On the harmonic technique to measure electron temperature with high time resolution

A detailed study of the harmonic technique, which exploits the generation o f harmonics resulting from excitation of the nonlinearity of the single Lan gmuir probe characteristic, is presented. The technique is used to measure electron temperature and its fluctuations in tokamak plasmas and the techni cal issues relevant to extending the technique to high bandwidth (200 kHz) are discussed. The technique has been implemented in a fast reciprocating p robe in the TEXTOR tokamak, gaining the ability to study denser and hotter plasmas than previously possible. A corrected analytical expression is deri ved for the harmonic currents. Measurement of the probe current by inductiv e pickup is introduced to improve electrical isolation and bandwidth. The t emperature profiles in the boundary plasma of TEXTOR have been measured wit h high spatial (similar to 2 mm) and temporal (200 kHz) resolution and comp ared to those obtained with a double probe. The exact expansion of the prob e characteristic in terms of Bessel functions is compared to a computationa lly efficient power series. Various aspects of the interpretation of the me asurement are discussed such as the influence of plasma potential and densi ty fluctuations. The technique is well suited to study fast phenomena such as transient plasma discharges or turbulence and turbulent transport in pla smas. (C) 1999 American Institute of Physics. [S0034-6748(99)04507-4].