A fast-moving Langmuir probe array for the divertor of the Tokamak à Configuration Variable

In next step tokamaks such as ITER and future fusion power plants, wall materials will be subjected to considerably higher heat and particle fluxes than in present-day machines. Maintaining plasma facing components in good condition and simultaneously assuring acceptable core performance will be an outstanding challenge. This situation is further complicated by the fact that extrapolations towards future tokamaks are difficult and the mechanisms governing transport of energy and particles are still being investigated. Proper diagnostic coverage of the boundary plasma of currently operational machines is therefore crucial. In the Tokamak à Configuration Variable (TCV), wall-embedded Langmuir probes (LPs) are one of the most powerful diagnostics to describe local properties in the boundary region of the plasma. The present thesis work includes operation, maintenance and upgrades of this LP system. In particular, the TCV wall probe coverage has been extended from 114 to 194 probes. Furthermore, a new set of amplifiers has been installed, featuring a number of improvements. In parallel, a novel fast probe array, the Reciprocating Divertor Probe Array (RDPA), has been designed, built and commissioned at TCV, providing unprecedented Langmuir probe measurements across the divertor volume. In contrast to reciprocating probes accessing the divertor region of other major tokamaks, the RDPA provides 2D measurements in a single reciprocation, enabled by a radial array of Mach probe tips moved vertically across an extended region of the divertor volume. The RDPA capabilities have allowed for a detailed particle balance study in the outer TCV divertor in attached and detached conditions and both with and without the recently installed divertor baffles. It has been observed that the contribution of the ExB drift to the poloidal ion flux is comparable and sometimes larger than that due to the ion flux parallel to the magnetic field. Except for strongly detached conditions, most of the particle flux arriving at the outer target is shown to be due to ionization along the outer divertor leg, justifying the 'closed-box approximation' frequently used in detachment models and previously inferred on TCV from spectroscopic measurements. ExB drifts also play an important role in terms of heat transport, as found from a divertor heat flux balance study in attached conditions. Together with parallel heat conduction, the ExB-convected heat flux dominates the poloidal heat flux, while parallel heat convection plays a lesser role in the example studied. A new feature of RDPA is also to provide detailed, local measurements of fluctuation characteristics across the divertor volume of TCV, thanks to acquisition frequencies of up to 2MHz. The first findings include the development of a strong poloidal gradient of the density fluctuation level, observed towards the onset of detachment, and the surprising presence of high fluctuation levels in the narrow region of downward ExB (on the high-field side of the peak electron temperature) for well attached discharges. Ongoing efforts focus on using RDPA results to validate the global 3D two-fluid code GBS in diverted TCV simulations and thus helping to improve its predictive capabilities.

