Matteo Fontana
In this thesis work, the flexibility of the Tokamak à Configuration Variable (TCV) is exploited to study the influence of plasma parameters on turbulent fluctuations. The correlation electron cyclotron emission (CECE) diagnostic is used to measure low amplitude, large bandwidth radiative temperature fluctuations, associated with the anomalous transport terms that constitute the largest contribution to heat and particle loss in tokamak plasmas. In a series of L-mode limited plasmas, fluctuations are characterized over a large range of plasma parameters, obtained varying collisionality, Te/Ti and plasma triangularity. The influence of negative triangularity on confinement and fluctuations is also investigated.
Temperature fluctuations profiles have been measured in low density, ohmic, positive and negative triangularity discharges, with matched current and density profiles. Strong suppression of fluctuations is observed in negative triangularity discharges from the plasma edge to the plasma mid radius. This is despite triangularity quickly decreasing when moving away from the edge. The existence of a region of low stiffness at the plasma edge is postulated to be responsible for this. Negative triangularity is confirmed having beneficial effects on confinement and reducing fluctuations amplitude in regimes with high Te/Ti.
The neutral beam injector (NBI) has been used on positive and negative triangularity discharges in order to study the effect of negative triangularity on plasmas where Te/Ti~1. This condition is of particular interest since future reactor-like tokamaks are expected to work with thermalized electrons and ions. A pair of discharges with positive and negative triangularity has been realized, where matched temperature profiles are attained with ~385 kW and ~145 kW of NBI power respectively for the two shapes. This is evidence of negative triangularity improving plasma confinement also in conditions of low Te/Ti. In these discharges, CECE measurements find suppressed fluctuations in the negative triangularity discharges.
Linear, flux tube, gyrokinetic simulations show that, in the same plasmas, the dominant turbulent regime is a mix of ion temperature gradient (ITG) instabilities and trapped electron modes (TEM). This is different with respect to all previous works in TCV, in which no ion heating had been available and only pure TEM regimes were observed. The results of the linear simulations indicate that, also in this mixed turbolence regime, negative triangularity has a stabilizing effect on instabilities, more visible for radial positions closer to the plasma edge, where the magnitude of triangularity is higher.
A database of fluctuations measurements has been constructed from a series of discharges with positive and negative triangularity covering a large range of plasma conditions, particularly focusing on the effects of different combinations of collisionality and the Te/Ti ratio. Collisionality is found to be the main parameter to control the relative suppression of fluctutations in negative triangularity plasmas. No direct effect of Te/Ti on the fluctuation amplitudes is observed in the explored parameter range. Measurements taken in positive and negative triangularity plasmas, with comparable conditions and matched normalized temperature scale lengths still show reduced fluctuation levels for negative triangularity, suggesting influence on the threshold gradients for the onset of fluctuations.
EPFL2018
