A detailed study of the harmonic technique, which exploits the generation of harmonics resulting from excitation of the nonlinearity of the single Langmuir probe characteristic, is presented. The technique is used to measure electron temperature and its fluctuations in tokamak plasmas and the technical issues relevant to extending the technique to high bandwidth ͑200 kHz͒ are discussed. The technique has been implemented in a fast reciprocating probe in the TEXTOR tokamak, gaining the ability to study denser and hotter plasmas than previously possible. A corrected analytical expression is derived for the harmonic currents. Measurement of the probe current by inductive pickup is introduced to improve electrical isolation and bandwidth. The temperature profiles in the boundary plasma of TEXTOR have been measured with high spatial ͑ϳ2 mm͒ and temporal ͑200 kHz͒ resolution and compared to those obtained with a double probe. The exact expansion of the probe characteristic in terms of Bessel functions is compared to a computationally efficient power series. Various aspects of the interpretation of the measurement are discussed such as the influence of plasma potential and density fluctuations. The technique is well suited to study fast phenomena such as transient plasma discharges or turbulence and turbulent transport in plasmas.

Pick-up si@s f " h g m u i r pmbes have been measured in the boundary of the TEXTOR tokamak during discharges with different transp~n pmperlies namely attached. detached and electrically induced H-mode plasmas. These signals have then been analysed with respect to their dimensionality. The dimensionality is found to be high for attached plasmas, even higher for the H-mode discharges induced by polarizaIion and it is low for detached plasmas. It is found that lhe dimensionality depends on the radial position of the probes and on the above discharge conditions. Changes in the dimensionality are accompanied by changes in the frequency specua of lhe pick-up signals and also by changes in such plasma parameters as rotalion speed, rotation shear and radii particle and heat transport

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Direct current (DC) driven and turbulence driven cross-field particle fluxes have been deduced from Langmuir probe measurements during discharges with ion cyclotron resonance heating (ICRH) in the TEXTOR tokamak. Spectral analysis of fluctuations of the floating potential at spatially separated probe pins has been used to determine the velocity associated with the poloidal rotation of the boundary plasma. Characteristics of the turbulence spectra of density and potential fluctuations at the edge during ICRH have been measured and compared with the corresponding spectra during standard ohmically heated discharges. It has been observed that both DC driven and turbulence driven fluxes increase significantly during ICRH. The measured high particle fluxes indicate motion in DC convective cells. The DC convective flux and the turbulence driven flux are partly counteracting.