The radiative improved (RI) mode is a tokamak regime offering many attractive reactor features. In the article, the RI mode of TEXTOR-94 is shown to follow the same scaling as the linear ohmic confinement regime and is thus identified as one of the most fundamental tokamak operational regimes. The current understanding derived from experiments and modelling of the conditions necessary for sustaining the mode is reviewed, as are the mechanisms leading to L-RI mode transition. The article discusses the compatibility of high impurity seeding with the low central power density of a burning reactor, as well as RI mode properties at and beyond the Greenwald density.

Density limits and the evolution of disruptions were studied on TEXTOR for different wall materials facing the plasma surface. A systematic extension of the density limit was found over a large current range when the metallic wall was first exchanged by a carbon coated wall and later by a boron-carbon coated wall. The maximum attainable density nearly doubled when metallic impurities were replaced by low Z impurities such as boron and carbon.With a neutral beam additional heating power of P,, = 3.2 MW, average densities exceeding 5, = lo2' m-3 were obtained. The maximum density observed scales like the square root of the auxiliary heating power. The sequence of processes leading to a density limit disruption was experimentally analysed in detail for ohmically heated plasmas. During the density increase in the gas feeding phase, boundary cooling by radiation and steepening of the electron temperature gradient were observed. The plasma detaches from the limiters and the magnetohydrodynamic activity is launched in the edge plasma. An m = 2 mode in the outer plasma region couples with an m = 1 mode deeper in the plasma. During a first mode locking of the m = 2 mode, a strong heat pulse is emitted and a sharp increase of the impurity lines C V and 0 VI is found. The final energy quench is believed to be triggered by the overlapping of two coupled modes, Enhanced density fluctuations are observed by collective millimetre wave scattering in this phase of the discharge. The energy quench in TEXTOR lasts approximately 1 ms or less. Then the electron temperature profile evolution shows a structure which could be interpreted as a filamentation of the plasma current. The decay of the plasma current after the energy quench is governed by the resistance of the cold plasma.

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

No abstract

An experimental investigation into electron temperature profile perturbations due to MHD tearing modes is performed on the TEXTOR tokamak. By means of an ECE diagnostic with a good temporal and radial resolution, m = 1 and m = 2 precursors to sawtooth crashes and major disruptions, respectively, can be studied.The specific temperature perturbations due to a rotating m = 2 magnetic island prior to a disruption have been studied. The phase of the m = 2 MHD oscillation reverses at the rational q surface. The amplitude distribution of the perturbation has been determined. A frequency analysis of the MHD oscillations on different radial positions close to the rational surface shows an increase of higher harmonics in the frequency spectrum due to the difference in magnetic field topology.Temperature profiles over magnetic islands could be measured. A secondary temperature maximum in the centre of the m = 2 magnetic island has been found.Because of the good radial resolution of the ECE diagnostic an estimate of the island width and the position of the q = 2 surface could be obtained. In one case a movement of the q = 2 surface, due to peaking of the current profile, of more than 7 cm, i.e. 15% of the minor radius, was found. Island widths of w/a = 0.07-0.2 have been observed.