A Far-Infrared Polarimeter Diagnostic for the Measurement of the Poloidal Magnetic Field in the TCV Tokamak

EPFL, 2011
EPFL thesis

Abstract

Information about the poloidal field inside the plasma and about the current density profile is essential for the understanding of the behavior of tokamak plasmas. The poloidal field outside the plasma edge is easily measured by a set of magnetic coils, whereas measurements of the magnetic field distribution inside the plasma requires dedicated and often complex diagnostic systems. One of the methods is polarimetry using beams of electromagnetic waves traversing the plasma. It makes use of the fact that the magnetic field modifies the refractive index of the plasma and as a consequence affects the state of polarization of the passing waves. Specifically, Faraday rotation refers to the case of a linearly polarized wave propagating parallel to a magnetic field. In this case, the rotation of the polarization vector is proportional to the product of electron density and parallel magnetic field component. This thesis is devoted to the design of a specific polarimeter system and its implementation on the TCV tokamak. The conceptual design took into account the particular requirements of the TCV experiments and its experimental program with particular interest in measurements of current density profiles in low-density, EC-heated plasmas. Aiming at optimum performance of the polarimeter in this parameter range, the FIR laser wavelength of 432.5 µm was chosen. Since TCV is already equipped with an interferometer (at 214 µm) for measurements of the line-integrated density along 14 chords, the polarimeter was built as a separate instrument. The number of 10 spatial channels to cover the plasma diameter was found as a compromise between desired spatial resolution, access constraints and cost. The measurement of the Faraday rotation angle is based on a method suggested by Dodel and Kunz [1], which uses optical beams with rotating linear polarization. In this case, the Faraday rotation angle is retrieved from a phase measurement comparing the modulated signals from the probe detectors with that from a reference detector and to first order is insensitive to amplitude variations of the signals. Waveguide detectors based on Schottky barrier diodes are used as detectors for their sensitivity and large electrical bandwidth. Signal processing and analysis is performed in two branches : a) using specifically built analog electronic phase detectors followed by slow (250 kHz) ADCs b) using fast (5 MHz) ADCs and numeric signal processing on a mainframe computer. The thesis covers the description of the design and the various components of the polarimeter, presents initial tests and an evaluation of its performance based on simulations using real plasma configurations of TCV. The analysis of specific tests with the system installed on TCV revealed the presence of perturbations leading to parasitic contributions to the measured phase angles. The results of first measurements of the Faraday rotation angle for different plasma conditions on TCV are presented. Comparing the measured Faraday rotation angles with the results of calculations showed qualitative agreement, in particular the effects of an increase in electron density and of the reversal of the current direction were clearly seen. The system is also capable to detect a radial displacement of the magnetic axis of the plasma, as was demonstrated by comparing measurements from two specific plasma configurations. However, the absolute values still show significant deviations from the expected ones based on calculations. The discrepancies increase with increasing Faraday angle and depend on the direction of the plasma current. At high electron densities beam refraction becomes a problem and may lead to significant errors in the measurements and eventually to complete loss of the signal in several channels. In its present status, the polarimeter cannot yet provide results that are suitable to reconstruct the poloidal field or the current density profile of the plasma. Tentative explanations for this problem are given, but further specific tests are necessary to confirm them. Even in its present state, the polarimeter may be used to detect transient relative changes in the profiles of electron density and current. This was demonstrated by a series of experiments in plasmas with sawtooth activity. Comparing the signals from the polarimeter with those from other multi-chord diagnostics (interferometer and soft X-ray detectors) clearly revealed correlations but also specific differences. These experimental studies showed that the Far-Infrared Polarimetry still requires further improvements, but has potential to become a valuable diagnostic capable of directly measuring the current density profile in the TCV tokamak.

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EPFL, 2011
EPFL thesis

Abstract

Official source

https://infoscience.epfl.ch/record/151492?ln=en

About this result

Related concepts (38)

