Spatial structure of nonthermal fluctuating electric fields in the TJ-I tokamak

Spatial-resolved measurements of Doppler temperature of protons and impurities in the TJ-I tokamak have revealed that they cannot fit in the generally accepted frame used for line width data interpretation in tokamaks. Within this frame, it is assumed that due to the density and toroidal field ranges of these experiments, the line width is dominated by thermal Doppler broadening, i.e. Stark effects due to plasma microfields, Zeeman effect and the influence of plasma turbulence are negligible. The strong collisional coupling among different type of ions is used to justify that protons and impurities must have the same temperature. By contrast, we claim in this work based on TJ-I tokamak data, that low level of stochastic electric fields, even being weak enough not to produce quasi-static Stark broadening, can induce observable effects on the ion spectral line widths. The non-thermal velocity fluctuations due to these fields produce a differential effect on the apparent temperature of protons and impurities. Experimental profiles of proton and impurity temperature, in combination with a data analysis code have been used to deduce in the TJ-I tokamak the spatial structure of radial electric field fluctuations and their behaviour at three different plasma densities. First, we summarize the experimental and the data analysis methods and second, we present typical results obtained in this tokamak. The influence of magnetic fluctuations is also discussed. (author) 7 refs., 2 figs