Experimental and theoretical study of particle transport in the TCV tokamak

The main scope of this thesis work is to compare theoretical models with experimental observations on particle transport in particular regimes of plasma operation from the Tokamak à Configuration Variable (TCV) located at CRPP–EPFL in Lausanne. We introduce the main topics in Tokamak fusion research and the challenging problems in the first Chapter. A particular attention is devoted to the modelling of heat and particle transport. In the second Chapter the experimental part is presented, including an overview of TCV capabilities, a brief review of the relevant diagnostic systems, and a discussion of the numerical tools used to analyze the experimental data. In addition, the numerical codes that are used to interpret the experimental data and to compare them with theoretical predictions are introduced. The third Chapter deals with the problem of understanding the mechanisms that regulate the transport of energy in TCV plasmas, in particular in the electron Internal Transport Barrier (eITB) scenario. A radial transport code, integrated with an external module for the calculation of the turbulence-induced transport coefficients, is employed to reproduce the experimental scenario and to understand the physics at play. It is shown how the sustainment of an improved confinement regime is linked to the presence of a reversed safety factor profile. The improvement of confinement in the eITB regime is visible in the energy channel and in the particle channel as well. The density profile shows strong correlation with the temperature profile and has a large local logarithmic gradient. This is an important result obtained from the TCV eITB scenario analysis and is presented in the fourth Chapter. In the same chapter we present the estimate of the particle diffusion and convection coefficients obtained from density transient experiments performed in the eITB scenario. The theoretical understanding of the strong correlation between density and temperature observed in the eITB is detailed in the fifth Chapter. Being the main topic of this work, it is given more space to introduce the basic theory and to compare the simulation results with the experimental data. Impurity transport constitutes the topic of the sixth Chapter, where we demonstrate the physical mechanisms that can sustain a peaked carbon density profile in TCV L-mode plasmas. Finally, the seventh Chapter summarizes the work done with conclusions and a discussion of the possibilities to further improve the results

Plasma Turbulence studied by means of Correlation-ECE in the TEM domain in TCV

Plasma Turbulence studied by means of Correlation-ECE in the TEM domain in TCV Electron temperature fluctuations in the TEM domain have been measured in TCV using the correlation-ECE diagnostics [1]. Significant broadband electron temperature fluctuations are found radially extending between 0.3<ρ<0.7 on the equatorial LFS. Their amplitude decreases with collisionality (with increasing density in OH discharges), in qualitative agreement with predictions from local, linear gyrokinetic GS2 calculations. Thus the mixing length heat diffusivity calculated from GS2 decreases with collisionality, as does the measured heat diffusivity from power balance [2]. This diagnostics allows now the study at the microscopic, turbulence level, of the previously found heat transport triangularity scaling [3], linked to changes of the resonance of TE with the TEM [4]. The decrease of transport when going from positive to negative triangularity found in TCV L-mode can now be investigated and compared to gyrokinetic predictions of instabilities or turbulence (linear/non-linear, local/global). TEM features, like the orientation of the potential cells, predicted to change with plasma shape, up/down-asymmetries, can now be measured through the correlation lengths along a horizontal line of sight, or obliquely, using a mobile mirror arrangement (ECRH launcher in reception mode). [1] V.S. Udintsev et al., Fusion Science and Technology 52 (2007) 161. [2] V.S. Udintsev, E. Fable et al., in preparation. [3] Y. Camenen et al. Nucl., Fusion 47 (2007) 510. [4] A. Marinoni et al., Plasma Phys. Control. Fusion 51 (2009) 055016. 1 present address ITER-IO, Cadarache, St Paul-lez-Durance, F 2 present address CFSA, Dept of Physics, University of Warwick, UK This work was supported in part by the Swiss National Science Foundation

Transport and turbulence reduction with negative triangularity : Correlation ECE measurements in TCV

Turbulence and Transport Reduction with Negative Triangularity : Correlation ECE Measurements in TCV Due to turbulence, core energy transport in fusion devices such as tokamaks generally exceeds collisional transport by at least an order of magnitude. It is therefore crucial to understand the instabilities driving the turbulent state and to find ways to control them. Plasma shape is one of these fundamental tools. In low collisionality plasmas, such as in a reactor, changing the plasma shape from Dee-shape to inverse Dee-shape (from positive to negative triangularity δ) reduces the energy transport by a factor two: the heat flux necessary to sustain the same profiles and stored energy in a discharge with δ=-0.4 is only half of that at δ=+0.4. This is significant, since it opens the possibility of having Hmode-like confinement time within an L-mode edge; or at least with smaller ELMs. Recent correlation ECE measurements show that this reduction of transport at negative δ is reflected in a reduction by a factor of two of both 1) the amplitude of temperature fluctuations in the broadband frequency range 30-150 kHz, and 2) the fluctuation correlation length, measured at mid-radius (ρv~0.6). In addition, the fluctuations amplitude is reduced with increasing collisionality, consistent with theoretical estimates of the collisionality effect on Trapped Electron Modes (TEM). The correlation ECE results are compared to gyrokinetic code results: 1) global linear gyrokinetic simulations (LORB) have predicted shorter radial TEM wavelength λ⊥ for negative triangularity plasmas, consistent with the shorter radial turbulence correlation length λc observed. 2) At least close to the strongly shaped plasma boundary, local nonlinear gyrokinetic simulations with the GS2 code predict that the TEM induced transport decreases with decreasing triangularity and increasing collisionality, in fair agreement with the experimental observations. 3) Calculations are now extended to global nonlinear simulations (ORB5). This work was supported in part by the Swiss National Science Foundatio