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Thermonuclear controlled fusion research is a highly active branch of plasma physics. The main goal is the production of energy from the fusion reaction of hydrogen isotope nuclei, the same reaction that powers stars. The most promising present approach are Tokamaks, toroidal devices where high temperature plasmas are confined by means of magnetic fields. This thesis is devoted to a detailed and systematic study of plasma rotation nthe Tokamak à Configuration Variable (TCV), at the Centre de Recherches en Physique des Plasmas (CRPP) in Lausanne Switzerland. In a tokamak, confinement is limited by particle and energy transport from the hot core to the cold edge and by macroscopic perturbations of magnetic equilibrium. Recently, plasma rotation has been demonstrated to beneficially affect both confinement and stability, explaining the great recent interest in plasma rotation studies Relatively little is understood about plasma rotation physics and, in particular, the so called "intrinsic" rotation that will constitute the main component of plasma rotation in next generation machines. That is why a great theoretical and experimental effort is being deployed in studying intrinsic rotation and this work is part of this context. In TCV, plasma rotation is measured by the Charge eXchange Recombination Spectroscopy diagnostic (CXRS). The spectroscopic signal comes from the perpendicular observation of a low power Diagnostic Neutral Beam Injector (DNBI), which applies a negligible torque to the plasma. Hence, the DNBI/CXRS pair is an effective tool for the experimental study of intrinsic tokamak plasma rotation. During this work, the pre existing toroidal observation view was complemented with two new systems, permitting the measurement of toroidal rotation, on inboard plasma radius, and poloidal rotation in the plasma periphery. The implementation of an automated wavelength calibration procedure, based on reference Neon spectra, permitted the first viable (toroidal and poloidal) rotation measurements of TCV, with uncertainties down to 1km/s. Using upgraded light collection optics and fiber optic transmission lines, simultaneous measurement of core and edge plasma was achieved, with a doubling of of the radial resolution of the toroidal rotation measurements. The measurable range of plasma parameters was also extended to higher densities by the installation of back illuminated CCD detectors. In the present configuration (CXRS09), the diagnostic is capable of routinely measuring toroidal and poloidal plasma rotations with a radial resolution of ≲ 1 cm and a sample frequency of 10 ÷ 20 Hz, for plasma densities of 0.8 ≲ ne,av ≲ 8 × 1019 m-3. The basic scenario of Ohmically heated discharges in limiter L-mode configuration was initially addressed. A large core toroidal rotation up to uφ ≈ 50 km/s in the counter current direction is measured, reversing nearly exactly upon reversal of plasma current Ip. The toroidal rotation profile may be schematically divided into a core, a peripheral and an intermediate region. For qe ≈ 3 (magnetic safety factor at the plasma edge) the core region velocity is relatively flat or slightly "bulged" in the co current direction inside the sawtooth inversion radius. In the peripheral region, the toroidal rotation is small with a monotonic intermediate region. The central rotation appears to be limited to approximately its value at the sawtooth inversion radius. Poloidal rotation uθ ≲ 3 km/s, measured in Ohmic discharges, is only weakly dependent on plasma parameters but reverses with reversed magnetic field, with values and direction coherent with neoclassical predictions. Combining uφ and uθ measurements, the profile of the radial electric field Er was estimated through the radial force balance equation. Er down to 8 kVm (inward directed) is found in the plasma bulk, and close to zero at the plasma edge. A spontaneous reversal of the toroidal rotation profile is observed when the average density exceeds ne,av ≈ 4 × 1019 m-3 at low qe ≈ 3, with the plasma now rotating in the co current direction. The transition between the co and counter rotation regimes is studied dynamically using ne and Ip ramps and with the application of Electro Cyclotron Heating (ECH). The rotation reversal is weakly sensitive to impurity concentration and positive plasma triangularity appears to be a key ingredient. Whilst a physical explanation has not been identified yet, dynamic uφ reversal observations indicate that it results from a changed balance of radial non-diffusive fluxes of toroidal momentum. The study was extended to the divertor magnetic configuration in which the plasma column rotates in the co current direction at low ne, and uφ reverses at high ne opposite from the limited configuration. The rotation profile may again be divided into three regions, although the rotation at the plasma periphery does not always remain close to zero but evolves with the plasma parameters. In particular, independently on the core rotation regime, peripheral uφ decreases with ne and Ip and is strongly sensitive to the ion B→ × ∇B direction, suggesting a link with parallel fluxes in the Scrape-Off Layer (SOL). Combined measurements of of CXRS and Mach probe indicate that in the plasma edge toroidal rotation matches the toroidal component of SOL flows. From the analysis of toroidal rotation in stationary and transient phases, a characterization of the momentum transport is presented. The resulting radial momentum diffusivity, of the order of Χφ ∼ 0.1 – 0.3 m2/s, exceeds by 2 orders of magnitude the neoclassical estimation. A remarkable result is the existence of a "residual stress" component, which sustains a substantial stationary rotation gradient for null background rotation. Conversely, the analysis suggests a minor role for the convective (pinch) component, that is preliminarily confirmed by gyro-kinetic simulations including turbulent Coriolis convective pinch. Neoclassical predictions are in quantitative and qualitative disagreement with the experimental observations. The effect of the sawtooth instability on the core rotation was addressed in a specific experimental scenario where the inter crash evolution of the core uφ could be measured. The measurement required the development of a fast CXRS acquisition scheme based on a trigger constructed in real time from a Soft X-ray measurement. At the sawtooth crash the plasma core undergoes a strong acceleration in the co current direction (∆uφ ≈ 9 km/s in the experimental scenario), possibly related to a strong transient toroidal electric field. The systematic and varied observations reported in this work extend the experimental knowledge of bulk plasma rotation in low confinement regimes. In particular, the rotation reversal phenomena constitute an important test for momentum transport models of tokamak plasmas.

