the edge, which is commonly correlated with edge turbulence suppression. With the gas puff imaging diagnostic I observed a drastic reduction in the linear density of edge structures during the transition to the H-mode. Differences between the two plasma regimes are also observed in the perpendicular k-spectrum, which can be interpreted in the L-mode with the presence of an inverse energy cascade toward the small k that is not observed in the H-mode. All these studies highlight the importance of the reduction of the edge turbulence in the development of the edge transport barrier and in the consequent formation of the H-mode.
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