Measurement of edge electrostatic turbulence in the TCV tokamak plasma boundary

Jan Horacek

Almost since the first density profile measurements were made in the scrape-off layer (SOL) of the early tokamaks, it has been recognized that the rate of particle transport perpendicular to magnetic surfaces exceeds that expected on the basis of classical collisional diffusion by as much as three orders of magnitude. Plasma turbulence has rightfully been claimed as the origin of such large discrepancies, much as it has for enhanced (over classical or neoclassical) transport rates observed in the confined plasma. But in the SOL, the "bursty" or "blobby" nature of the measured density fluctuations is of a much higher amplitude than that found in the core, making large-scale, convective fluid turbulence a strong candidate mechanism. This thesis will demonstrate quantitatively and unequivocally, for the first time, that such interchange motions are indeed the driver for the edge density and particle flux fluctuations observed on the Tokamak a Configuration Variable (TCV). Since the principle driver of this turbulence is a curved magnetic field, with gradient direction matching that of the local edge plasma pressure profile, together with a region of open magnetic field lines, the interchange mechanism identified here is very likely to be the very same process at the root of transport in all tokamak SOLs. In showing that the measured turbulence driven cross-field particle flux in TCV is quantitatively consistent with interchange physics, a path is opened by which the anomalous transport rates might be estimated in a predictive way for larger tokamaks, like the ITER device, which are yet to be built but for which concerns are now being raised that such transport might lead to excessive plasma-wall interactions. Using a fast reciprocating Langmuir probe, fluctuation measurements have been made in the TCV low-field-side SOL across a wide range of ohmic discharges comprising variations in plasma shape and configuration (limiter and divertor), plasma current, confinement mode (L and H), plasma density, toroidal magnetic field direction and plasma fuel species (deuterium and helium). Statistical analysis of the time series is used to demonstrate a remarkable degree of similarity across the database and to show that the radial dependence of the probability distribution functions (PDFs) of flux and density fluctuations can be well approximated by the known Gamma and Lognormal analytic PDFs, characterized in terms only of the relative fluctuation levels. In the vicinity of the SOL-main chamber interface, where particles interact with the walls, the density fluctuations exhibit clear evidence of self-similarity over two orders of magnitude in frequency and a PDF which is universal in shape. The observed constancy of the correlation between density and poloidal field fluctuations in turn implies a universal PDF for the radial particle flux which moreover is found to scale almost linearly with the local mean density. Careful comparison of one particular case inside the experimental database with the results of 2D fluid turbulence simulations of the TCV SOL using the ESEL code developed at the Risø National Laboratory, Denmark has shown a remarkable level of agreement between theory and experiment when the simulation output time series is analyzed in exactly the same way as that applied to the tokamak data. Quantitative agreement between model and experiment has been found for radial profiles of mean values, fluctuation levels, PDF shapes, timescales and power spectra of both density and turbulent driven flux throughout the main SOL and even partially inside the separatrix. Automatically, this level of agreement also implies that the code output conforms quite closely to the Gamma and Lognormal distributions. Parallel SOL flow data have also been gathered simultaneously with the turbulence measurements. An extensive database of radial Mach flow profiles has been assembled, most notably including a direct comparison of the density dependence of the flow dynamics in carefully matched discharges with forward and reversed toroidal field. These constitute the first measurements of their kind in TCV and reveal the presence of very strong flows, up to Mach numbers of ∼ 0.6. The magnitude and direction of the measured flows is found to be surprisingly well described by neoclassical return parallel flows (Pfirsch-Schlüter) compensating the poloidal ExB and diamagnetic drifts. Combining the forward and reversed field data uncovers a slight, field independent offset thought to originate from the excess transport, driven by the interchange motions, in the outboard midplane vicinity. The flow and fluctuation data have been combined to test the possible link between flow generation and turbulence first demonstrated from similar data on JET. No such correlations have been found on TCV throughout most of the SOL, supporting the finding that the neoclassical component can account for the majority of the measured parallel flow.

EPFL2006

Étude du transport d'énergie thermique dans les plasmas du tokamak à configuration variable au moyen de chauffage électronique cyclotronique

Yann Camenen

The development of controlled thermonuclear fusion, a quasi-unlimited energy source suitable for large scale electricity production, is one of the main goals of plasma physics research. Among the directions explored to date, the use of toroidal devices called tokamaks to create and confine hot plasmas using strong magnetic fields is particularly promising. The energy used to heat the plasma must remain well confined in order to achieve plasma temperatures higher than one hundred millon degrees for a sufficiently long period to obtain numerous fusion reactions. In the frame of efficient electricity production, maximising the energy confinement is also essential to achieve the required temperature with the lowest heating power. In tokamak plasmas, energy losses are mainly due to radiation and radial energy transport from the plasma core to the edge. A significant fraction of plasma physics research is therefore dedicated to the study of radial transport in tokamaks and the exploration of new operation regimes characterised by a low transport level. The development and optimisation of diagnostics used to observe plasmas is also part of this work. This thesis work, performed on the Tokamak à Configuration Variable (TCV) in Lausanne, covers the implementation and exploitation of a multi-channel soft X-ray detector with high spatial and temporal resolution, together with the development of the tomographic inversion routines used for data analysis. The detector, comprised of two superposed wire chambers, has been tested and calibrated using an X-ray source and then installed onto the tokamak. The position of the detector was chosen such as to observe the whole plasma cross-section with maximum spatial resolution leading to high quality tomographic inversions. A mobile absorber holder was installed between the plasma and the wire chambers. The energy range of the soft X-ray emission observed by the detector was thus chosen by selecting the appropriate absorber. These various features have made possible the use of the detector for numerous studies and in particular for the spatial and temporal characterisation of the plasma internal transport barrier formation. Plasma shaping abilities covering a wide range of plasma elongations and triangularities, including negative values, are one of the strengths of the TCV tokamak. For instance, plasmas with elongated cross-sections offer higher energy confinement as well as higher plasma current and pressure limits. However, the increase of the plasma vertical instability growth rate with elongation makes the vertical control of elongated plasmas difficult, in particular if the plasma current profile is too peaked. As the current profile is usually peaked for low plasma currents, current profile broadening is required there to achieve high elongation. During this thesis, a current profile broadening method based on temperature profile modification by localised EC heating has been studied in detail. The mechanism of this method has been documented and the optimal conditions for the EC power deposition determined. Using these conditions, the TCV operational space has been extended towards higher elongation at low current. The highest elongation obtained at low current has been increased by over 25% permitting the exploration of the plasma transport properties in this regime. The flexibility of the TCV EC heating system has also been used to investigate radial electron heat transport in L-mode plasmas. For the first time, the normalised temperature gradient has been varied by a factor of four and its influence on electron heat transport has been separated from that of the electron temperature. Electron heat transport increases strongly with the normalised temperature gradient, for values between 6 and 10, and then becomes independent of this parameter. In addition, electron heat transport increases with increasing electron temperature, decreasing density and increasing effective charge. The electron heat transport dependence on these three parameters can be cast as a single dependence on the plasma collisionality. TCV shaping abilities have then been used to test the influence of plasma triangularity. The main variations of the level of electron heat transport are described by a decrease of the electron heat diffusivity towards negative triangularity and high collisionality. At constant collisionality, electron heat transport is two times lower at a negative triangularity of –0.4 than at a positive triangularity of +0.4. Concerning micro-instabilities, gyro-fluid and gyro-kinetic simulations indicate that TEM and ITG instabilities are at play in these plasmas. The good qualitative agreement between the observed experimental dependencies and the predictions of simulations suggests strongly that the TEM instabilities are involved in the transport of electron heat. The experimental study provides dependable scaling of the electron heat transport on plasma parameters that can now be used to test the prediction of transport simulations. New elements such as the saturation of electron heat transport at high values of the normalised temperature gradient and the decrease of electron heat transport towards negative triangularities have been demonstrated.