EPFL2009

Bootstrap current compensation with electron-cyclotron waves in 3D reactor-size configurations

Sergi Ferrando I Margalet

In magnetic confinement devices, the inhomogeneity of the confining magnetic field along a magnetic field line generates the trapping of particles (with low ratio of parallel to perpendicular velocities) within local magnetic wells. One of the consequences of the trapped particles is the generation of a current, known as the bootstrap current (BC), whose direction depends on the nature of the magnetic trapping. The BC provides an extra contribution to the poloidal component of the confining magnetic field. The variation of the poloidal component produces the alteration of the winding of the magnetic field lines around the flux surfaces quantified by the rotational transform ι. When ι reaches low rational values, it can trigger the generation of ideal MHD instabilities. Therefore, the BC may be responsible for the destabilisation of the configuration. This thesis is divided into two parts. In the first part, we present a self-consistent method to calculate the BC and assess its effect on equilibrium and stability in general 3D configurations. This procedure is applied to two reactor-size prototypes (both with plasma volumes ∼ 1000m3): a quasi-axisymmetric (QAS) system and a quasi helically symmetric (QHS) system with magnetic structures that develop BC in opposite directions. The BC increases with the plasma pressure, therefore its relevance is enhanced when dealing with reactor-level scenarios. The behaviour of both prototypes at reactor level values of β ≡ (kinetic plasma pressure)/(magnetic pressure) is assessed, as well as its alteration of the equilibrium and stability. In the QAS prototype, BC-consistent equilibria have been computed up to β = 6.7% and the configuration is shown to be stable up to β = 6.4%. Convergence of self-consistent BC calculations for the QHS case is achieved only up to β = 3.5%, but the configuration is unstable for β ≥ 0.6%. The relevance of symmetry breaking modes of the Fourier expansion of the confining magnetic field on the generation of BC is studied for each prototype. This proves the close relationship between magnetic structure and BC. Having established the potentially dangerous implication of the BC, principally, in reactor prototypes, a method to compensate its harmful effects is proposed in the second part of the thesis. It consists of the modelling of the current driven by externally launched ECWs within the plasma to compensate the effects of the BC. This method is flexible enough to allow the identification of the appropriate scenarios in which to generate the required CD depending on the nature of the confining magnetic field and the specific plasma parameters of the configuration. Both the BC and the CD calculations are included in a self-consistent scheme which leads to the computation of a stable BC+CD-consistent MHD equilibrium. This procedure is applied in this thesis to simulate the required CD to stabilise the QAS and QHS prototypes introduced in the first part. The estimation of the input power required and the effect of the driven current on the final equilibrium of the system is performed for several relevant scenarios and wave polarisations providing various options of stabilising driven currents. Several scenarios have been devised for each prototype in order to drive current at the appropriate location and with the desired direction. Different polarisations and launching conditions have been employed to this purpose. In particular, a HFS launched X2 ECW with an input power of 1.5MW has been shown to drive sufficient current to maintain the rotational transform below the critical value 2/3 at β = 6.4% for the QAS reactor. Correspondingly, in the QHS reactor, an X3-mode ECW of 100KW was sufficient to drive the current required to push the rotational transform below unity near the magnetic axis at β = 3%. Thus, stabilisation of BC-driven instabilities with externally launched ECWs has been achieved for both contrasting configurations. The method proposed in this thesis allows also the utilisation of EBW in the generation of CD. The possible advantages of EBCD for compensation studies are described as well as their possible application to the two prototypes under consideration. The BC+CD procedure is particularly interesting to investigate new magnetic geometries as potential candidates for fusion reactors. With this numerical tool, it is possible to assess the implications of their consistent BC when operating at reactor level. It also allows to quantify how much power would be required to maintain the system MHD stable in these circumstances. Nevertheless, this method is flexible enough to be applicable to any configuration.

