Third-harmonic X-mode, real-time controlled top-launch ECW experiments on TCV Tokamak

Stefano Alberti, Gilles Arnoux, Alessandro Bortolon, Alexander Karpushov, Yves Martin, Richard Pitts, Laurie Porte
2005
Article

Résumé

In the moderate magneticfield of TCV (1.5T), the recently installed X3 system (3 gyrotrons @118 GHz, 0.45 MW/each, 2s) broadens the operational space with the possibility of heating plasmas at high density, well above the cutoff density of the X2 system (X2 cutoff at ne = 4.2 × 1019m-3). To compensate the significantly weaker absorption coefficient compared to the absorption of X2, the top-launch injection allows to maximize the ray path along the resonance layer thus maximizing the optical depth. To maintain the maximum absorption in plasma discharges with a dynamic variation of both density (refraction) and temperature (relativistic shift) a real time control system on the poloidal injection angle has been developped and succesfully tested on TCV. With a total injected power of 1.35 MW and using the mirror realtime control, full-single pass absorption has been measured in an L-mode plasma. A significant fraction of the absorbed power is associated to the presence of suprathermal electrons generated by the X3 wave itself. In X3 heating experiments of H-mode plasmas it has been possible to enter into a different ELMy regime compared to the ohmic/low-power-heating ELMy regime. In these experiments a significant increase of the plasma energy is obtained with nearly fullsingle pass absorption. Results on the comparison of the absorbed fraction calculated with the TORAY-GA ray-tracing code and the beam-tracing code, ECWGB, which includes difiraction effects, are discussed.

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https://infoscience.epfl.ch/record/120272?ln=fr

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Stefano Alberti, Gilles Arnoux, Alessandro Bortolon, Alexander Karpushov, Yves Martin, Richard Pitts, Laurie Porte
2005
Article

Résumé

Official source

https://infoscience.epfl.ch/record/120272?ln=fr

À propos de ce résultat

Concepts associés (17)

Concepts associés

Chargement

Publications associées (36)

Publications associées

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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

Chargement

Overview of edge electrostatic turbulence experiments on TCV

Jonathan Graves, Jan Horacek, Richard Pitts

Experiments on edge turbulence in the TCV tokamak have been performed for the first time at the beginning of 2003. This paper presents an overview of some of the results obtained, concentrating in particular on two areas: universality of density fluctuations and the dynamical coupling between radial turbulent-driven fluxes and parallel flows. Plasma fluctuations in the edge of the TCV tokamak have been found to exhibit statistical properties which are universal across a broad range of discharge conditions. Analysis of the time series of density fluctuations in the entire scrape-off layer (SOL) region from just inside the magnetic separatrix to the plasma-wall interface, yields a probability distribution function (PDF) of density which conforms closely to a Gamma distribution. In the wall vicinity, the density fluctuations exhibit clear evidence of self-similarity and are characterised by a PDF with universal shape and with a standard deviation proportional to the mean density. It is also found that radial particle-flux fluctuations scale solely with the mean density. Such findings indicate that it may be possible to improve the prediction of transport in the critical plasma-wall interaction region of future large-scale tokamaks. Recent experiments on JET [C. Hidalgo et al.: Phys. Rev. Lett. 91 (2003) 065001] have investigated a possible link between turbulent transport and the parallel flows. Similar experiments have been performed on TCV for a variety of plasma conditions and flow magnitudes. Although correlations have been found as seen on JET, especially in the wall vicinity, it appears that the magnitude of the coupling is insufficient to drive any significant flow.

2005

Chargement

Controlled nuclear fusion is the most promising candidate for being an inexhaustible, clean and intrinsically safe energy source. In the tokamak fusion reactor concept, a high temperature plasma is confined by magnetic fields. Turbulence diffuses the confined plasma to the tokamak outermost region, the Scrape-Off Layer (SOL), featuring open field lines. In the SOL, the plasma is convected along the field lines and deposited on the solid surfaces of the tokamak wall. The plasma-wall interaction through the SOL strongly affects the reactor performances, and the elevated heat loads on the solid surfaces are one of the most limiting factors for fusion. SOL physics is not completely understood, not even in the simple

limited'' configuration, where the main plasma touches the reactor wall, and the contact point defines the Last Closed Flux Surface (LCFS). In this thesis, we advance the understanding of SOL physics in limited plasmas, combining experiments and numerical simulations. In particular, two topics are addressed. First, the separation between the

near'' and ``far'' SOL is investigated. The near SOL extends typically a few mm from the LCFS, features steep radial profiles of parallel heat flux and is responsible for the peak of heat deposition on the tokamak first wall. The far SOL, typically a few cm wide, features flatter heat flux profiles, and accounts for the majority of the heat deposited on the first wall. Secondly, blob dynamics is investigated. Blobs are high density plasma filaments generated by turbulence, and are an ubiquitous feature of plasmas in open magnetic field lines. The blobs travel outwards to the reactor walls, increasing the cross field transport. Dedicated experiments have been performed on the TCV tokamak. Inboard-limited Deuterium (D) and Helium (He) plasma discharges are performed, varying the main plasma parameters (current, density and shaping). The parallel heat flux radial profiles are determined with infrared thermography. For the first time, the presence of a near SOL in TCV limited plasmas is reported, both for D and He discharges. The near SOL is found to disappear for high plasma resistivity. Non-ambipolar currents are measured to flow to the wall in the near SOL using Langmuir probes, and their presence is found to correlate with the strength of the near SOL heat fluxes. A simple interpretation is given. The heat fluxes and electric potentials are also measured on the low field side using a reciprocating Langmuir probe, and compared with the ones measured on the tokamak wall. A method for the mitigation and suppression of the near SOL heat fluxes through impurity seeding is proposed, and first experimental evidences are presented. The experiments are compared with numerical simulations of the TCV SOL, performed with the GBS code. The simulated parallel heat flux profiles qualitatively agree with the experimental ones, showing the presence of a near and far SOL. Also, non-ambipolar currents are observed to flow to the wall. The effect of resistivity is investigated through a second simulation with a 40 times higher resistivity. The blob dynamics in TCV is investigated using a conditional average sampling technique on the reciprocating Langmuir probe data. The results for two discharges, for low and high resistivity respectively, are discussed. A blob detection and tracking algorithm is applied to the numerical simulation outputs, and the results are discussed.

EPFL2017

Chargement

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

Yann Camenen

EPFL2006

limited'' configuration, where the main plasma touches the reactor wall, and the contact point defines the Last Closed Flux Surface (LCFS). In this thesis, we advance the understanding of SOL physics in limited plasmas, combining experiments and numerical simulations. In particular, two topics are addressed. First, the separation between the

EPFL2017