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.

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.

Spatial–temporal intermittency of floating potential and ion saturation current fluctuations is analyzed by using data obtained from two probes arrays in the scrape-off layer of the CASTOR tokamak [Proceedings of the 1996 International Conference on Plasma Physics (Nagoya) (International Atomic Energy Agency, Vienna, 1997), Vol. I, p. 322]. For these ion saturation current fluctuations with non-Gaussian probability density functions, a conditional averaging analysis shows coherent structures with correlation lengths and lifetimes larger for larger amplitude conditions. Nevertheless, there is no evidence of such large structures in the potential fluctuations. Furthermore, wavelet transforms are used to analyze these nonstationary fluctuations and obtain details not observed with the Fourier technique. So, examining wavelet power and coherence spectra, strong intermittency is found for both kinds of fluctuations, in a time scale two orders of magnitude higher than that observed in the conditional analysis. Moreover, during the discharges the observed wavelet quadratic coupling alters intermittently for some low-frequency components.

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Wavelet spectrum and bispectrum techniques are applied to study the development of temporal turbulence induced by a confinement toroidal magnetic field in a toroidal magnetoplasma created by radio frequency waves. For low magnetic fields the plasma is roughly uniform and the analyzed electrostatic linear frequency spectra are essentially determined by the driven radio frequencies. However, by increasing the toroidal magnetic field, gradients in the plasma radial profiles and broader frequency spectra are observed. Thus, spectral components with frequencies higher than those injected in the plasma are excited. Moreover, this variation of magnetic field also induces nonlinear phase coupling between low frequency coherent peaks and continuous high frequency spectral components.