EPFL2006

MHD stability limits in the TCV tokamak

Holger Reimerdes

Magnetohydrodynamic (MHD) instabilities can limit the performance and degrade the confinement of tokamak plasmas. The Tokamak à Configuration Variable (TCV), unique for its capability to produce a variety of poloidal plasma shapes, has been used to analyse various instabilities and compare their behaviour with theoretical predictions. These instabilities are perturbations of the magnetic field which usually extend to the plasma edge where they can be detected with magnetic pick-up coils as magnetic fluctuations. A spatially dense set of magnetic probes, installed inside the TCV vacuum vessel, allows for a fast observation of these fluctuations. The structure and temporal evolution of coherent modes is extracted using several numerical methods. In addition to the setup of the magnetic diagnostic and the implementation of analysis methods, the subject matter of this thesis focuses on four instabilities which impose local and global stability limits. Al1 of these instabilities are relevant for the operation of a fusion reactor and a profound understanding of their behaviour is required in order to optimise the performance of such a reactor. Sawteeth, which are central relaxation oscillations common to most standard tokamak scenarios, have a significant effect on central plasma parameters. In TCV, systematic scans of the plasma shape have revealed a strong dependence of their behaviour on elongation κ, and triangularity δ, with high κ and low δ leading to shorter sawteeth with smaller crashes. This shape dependence is increased by applying central electron cyclotron heating (ECH), which increases or decreases the sawtooth period, depending on the plasma shape. The response to additional heating power is determined by the role of ideal or resistive MHD in triggering the sawtooth crash. For plasma shapes where additional heating and consequently, a faster increase of the central pressure shortens the sawteeth, the low experimental limit of the pressure gradient within the q = 1 surface is consistent with ideal MHD predictions. The observed decrease of this limit with elongation is also in qualitative agreement with ideal MHD theory. Edge localised modes (ELMs), occurring in TCV Ohmic high-confinement mode discharges, were observed to be preceded by coherent magnetic oscillations. The precursors prior to small ELMs, believed to be of type III, and prior to larger ELMs, previously referred to as TCV large ELMs, show the same characteristics, which allows for an identification of both ELMs to be of type III according to the usual classification scheme. The detected poloidal and toroidal mode structures are consistent with a resonant flux surface close to the plasma edge. Unlike conventional MHD modes, these precursors start at a random toroidal location and then grow in amplitude and toroidal extent until they encompass the whole toroidal circumference. Thus, the asymmetry causing and maintaining the toroidal localisation of the ELM precursor, must be intrinsic to the plasma. Soft X-ray measurements show that the localised precursor always coincides with a central m = 1 mode, which can usually be associated with the sawtooth pre- or postcursor mode. A comparison of the phases indicates a correlation with the maximum of the central mode preceding the toroidal location of the ELM precursor and, therefore, a hitherto unobserved coupling between central modes and ELMs. Highly elongated plasmas promise several advantages, among them higher current and beta limits. During TCV experiments dedicated to an increasing of the plasma elongation, a new disruptive current limit, at values well below the conventional current limit corresponding to qa >2, was encountered for κ >2.3. This limit, which is preceded by a kink-type mode, is found to be consistent with ideal MHD stability calculations. The TCV observations, therefore, provide the first experimental confirmation of a deviation of the linear Troyon-scaling of the ideal beta limit with normalised current at high elongation, which was predicted over 10 years ago. In addition to the ideal beta limit, several other MHD events are observed in highly elongated plasmas. The axisymmetric mode causes vertical displacement events and thereby, imposes a lower current limit. This operational limit is in good agreement with theoretical predictions of the growth rate of the axisymmetric mode. Furthermore, minor disruption are occasionally observed. They are caused by bursts of tearing modes located close to the q = 1 surface, which are destabilised by flat central current profiles, typical for highly elongated plasmas. Neoclassical tearing modes (NTMs), which have been observed to limit the achievable beta in a number of tokamaks, arise from a helical perturbation of the bootstrap current caused by an existing seed island. Neoclassical m/n = 2/1 tearing modes have been identified in TCV discharges which are characterised by a low electron collisionality νe*, a medium ion collisionality ν*, a medium value of beta and strongly peaked pressure and current profiles. In contrast to other tokamak experiments, where sawteeth, fishbones or ELMs generate the seed island, the required seed island is provided by a conventional tearing mode. The island clearly shows a conventional and a neoclassical growth phase. The TCV results provide the first clear observations of such a trigger mechanism which could also explain the occurrence of "triggerless" NTMs observed in other experiments. The slowly growing seed island also allows a measurement of the critical seed island width ωcrit, which is observed to increase with increasing density. In conclusion, several local and a global stability limits are analysed. These instabilities can limit the pressure gradient and thereby, the performance of the plasma. The presented results reveal several previously unobserved features of commonly observed instabilities. Since the most of the new observations can be explained by theory, they improve the predictive capabilities with respect to new experiments. The experiments have also shown some new interactions among different instabilities, which add to the already crowded complexities of MHD phenomena in fusion plasmas.

EPFL2001