Exploring fusion-reactor physics with high-power electron cyclotron resonance heating on ASDEX Upgrade

The electron cyclotron resonance heating (ECRH) system of the ASDEX Upgrade tokomak has been upgraded over the last 15 years from a 2MW, 2 s, 140 GHz system to an 8MW, 10 s, dual frequency system (105/140 GHz). The power exceeds the L/H power threshold by at least a factor of two, even for high densities, and roughly equals the installed ion cyclotron range of frequencies power. The power of both wave heating systems together (>10MW in the plasma) is about half of the available neutral beam injection (NBI) power, allowing significant variations of torque input, of the shape of the heating profile and of Qe/Qi, even at high heating power. For applications at a low magnetic field an X3-heating scheme is routinely in use. Such a scenario is now also forseen for ITER to study the first H-modes at one third of the full field. This versatile system allows one to address important issues fundamental to a fusion reactor: H-mode operation with dominant electron heating, accessing low collisionalities in full metal devices (also related to suppression of edge localized modes with resonant magnetic perturbations), influence of Te/Ti and rotational shear on transport, and dependence of impurity accumulation on heating profiles. Experiments on all these subjects have been carried out over the last few years and will be presented in this contribution. The adjustable localized current drive capability of ECRH allows dedicated variations of the shape of the q-profile and the study of their influence on non-inductive tokamak operation (so far at q$_{95}$>5.3). The ultimate goal of these experiments is to use the experimental findings to refine theoretical models such that they allow a reliable design of operational schemes for reactor size devices. In this respect, recent studies comparing a quasi-linear approach (TGLF) with fully non-linear modeling (GENE) of non-inductive high-beta plasmas will be reported

Verification of the equilibrium and MHD stability codes within the Integrated Tokamak Modeling Task Force framework.

Validation of the numerical tools used for modeling of the fusion plasma is an important step in the interpretation of experimental results. As several codes are usually used in the fusion community to model the same plasma processes, prior verification of the codes should be done in order to avoid discrepancies in the results treatment. Often numerical codes use different post- and pre-processing routines, coordinate system conventions, etc. These make such comparison complicated.One of the main efforts within the Integrated Tokamak Modeling Task Force (ITM TF) project is the verification and validation of the existing numerical tools on the existing tokamak experiments. The framework is developed in ITM that provides common standard interfaces for accessing, storing and exchanging data. All codes integrated in ITM framework use this common interface which makes verification process straightforward. Analysis of the plasma equilibrium and MHD stability is one of the subjects covered by the ITM. Several equilibrium and MHD stability codes are integrated in the ITM framework presently. Initial verification of the fixed-boundary equilibrium codes CHEASE, HELENA, SPIDER was performed using Tore Supra equilibrium reconstructed with EQUAL code. The compared profiles used as input to the MHD stability and transport codes are in agreement. In this work reconstruction of the JET equilibrium will be used for further verification of the equilibrium codes. Equilibrium unstable to the internal or external kink modes will be used for the verification of the MHD stability codes MARS, MARS-F, KINX. ITM tools will be highlighted available in the area of the equilibrium reconstruction and MHD stability analysis

Inward thermodiffusive particle pinch in electron internal transport barriers in TCV

Electron internal transport barriers (eITBs) are obtained in TCV with different heating and current drive schemes. They are sustained in steady-state conditions for several energy confinement and current redistribution times. In these scenarios, the density profile displays a different behaviour with respect to normal L-mode plasmas, with or without auxiliary heating. In fully non-inductive discharges developing an eITB, the density profile shape is strongly correlated with the electron temperature profile, with the normalized density gradient equal to 0.45 times the normalized temperature gradient, revealing the existence of a significant inward pinch of a thermodiffusive type. The coupling of the two profiles is observed from the 'foot' of the barrier inwards. The effect of small inductive current perturbations on fully non-inductive sustained eITBs shows that ∇n/n is only indirectly coupled to the current profile, through its effect on local confinement, contrary to the standard L-mode