Reflectometry applied to the measurement of density profiles on fusion plasmas has been subject to many recent developments. After a brief reminder of the principles of reflectometry, the theoretical accuracy of reflectometry measurements is discussed. The main difficulties limiting the performance, namely the plasma fluctuations and the quality of the transmission lines, are analysed. The different techniques used for reflectometry are then presented grouped into three different categories, depending on the frequency spectrum of the probing wave: single frequency, few discrete frequencies, or broad spectrum. The present status and achievements of actual implementations of these techniques are demonstrated, with an analysis of their respective limitations and merits, as well as foreseen developments. Finally, a discussion of the various reflectometry techniques is made, in particular their ability to cope with plasma fluctuations and complex transmission lines, in view of the application to next step machines and very severe environments.

Experiments with strong localized electron cyclotron heating (ECH) in the RTP tokamak show that electron heat transport is governed by alternating layers of good and bad thermal conduction. For central deposition hot T e filaments are observed inside the q = 1 radius. Moving the ECH resonance from the centre to the edge of the plasma results in discrete steps of the central electron temperature. The transitions occur when the minimum q value crosses q = 1, 2, 5/2 or 3, and correspond to the loss of a transport barrier situated close to the rational q value. Close to the transitions a new type of sawtooth activity is observed, characterized by the formation of sharp off-axis maxima on the T e profile, which collapse abruptly. The formation of the off-axis maxima is attributed to heat deposition precisely 'on top of' a transport barrier.

The newly installed electron cyclotron emission imaging diagnostic on ASDEX Upgrade provides measurements of the 2D electron temperature dynamics with high spatial and temporal resolution. An overview of the technical and experimental properties of the system is presented. These properties are illustrated by the measurements of the edge localized mode and the reversed shear Alfvén eigenmode, showing both the advantage of having a two-dimensional (2D) measurement, as well as some of the limitations of electron cyclotron emission measurements. Furthermore, the application of singular value decomposition as a powerful tool for analyzing and filtering 2D data is presented.

The main challenge for the Thomson scattering (TS) diagnostic on the TEXTOR tokamak is the detailed study of fast plasma events at a high spatial resolution and a high repetition rate of the measurements. The diagnostic uses intra-cavity probing of the plasma with a repetitively pulsed ruby laser and a fast CMOS camera as detectors. Since 2004, the TS system on TEXTOR has been gradually and systematically enhanced for the measurements of fast plasma events. For that it has recently been upgraded to obtain a multi-pass configuration. Two spherical mirrors have been installed that force the laser beam to probe the plasma a specified number of times before it is directed back into the laser medium. The diagnostics with the upgraded probing system have achieved the measurement accuracy of 3% for the electron temperature and 1.5% for the electron density at <1 cm spatial resolution and 3 × 10 19 m −3 plasma density and can measure at 5 kHz during an interval up to 8 ms. This makes it possible to detect, amongst others, fine structures of magnetic islands and variations of the edge pedestal in the ELMy limiter H-mode.

High resolution (temporal and spatial), two-dimensional images of electron temperature fluctuations during sawtooth oscillations were employed to study the crash process and heat transfer in magnetically confined toroidal plasmas. The combination of kink and local pressure driven instabilities leads to a small poloidally localized puncture in the magnetic surface at both the low and the high field sides of the poloidal plane. This observation closely resembles the "fingering event" of the ballooning mode model with the high- mode only predicted at the low field side.

High temporal and spatial resolution two-dimensional (2D) images of electron temperature fluctuations were employed to study the sawtooth oscillation in the Toroidal Experiment for Technically Oriented Research tokamak plasmas. The 2D images are directly compared with the expected 2D patterns of the plasma pressure (or electron temperature) from various theoretical models. The observed experimental 2D images are only partially in agreement with the expected patterns from each model: The image of the initial reconnection process is similar to that of the ballooning mode model. The intermediate and final stages of the reconnection process resemble those of the full reconnection model. The time evolution of the images of the hot spot or island is partially consistent to those from the full reconnection model but is not consistent with those from the quasi-interchange model. The ''sawtooth oscillation'' was discovered in the early days of fusion plasma research [1] and is known as the m=n 1=1 internal kink mode, where m and n are poloidal and toroidal mode numbers, respectively. An excellent review of recent research in the field of sawtooth oscillations is given in Ref. [2]. This Letter presents a direct comparison study between experimentally measured highresolution 2D images of electron temperature fluctuations with the relevant 2D pattern from prominent physical models developed for the sawtooth oscillation physics in high temperature plasmas. The experiment was performed in the Toroidal Experiment for Technically Oriented Research (TEXTOR) tokamak plasma, which has a circular cross section, a major radius of 175 cm, and a minor radius of 46 cm [3]. The toroidal magnetic field in the present work was in the range 1.9-2.4 T, and the corresponding plasma current was <305 kA. Key plasma parameters were as follows: The central electron density and temperature are in the range 1:5-2:5 10 19 m ÿ3 and from 1.2 to 1.6 keV, respectively. The corresponding peak toroidal beta is 1:0%, and the average poloidal beta is between 0.3 and 0.5. The toroidal rotation of the plasma varied from 1 to 8 10 4 m=s. The speed of a thermal electron is 6 10 7 m=s. The Alfvén and ion acoustic speeds are 5 10 6 and 7 10 5 m=s, respectively. Using plasma parameters close to the q 1 surface, the characteristic reconnection time ( c ) is 700 s.High-resolution 2D images of the electron temperature fluctuations in TEXTOR have been measured with a 2D electron cyclotron emission imaging (ECEI) system. The basic principle of the technique is similar to that of conventional 1D ECE radiometers [4,5]. The new feature of the ECEI diagnostic is that measurements are done in a 2D matrix of sample volumes. Since the ECEI diagnostic has recently been published elsewhere [6,7], we only briefly mention it here as an introduction to the comparison with theoretical models. The system has 16 vertical 8 horizontal sampling volumes arranged in a 2D matrix of 16 cm vertical 7 cm radial . The vertical full width at half maximum of the central antenna pattern is 2 cm...

An experimental investigation into rotating MHD modes has been performed in the TEXTOR tokamak. The effects on the stability of the MHD tearing modes of coupling between m/n = 2/1 and 1/1 modes and of the slowing down of the mode rotation by wall friction have been studied. Tangential neutral beam injection (NBI) has been used to change the toroidal rotation to observe the influence of plasma rotation on the development of MHD modes.Two phases in the rotation frequency behaviour of the modes can be distinguished: a slow reversible decrease and a fast exponential decay, which starts with a short transient increase in frequency. The last phase finally results in mode locking, which mostly leads to a disruption.By injecting low power NBI in the last phase of the slowing-down process, mode locking could be prevented. The mode amplitude and rotation frequency have been stabilized, without major changes to the pressure profile. By decreasing the central plasma velocity, using NBI, a coupling of the two modes has been forced. The coupling of the modes had a destabilizing effect on the MHD activity. * This paper is an extended version of a contribution to the 22nd EPS conference on Plasma Physics and Controlled

Recent 2D spatially and temporally resolved measurements of electron temperature fluctuations in the tokamak core have revealed new information on the dynamics of the sawtooth crash. Measures of poloidal localization of the reconnection zone are achieved through direct analysis of the 2D data and through an interpolated projection of the q = 1 region. An estimate of the toroidal coverage of the reconnection zone is achieved through analysis of shots exhibiting toroidal rotation in which the helically localized reconnection occurs both within and outside of the view of the measurement window. The localized trigger of the crash instability exhibits a dominant toroidal mode number of n ≈ 3, and a dominant poloidal mode number of m ≥ 10.