Experimental data obtained on the TCABR tokamak (R = 0.61 m, a = 0.18 m) with an electrically polarized electrode, placed at r = 0.16 m, is reported in this paper. The experiment was performed with plasma current of 90 kA (q = 3.1) and hydrogen gas injection adjusted for keeping the electron density at 1.0 × 1019 m−3 without bias. Time evolution and radial profiles of plasma parameters with and without bias were measured. The comparison of the profiles shows an increase of the central line-averaged density, up to a maximum factor of 2.6, while Hα hydrogen spectral line intensity decreases and the C III impurity stays on the same level. The analysis of temporal behaviour and radial profiles of plasma parameters indicates that the confined plasma enters the H-mode regime. The data analysis shows a maximum enhanced energy confinement factor of 1.95, decaying to 1.5 at the maximum of the density, in comparison with predicted Neo–Alcator scaling law values. Indications of transient increase of the density gradient near the plasma edge were obtained with measurements of density profiles. Calculations of turbulence and transport at the Scrape-Off-Layer, using measured floating potentials and ion saturation currents, show a strong decrease in the power spectra and transport. Bifurcation was not observed and the decrease in the saturation current occurs in 50 µs.

Investigations of chaotic particle transport by drift waves propagating in the edge plasma of tokamaks with poloidal zonal flow are described. For large aspect ratio tokamaks, the influence of radial electric field profiles on convective cells and transport barriers, created by the nonlinear interaction between the poloidal flow and resonant waves, is investigated. For equilibria with edge shear flow, particle transport is seen to be reduced when the electric field shear is reversed. The transport reduction is attributed to the robust invariant tori that occur in nontwist Hamiltonian systems. This mechanism is proposed as an explanation for the transport reduction in Tokamak Chauffage Alfvén Brésilien [R. M. O. Galvão et al., Plasma Phys. Controlled Fusion 43, 1181 (2001)] for discharges with a biased electrode at the plasma edge.

Alfvén eigenmodes (AEs) were studied in neutral beam injection (NBI) heated plasmas in the TJ-II stellarator using a heavy ion beam probe (HIBP) in the core, and by Langmuir (LP) and Mirnov probes (MP) at the edge. AEs were detected over the whole plasma radius by the HIBP with a spatial resolution of about 1 cm. AE-induced oscillations were detected in the plasma density n e, electric potential φ and poloidal magnetic field B pol with frequencies 50 kHz < f AE < 300 kHz. The LP, MP and HIBP data showed a high level of coherency for specific branches of AEs. Poloidal mode wave-vectors k θ , mode numbers m(m < 8) and propagation velocities V θ  ∼ 30 km s−1 were detected for various branches of AEs, having different radial locations. When the density rose due to NBI fuelling, the AE frequency decreased as predicted by the Alfvén law . During the AE frequency decay the following new AE features were observed: (i) the poloidal wave-vector k θ and mode number m remained constant, (ii) the cross-phases between the oscillations in B pol, n e and electric potential remained constant, having an individual value for each AE branch, (iii) V θ decreased proportional to the AE frequency. The interaction of the AEs with the bulk (thermal) plasma resulted in clearly pronounced quasi-coherent peaks in the electrostatic turbulent particle flux spectra. Various AE branches exhibited different contributions to the particle flux: outward, inward and also zero, depending on the phase relations between the oscillations in E pol and n e, which are specific for each branch. A comparison with MHD mode modelling indicated that some of the more prominent frequency branches can be identified as radially extended helical AEs.

A new regime of runaway discharges has been found in TCABR (Tokamak Chauffage Alfvén Brésilien). This regime is obtained by initiating the discharge with low filling pressure and, after the initial current rise, maintaining a large filling rate. The line density reaches a maximum value around 2 × 10 19 m −3 , during the current ramp-up phase, and then drops by a factor of around four in the quasi-stationary phase of the discharge, when a new regime is achieved. The most distinctive features of this regime, as compared to 'conventional' runaway discharges reported in the literature, are (i) maintenance of the runaway discharge, with the plasma current almost entirely provided by the runaway beam, in a cold background plasma and with strong neutral gas injection; (ii) enhancement of the relaxation instability with strong spikes in the Hα emission and loop voltage correlated with sawtooth relaxation of the line density; and (iii) plasma detachment from the limiter. A simple phenomenological model, based upon straightforward particle and energy balance calculations, is proposed to explain the experimental observations. According to this model, the plasma is rather cold and the short pulses of gas ionization and the related density spikes are due to sudden plasma heating caused by the relaxation instability. Furthermore, it seems that the runaway generation for the conditions of the experiments can be explained only if the secondary generation process is invoked.

Using a voltage biased electrode inside the edge of the tokamak TCABR, H-mode discharges with strong, Regime I, and partial or total suppressed MHD activity, Regime II, were obtained. In this paper we present the results of the study of these two regimes of operation. The experiment was carried out adjusting the tokamak parameters to obtain discharges with strong or weak MHD activity, without biasing in both cases. During the shots the plasma current varied to cover a range of safety factor from 2.8 up to 3.6, allowing the Hmode barrier to interact with the magnetic islands. Subsequently, the bias was applied and shots with and without MHD activity were obtained. An array of 22 Mirnov coils was used to detect the magnetic oscillations and a triple Langmuir probe to measure plasma floating potentials and ion saturation currents. The results show that the dominant modes are m=2, n=1 and m=3, n=1 for partial excitation and suppression, respectively. In both cases strong decrease of the radial electric field is detected with destruction of the transport barrier and of the H mode regime. The measurements include temporal behaviour of edge transport, turbulence, poloidal electric and magnetic fields, edge density, radial electric fields and radial profile of H α line intensity. The explanation of the excitation and suppression processes is discussed in the text.