Search

The amplification of a turbulence driven seed magnetic island by the bootstrap current is investigated numerically in a reduced magnetohydrodynamic model as a possible additional channel for the excitation and development of a neoclassical tearing mode (NTM) in a tokamak. The novel mechanism for such an excitation involves a two-step process, namely, the formation of a seed island due to a nonlinear beating of interchange turbulence modes followed by an amplification of the seed island by the bootstrap current leading to a large saturated island. Our study indicates that the level of turbulence controls both the critical seed island size and the saturated island size.

International audience; This paper presents an overview of recent advances in modeling and measurements of turbulence in Tore Supra plasmas. Spatio‐temporal properties of turbulence are discussed in relation with transport models. An explanation is proposed for the k−3 decay of wave number spectra, which are observed by using both coherent laser scattering and Doppler reflectometry. Also the interplay between turbulence and flows has been investigated. It is shown that toroidal rotation results from a competition between neoclassical and turbulence flow generation. Finally a new method has been proposed to determine the transport coefficients of impurities. These coefficients are usually well above the neoclassical prediction.

Impurity transport in tokamak core plasmas is investigated with a three-dimensional fluid global code. It is shown that, in the presence of an internal transport barrier (ITB) created by a reversed magnetic shear configuration, one can obtain a reversal of the impurity pinch velocity which can change from the inward direction to the outward direction. This scenario is favorable for expelling impurities from the central region and decontaminating the core plasma. The mechanism of pinch reversal is attributed to a change of direction of the curvature pinch and to a modification of the dominant underlying instability caused by a change of the gradient of the ion temperature and consequently of the ITB formation.

Impurity accumulation has been observed in many tokamak plasma experiments and predicted by collisional transport theory. However in most experimental cases observed transport departs from the collisional predictions ('anomalous transport'), admittedly due to turbulent effects. Diffusion is used as a criterion to assess the relative contributions of collisions and turbulence to observed impurity transport in the published literature. In the ITER relevant confinement modes (H-mode and internal transport barrier scenarios) turbulence always contributes but with large variations. The predicted parametric dependences of impurity transport are reviewed when available. Information on turbulent diffusion is scarce. Predicted collisional and turbulent convection velocities can be directed either inwards or outwards. The collisional predictions match satisfactorily a number of observations. Theoretical predictions of the turbulent convection velocity including recent quasilinear gyrokinetic results are in qualitative agreement with a dedicated experiment. This is only a first step toward a complete validation of the turbulent impurity convection theoretical models and predictive modelling of impurity transport in tokamaks.

This work presents a comparative study of the experimental observation of MHD instabilities identified as electron fishbone-like modes that appear in plasmas with lower hybrid current drive in FTU and Tore Supra tokamaks. Initially, the mode-induced electronic temperature oscillations measured by electron cyclotron emission were used to study the evolutions of the frequency and position of these modes. In FTU, where fishbones with and without bursting behaviour are observed, it was found that the evolutions of the mode position and frequency follow opposite trend in the two regimes. In Tore Supra, where abrupt changes between modes with different mode structures are often observed, it was found that the mode position evolves continuously and the wavenumbers follow an inverse cascade starting from an m/n = 4/4 mode and finishing in an 1/1 mode. In a second step, the energy of resonant electrons was estimated from the resonant condition of the precession drift frequency. It was found that in Tore Supra the resonant condition does not change during the frequency jumps. The relevance of the correction due to the pitch angle of the resonant electrons and the possible role of the energetic passing particles in the drive of these modes are discussed from the results obtained in both machines.

Tore Supra ECE diagnostic has been recently upgraded to study MHD modes driven by energetic particles up to 400 kHz. To improve the measurement sensitivity, the ECE signals of the 32 channels radiometer were amplified just below the saturation limit and sources of noise were investigated in order to keep it as low as possible. With such an improvement, fast particle driven modes with frequencies up to 200 kHz were detected. A 4-channel correlation ECE system using YIG filters with tuneable frequency was also installed. It allows fine radial scans of MHD modes and correlation length measurements. For the two kinds of YIG filter in use, the minimum frequency separation between two ECE channels that could be achieved was established measuring the correlation coefficient between the respective radiation noises. Finally, by modelling the ECE radiometer taking into account the antenna radiation pattern and the vertical position of the ECE beam relative to the plasma centre we improved the data analysis tools, thus giving a better determination of the phase radial structure of ECE oscillations. The poloidal structure of MHD modes can then be identified from ECE data and, for off axis ECE lines of sight, the direction of the plasma rotation can also be determined. This method allows identifying the occurrence of an inverse cascade of electron fishbone modes ranging from m/n=4/4 to 1/1 (m and n are the poloidal and toroidal mode numbers, respectively) which appears in lower hybrid current drive plasmas.

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.

Impurity transport in tokamak core plasmas is investigated with a three-dimensional global fluid code. The diffusion coefficient and the pinch velocity of impurity transport in tokamaks are studied using the fluid model for ion temperature gradient and trapped electron mode driven turbulence in tokamak plasmas. It is shown that in the presence of an internal transport barrier created by a reversed magnetic shear configuration or external E×B shear flow, a reversal of impurity pinch velocity is obtained, which changes from inward direction to outward direction. This scenario is favorable for expelling impurities from the central region and decontaminating the core plasma. The pinch reversal is attributed to a change of sign of the curvature pinch velocity. This modification is mostly due to the reversal of magnetic shear for the hollow q profile. When a strong E×B shear flow is externally imposed, it is rather due to a change of the turbulence mean phase velocity.

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.

Impurity transport in tokamak plasmas is studied with a fluid turbulence code, which has been upgraded to implement two ion species and electrons. The (fixed-flux) simulations are compared to the predictions of a quasilinear model. These simulations mostly agree with quasilinear estimates; they indicate that a turbulent impurity pinch exists. Moreover, this pinch is found to be dominated by curvature terms, as thermodiffusion pinches are found to decrease as 1∕Z and observed parallel velocity effects remain weak. The sign of the pinch is also investigated.

International audience; A 6-field reduced-MHD model is derived for plasma dynamics. The new model describes coherently both Ion Temperature Gradient mode and Tearing mode, and includes neoclassical effects. The model allows the construction of an energy-like quantity with a linear pressure contribution that is conserved except for dissipative, finite Larmor radius and neoclassical terms. This model may be used to study the nonlinear interaction between ITG microturbulence and neoclassical tearing mode, which is responsible for large-scale magnetic islands in tokamaks, and opens the way to a coherent description of turbulent impurity transport in magnetic islands.