Comparison of measurements with different types of divertor Langmuir probe in EAST tokamak

Abstract

Divertor Langmuir probes (Div-LPs), embedded in the divertor target plates, are the important diagnostic tools to measure the plasma parameters in EAST divertor region. A series of measurement methods, i.e., single-probe, double-probe and triple-probe schemes, are applied and compared for the Div-LPs on EAST, to get the electron temperature Te and the electron density ne at the same poloidal position, respectively. It is found that the electron temperature Te and the electron density ne measured by the single-probe and the double-probe schemes are basically the same during the plasma discharge phase. Further analysis of the electron energy distribution function (EEDF) represented by the first derivative curve method shows that the electrons conform to a Maxwell distribution, which indicate that the three methods can be applied in the EAST divertor region. Besides, the domed and flush-mounted probe tips for measuring the plasma parameters are also compared, and the results show a good match. The sheath potential coefficient Λ is also calculated from the I-V characteristic curves by two ways, i.e. Λp=(Vplasma-Vf)/Te and ΛI= ln (|Ies/Iis|), and compared with the traditional value Λ=2.8, the results indicate that the ∼1.5ΛI and Λ=2.8 are available to replace the Λp under the different strength of toroidal magnetic field in EAST tokamak.