A study on the modification of density and plasma potential in presence of electron temperature fluctuations (abstract)

Abstract

Electron temperature fluctuations have been investigated in the plasma of “Thorello,” a small toroidal device (minor radius a=8 cm and major radius R=40 cm) which produces a magnetized plasma in steady-state condition without rotational transform (n≅1011 cm−3; Te≅2 eV; B=1000 G).1 In our experiment two techniques are applied to estimate T̃e: triple Langmuir probe technique and fast-sweep Langmuir probe method, in both cases, varying the density of the neutral source (in our plasma, H2). The so-called triple probe is the conventional method used to investigate T̃e, while fast-sweep Langmuir probe is a new diagnostic technique recently developed on Thorello.2 Our measurements have been performed in the frequency range between 0 and 500 kHz. The turbulent plasma exhibits a spectrum quite broad in frequency: power spectra of floating potential and density show the characteristic frequencies are in the range 0–100 kHz, while power spectra of electron temperature can extend up to 150–200 kHz. In the plasma core relative levels of electron temperature fluctuations are about 20%, but in the edge region these relative levels raise up to 50%. In situations where electron temperature fluctuations are not negligible, the measurement of T̃e is essential for obtaining a correct interpretation of density and potential fluctuations. In our case the level of electron temperature fluctuations is not negligible in the edge region and it was possible to separate ĨSi and ñ according to the hypothesis ĨSi/ISi=ñ/n+12T̃e/Te and to the theory of thermal drift wave turbulence. Then we can particularly compare the mean values of Te, the level of temperature fluctuations measured in the same plasma, the power spectrum, and the correlation between temperature, density, and potential fluctuations applying the digital correlation technique.3