Experimental results concerning the plasma response to the biasing of the tokamak ISTTOK localized limiters, on a strong flat-top plasma current reference discharge, are reported. Modifications of central beta as well as of energy confinement time are determined through timeresolved measurements of the line-averaged plasma density, electron density profile, electron temperature and ohmic power. Gross particle confinement variations are confirmed by the associated changes of the ratio between the line-averaged electron density and the H α radiation level. Plasma stability modifications are analysed by measurements of the plasma column transverse displacement, plasma poloidal rotation frequency and sliding fast Fourier transform spectra of both the magnetic and the electron density fluctuations. The evolution of the amplitude as well as the frequency of the most important tearing modes is determined. Negative bias leads to better particle and energy confinement, and improved stability. Positive bias reduces both confinement and stability, causing a significant transitory vertical displacement of the plasma column as well as of its current axis.

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This article describes the conventional Thomson scattering diagnostic that has been developed for multipoint measurements of the electron temperature and plasma density of the tokamak ISTTOK plasma. The design is based on a single laser pulse, two multi-input three channel spectrometers, one digital oscilloscope, and a time-delay technique. Special collection optics, optimized detectors, and high transmission monofibers with large numerical aperture have been used to increase the sensitivity and spatial resolution. The number of measurement points can be extended by increasing the number of fibers without altering the collection optics or light relay system. Results are in good agreement with those determined with other diagnostics.

Long-distance correlations (LDCs) of plasma potential fluctuations in the plasma edge have been investigated in the TCABR tokamak in the regime of edge biasing H-mode using an array of multi-pin Langmuir probes. This activity was carried out as part of the scientific programme of the 4th IAEA Joint Experiment (2009). The experimental data confirm the effect of amplification of LDCs in potential fluctuations during biasing recently observed in stellarators and tokamaks. For long toroidal distances between probes, the cross-spectrum is concentrated at low frequencies f < 60 kHz with peaks at f < 5 kHz, f = 13–15 kHz and f ∼ 40 kHz and low wave numbers with a maximum at k = 0. The effects of MHD activity on the LDCs in potential fluctuation are investigated.

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