Study of plasma fluctuations in new closed flux-surface configurations of the TORPEX experiment

The TORoidal Plasma EXperiment (TORPEX) is a Simple Magnetized Torus (SMT) where open helical magnetic field lines are obtained from the superposition of a small vertical component on the main toroidal field. The SMT configuration features the main driving mechanisms of plasma instabilities characterising the Scrape-Off Layer (SOL) of larger tokamak devices, making TORPEX a very attractive machine to perform fusion oriented studies of plasma turbulence. A new experimental set-up has been recently developed to generate a poloidal magnetic field. It consists in a toroidal copper wire suspended in the middle of the vacuum vessel by seven 1mm stainless steel wires, three vertical and four lateral, plus the vertical electrical coaxial feed-through. A current up to 1kA can be driven in the system, giving us the possibility to include a rotational transform and to deal with a SOL much closer to the one of a fusion device. Moreover plasma core and closed-to-open field line region studies could be performed. The technical details of the set-up will be given together with the experimental results obtained during the first campaigns with Hydrogen plasmas. A magnetic configuration with almost closed flux surfaces has been chosen, resulting from the poloidal magnetic field together with a small vertical component. The radial profiles of the main plasma parameters will be presented, such as plasma density n_e, plasma temperature T_e and plasma potential V_pl. The spectral properties of plasma dominant coherent modes at the position of maxima fluctuations will be provided too, including power spectral density estimates and measurements of vertical and toroidal wave-numbers obtained for different values of poloidal magnetic field. This allows us to calculate the poloidal and toroidal mode-numbers which can be used together with fluid simulations to asses the kind of instability we are dealing with. An overview of the new accessible magnetic field configurations will be showed as well, ranging from Single Null X-points to magnetic Snowflakes.

Exploration of candidate fusion reactor regimes by real time control of tokamak plasma shape

Himank Anand

The potential of nuclear fusion to provide a practically inexhaustible source of energy has motivated scientists to work towards developing nuclear fusion tokamak power plants. Stable operation of a tokamak at high performance requires simultaneous treatment of several plasma control problems. Moreover, the complex physics which governs the tokamak plasma evolution must be studied and understood to make correct choices in controller design. This mutual inter-dependence has informed this thesis, using control solutions as an experimental tool for physics studies, and using physics knowledge for developing new advanced control solutions. The TCV tokamak at SPC-EPFL is ideally placed to explore issues at the interface between plasma physics and plasma control, by combining a state-of-the-art digital real time control system with a flexible and diverse set of actuators including a full set of independently powered shaping coils. The recent deployment of the real time version of the Grad-Shafranov equilibrium reconstruction code LIUQE, with a sub-ms cycle time in the digital control system, has facilitated the design of a new generalised plasma position and shape controller, based on the information on poloidal flux and magnetic field provided by the real-time Grad-Shafranov solver. The first issue addressed in the thesis is the development and experimental testing of a new real time control strategy to construct a generalised control algorithm for not only controlling the position of the plasma but also to aid in the precise control of higher order shape moments, X-points and strike points, particularly in advanced plasma configurations such as negative-triangularity plasmas, snowflake and super-X divertors, and doublets. A controller formulation ensuring flexibility through an ordering of controlled variables from the most easily to the least easily controlled, while respecting the hardware limits on the poloidal field coil currents, is developed. The successful experimental implementation of the control algorithm has been demonstrated for both fixed and time varying plasma position and shape for limiter and divertor plasma discharges. In addition, the controller has provided satisfactory performance with respect to plasma scenarios involving complex changes in the plasma shape and position. The second issue addressed in the thesis is the application of the generalised plasma position and shape controller to a snowflake plasma configuration. A comparison between the optimised generalised plasma position and shape controller with the performance of the TCV hybrid controller for a given reference snowflake plasma discharge showed a marked improvement in various geometrical properties of the snowflake plasma configuration in the vicinity of the null point. However, strong control of the poloidal magnetic field at the two X-points resulted in a tradeoff on the upper part of the plasma boundary, where the overall precision was comparable to that of the legacy controller. In the experimental time available, the snowflake shots developed exhibited a boundary that was too close to the inner wall of the vessel, modifying the edge plasma behaviour (studied with infrared cameras and Langmuir probes) and making it difficult to study the physics properties of the exact snowflake. However, further optimisation should well be possible.

EPFL2017

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

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.

EPFL2011

Turbulence at the boundary of toroidal plasmas with open and closed magnetic flux surfaces

Fabio Avino

The control and confinement of fusion plasmas are currently limited by a lack of understanding of the physical mechanisms behind the evolution of the turbulent transport experienced by particles and energy. In-situ investigations of plasma turbulence in fusion experiments is strongly hampered by the high temperatures and densities. Basic plasma physics devices represent an alternative solution to perform turbulence studies with the possibility of rigorously validating numerical codes. One of these experiments is TORPEX, in which a comprehensive characterization of plasma turbulence has been conducted in the presence of open helical magnetic field lines in toroidal geometry. These reproduce the main features of the scrape-off layer (SOL), which is the open flux surface region at the edge of magnetically confined fusion plasmas. The SOL has a key role in the balance of the dynamics that determine the overall plasma confinement. The first achievement of this thesis work is a technical upgrade of TORPEX that consists in the installation of a copper toroidal conductor inside the TORPEX vacuum vessel. A poloidal magnetic field is produced by a current flowing inside the conductor, introducing a rotational transform. This allows studying plasma turbulence in magnetic geometries of increasing complexity, starting with the simplest configuration of quasi-concentric flux surfaces. The initial exploration of the main plasma properties, including plasma production mechanisms and particle confinement time, is followed by a detailed spectral characterization of the measured electrostatic quasi-coherent fluctuations. Measurements of the toroidal and poloidal mode numbers reveal field-aligned modes. These present a poloidal localization indicating a clear ballooning feature that is in agreement with the results of a linear fluid code. The first experimental measurements of plasma blobs in the presence of a single-null X-point are performed. Blobs radially propagating outwards across the X-point are conditionally sampled, which allows us to track and analyze in detail the corresponding dynamics. The ExB drifts induced by the background potential gradients and the fluctuating potential dipole are both responsible for the measured blob acceleration in the X-point region. The contribution of the potential dipole is explained on the basis of an analytical model, in which the variation of the magnetic field intensity close to the X-point plays a key role. This results in a blob speed scaling that is in good agreement with the measured values.

EPFL2015