Two-dimensional heat transport in tokamak plasmas

Abstract

A very promising source for future energy is nuclear fusion. Achieving energy production from fusion reactions on earth however is challenging as it requires that the fusion fuel is brought to high temperatures and densities so that sufficient fusion reactions occur to produce a net energy gain. Maintaining fusion conditions therefore requires low heat losses. The fusion machines of the Tokamak concept have already approached the required fusion conditions by means of magnetically confining the plasma fusion fuel. The plasma energy content and by extension the fusion performance is degraded by instabilities that form magnetic islands. Controlling the growth of magnetic islands is very important for controlling the fusion process. For island growth control a local modification of the current distribution by means of high-power mm-wave deposition is envisioned. In tokamaks, the currents are driven by high-power mm-waves. The currents are driven directly as well as inductively by means of a local modification of the electron temperature profile shape and the closely associated inductive current profile shape. While the effect of direct current drive is envisioned as the main means to modify the current-profile, the non-inductively driven current is not negligible. The efficiency of the inductive current drive near the island depends on the electron transport properties in and around the magnetic island. The transport properties near the island are however not well known. With the aim of characterizing these properties, transport experiments were carried out on the TEXTOR Tokamak. TEXTOR is well equipped with a set of experimental tools that allows a deeper investigation of the island transport properties. It has been investigated how much the electron temperature-profile and closely related electron current-profiles could be modified in plasmas without the formation of magnetic islands by the application of different sources of additional heating. The profiles of electron temperature and density have been measured using high resolution Thomson scattering. Centrally heated plasmas from different sources have been found to yield similarly shaped electron temperature profiles and current-profiles outside the plasma center. While the electron temperature profile shapes of the plasma center were found to be strongly affected by the sawtooth-instability, the profiles outside of the plasma center were both found to be very similar, independent of the central heating method used. Both the profile consistency model and a critical temperature gradient model could be used to describe the observed profile shapes outside of the plasma center that is affected by the sawtooth instability. Application of localized off-axis electron heating caused a localized modification of the shape of the electron temperature profile and the inductive current-profile. The change in electron temperature and inductive current profile shapes is however limited by the resilience of the profiles. For the control of island growth it has been shown that besides the non-inductively driven current, the inductively driven current also plays an important role. The electron transport properties of magnetic islands are however poorly known. For this reason, perturbative electron heat transport experiments in the vicinity of magnetic islands have been performed. From the analysis of the propagation of temperature perturbations inside the magnetic island that has a low temperature gradient, a low electron heat conductivity has been found. This is in contrast to previously found higher transport coefficients from peaked temperature profiles inside magnetic islands. The increase of the electron heat conductivity due to extra heating power limits how effective an inductive current can be driven inside the magnetic island. Driving current with a local deposition of mm-waves is also used to control the sawtooth-instability which affects the confinement of a large part around the plasma center. Different types of instability modes can cause sawtooth crashes. Control of the different instabilities requires different applications of additional current drive and heat. The sawtooth crash generally appears on a time-scale of less then a millisecond which makes it difficult to resolve the type of instability causing the sawtooth crash. Two main instability modes are held responsible for sawtooth-crashes; a resistive internal kink mode and a quasi-interchange mode. These modes can be distinguished from their characteristic two-dimensional temperature redistribution pattern during the sawtooth crash. Because it has been shown to be difficult to distinguish these instability modes with conventional one-dimensional temperature measurements or line-integrated measurements, the applicability of direct two-dimensional ECE-Imaging measurements has been evaluated. Different temperature normalization methods that can be used for uncalibrated two-dimensional ECE-Imaging measurements have been analyzed for their applicability in distinguishing the different mode structures. It has been shown that two normalization methods allow clear distinction of the temperature structures due to the quasi-interchange mode and the resistive internal kink mode. The application of these normalizations on two-dimensional ECE-Imaging data from sawtooth-crashes in TEXTOR showed clearly a two-dimensional temperature structure that is accurately described by the evolution of a resistive internal kink mode.