Search

In the Tore Supra tokamak, magnetohydrodynamic instabilities can be destabilized by energetic particles produced by radio-frequency heating systems. This paper reports on experimental observations of two types of energetic particle driven internal modes: the electron fishbone and the beta-induced Alfven eigenmodes (BAEs). The evolutions of these modes in frequency and radial position during core relaxation cycles were determined by their induced electronic temperature and density fluctuations. Electron fishbones were observed in the so-called oscillation regime, while BAEs' observations were performed in the sawtooth regime. The results show that the frequency and radial position of these two instabilities vary substantially during the core relaxation cycles. It indicates that an accurate description of these evolutions must consider the equilibrium profile alterations during the core oscillations.

This paper presents recent efforts to better understand and quantify charged particle transport in Hall effect thrusters (HETs). Particle-in-cell (PIC) models, hybrid models, laser induced fluorescence (LIF) measurements and collective scattering (CS) experiments are combined to get a better insight into anomalous electron transport in HETs and to increase the predictive capabilities of simulation codes.PIC models have demonstrated that plasma turbulence associated with the development of a high frequency, short wavelength azimuthal instability can be responsible for anomalous transport. Scaling laws for anomalous electron mobility have not yet been derived and hybrid models, which are more practical than PIC models for parametric studies, must use empirical, adjustable transport coefficients that can be inferred from PIC results or LIF measurements of the ion velocity distribution function. CS experiments are aimed at validating the PIC model predictions of the azimuthal instability. The CS results show the first direct experimental evidence of the azimuthal instability predicted by the PIC code. The paper illustrates the synergy between experiments and models toward a complete and quantitative understanding of the physics of HETs.

Recent numerical works on turbulent impurity transport raise a new interest in experimental studies on impurity transport and more especially a comparison of its parametric dependences with theoretical predictions. In this context, we have studied the behaviour of four different impurity species in Tore Supra lower-hybrid heated, sawtooth-free plasmas. We observe a decrease in the central impurity confinement time ?imp with increasing Z in the range explored (from 13 up to 32). This result is in contradiction to both neoclassical and turbulent theoretical predictions. An analysis with the 1D impurity transport code ITC shows the existence of a central zone with reduced diffusion and indication of an inward convection. These results are completed with theoretical simulations of neoclassical transport (NCLASS) and turbulent transport with the quasilinear gyrokinetic QuaLiKiz and nonlinear gyrofluid TRB models. These simulations do not provide an explanation of the confinement time dependence on the impurity charge. Transport outside r/a = 0.3 is well described by the nonlinear turbulent calculations but the central region is predicted to be below the turbulence threshold while the observed transport is one order of magnitude higher than neoclassical values. The gyrokinetic calculations evidence the dominant role of the curvature pinch in the total convective flux. This term is directed inwards and only weakly dependent on the impurity charge in the explored range of species.