At TEXTOR, an O-mode heterodyne reflectometer system is installed and operated for the measurement of plasma density fluctuations and turbulence investigations. With two antenna arrays in the equatorial and top positions having two and three horn antennae, respectively, poloidal correlations are investigated under different plasma scenarios. From the amplitude, cross-phase and coherency spectrum, differences in the ohmic and auxiliary heated discharges are investigated. Furthermore the dynamic behaviour of the turbulence is studied in the SOC-IOC transition and in the precursor phase of a disruption. For the latter an increased integrated power spectral density was observed at the X-point of the mode compared with the O-point. Stationary m = 2 mode activity is observed for the first time at TEXTOR by reflectometry. The fluctuation level is calculated for different conditions and rises significantly increasing heating power which is consistent with the L-mode confinement degradation. Correlation measurements yield the measured phase delays which are used to calculate the poloidal phase velocity perpendicular to the magnetic field. In ohmic plasmas the turbulence rotates like a 'rigid body' with constant angular velocity inside the q = 2 surface. The rigid body rotation is broken up during tangential neutral beam injection. From the deduced poloidal wavenumber of the turbulence, most likely ion temperature gradient modes are the driving mechanism of the turbulence.

An attempt is made to explain the type II irregularities observed by Balsley in the equatorial E region. Vertical gradients in the plasma density appear to be essential for the excitation of the instability responsible for the irregularities. An interesting by‐product of the calculations is that the Farley instability is recovered by making use of a fluid picture for both ion and electron components.

It is shown that existing neoclassical theories in the high collisionality regime are inadequate to describe the edge plasma, where the gradient length scales are of the order of (only) a few centimeters (the ‘‘edge’’ is defined here in relation to the neutrals ionization depth). Gyrostresses indeed modify considerably the continuity, energy, and parallel momentum equations if, as is usually the case, Λ1≡νi (qR)2/ΩiLψ r is not negligible compared to unity (Lψ=LN, LT, LV, where, e.g., LN is the density length scale). Particle transport, moreover, is no longer automatically ambipolar: the radial electric field −∇V is actually determined by the ambipolarity constraint. Another parameter also usually erroneously considered to be smaller than unity is Λ2≡(me/mi)1/2 (qRνi/ci)2. The theory presented here encompasses the more general conditions, where Λ1∼1, Λ2∼1.

The characteristics of the toroidal ion temperature gradient (ITG) instability, considered as the main source of anomalous transport in the low (L) confinement mode of tokamaks, are analysed for the conditions of the radiatively improved (RI) mode triggered by seeding of impurities. Based on experimental profiles from TEXTOR-94 we deduce that the ITG-induced turbulence is quenched in the RI-mode in a large part of the plasma radius. Under those conditions the dissipative trapped electron (DTE) instability dominates the transport. The peaking of the electron density, one of the most remarkable features of the L-RI transition, is explained by the difference in contributions of ITG and DTE unstable modes to the diffusive and convective components of the electron flow. The shearing of the plasma toroidal velocity observed in the RI-modes with unbalanced neutral injection is successfully modelled by taking into account both the anomalous plasma viscosity due to ITG-turbulence and predictions of the revisited neoclassical theory.

This paper is concerned with the nonlinear development of the Farley‐Buneman instability in the equatorial electrojet. The stabilization mechanism is as follows. A turbulent (negative) vertical electron flux 〈δυe,xδn〉 develops in the turbulent layer; to preserve charge neutrality, the (positive) vertical ion flux decreases accordingly. Hence the secondary electricfield and the electrojet electron current also decreases, thereby quenching the instability. The predictions of the theory agree quite well with available experimental evidence.