EPFL2006

Infrared measurements of the heat flux spreading under variable divertor geometries in TCV

Basil Duval, Benoît Labit, Roberto Maurizio, Federico Nespoli, Holger Reimerdes, Umar Sheikh, Christian Gabriel Theiler

The safe and stable operation of a future fusion reactor depends critically on the ability to control the heat loads on the material surfaces facing the plasma. The heat fluxes are particularly high at the strike points in the divertor, where the plasma interacts directly with the wall. By varying the divertor geometry it is possible to increase the power radiated or transferred to neutrals and to spatially extend the scrape-off layer (SOL), with the common goal of distributing the total power over a greater surface. In a diverted plasma, the heat flux profile at the divertor strike points is largely determined by three competing mechanisms: (I) transport of heat along the field lines, (II) cross-field transport in the SOL region (with the LCFS as a source of heat), (III) cross-field transport both in the SOL and in the private flux region (without source). Mechanisms II and III spread the heat flux profile at the divertor and the experimental profiles are well parametrised by the convolution of an exponential decay and a Gaussian, representing mechanisms II and III respectively [1]. Infrared (IR) thermography is an invaluable tool with which to measure the heat flux distribution independently of the plasma parameters. Langmuir probes and thermocouples in the graphite protection tiles provide independent measurements to cross-check IR estimates. The IR system of TCV was recently upgraded to provide coverage of a wider range of divertor configurations and simultaneous measurements at both strike points of a conventional divertor geometry. Using the magnetic shaping flexibility of TCV, multiple divertor configurations ranging from modifications of the classical single null to alternative ones have been tested under attached divertor leg conditions and are presented in this paper. While the outer strike point is generally well fitted with the decay length and an additional spreading in the divertor itself, the inner divertor view displays a double-peak heat flux profile in forward B field, which may be caused by drifts in the SOL [2] and has been previously detected in other tokamaks. In order to take into account the effect of such drifts on the target profile shape, an extension of the parametrisation from [1] representing a radial redistribution of heat is proposed. [1] Eich T et al. 2011 Physical Review Letters 107 215001 [2] Canal G et al. 2015 Nuclear Fusion 55 123023

2016

Étude du transport d'énergie thermique dans les plasmas du tokamak à configuration variable au moyen de chauffage électronique cyclotronique

Yann Camenen

EPFL2006

Infrared measurements of the heat flux spreading under variable divertor geometries in TCV

Basil Duval, Benoît Labit, Roberto Maurizio, Federico Nespoli, Holger Reimerdes, Umar Sheikh, Christian Gabriel Theiler

2016

Measurement of edge electrostatic turbulence in the TCV tokamak plasma boundary

Jan Horacek

EPFL2006