Application of the triple-probe technique to magnetized plasmas

Abstract

The triple-probe technique is a diagnostic widely used to determine the electron temperature in various devices and plasma conditions. It was developed for measurements in low-pressure gas-discharge plasmas in the absence of a magnetic field. This paper presents a comparison of the experimental results obtained by swept Langmuir probe measurements with those obtained by the triple-probe technique in magnetized plasmas in three experimental machines with different magnetic field magnitudes (0.01-1.15 T). The reliability of the triple-probe results for the electron temperature in tokamak plasmas at higher magnetic fields is discussed. It was found that the larger the magnetic field, the more the triple-probe technique overestimates the electron temperature compared with single swept Langmuir probes. The explanation proposed in this paper is based on a shift in the floating potential towards the plasma potential in the presence of a magnetic field, yielding a more positive voltage measured by the triple-probe technique and therefore higher electron temperatures. Using the extended formula for the electron probe current in the presence of a magnetic field a correction factor is derived such that the triple-probe technique yields a temperature similar to that of the swept Langmuir probe techniques.

Characterization of the TJ-II stellarator plasma by means of reciprocating Langmuir probes

“…A good agreement between the plasma parameters values is observed when using the four-parameter fit and the FDPT. However, the TPT overestimates the electron temperatures, which is to be expected as found in [7]; the values of the other parameters are in agreement with those obtained by the FDPT.…”

“…Another reason why the TPT provides higher Te values is that the magnetic field alters the electron branch of the I-V characteristics and moves the floating potential closer to the plasma potential; this has the result of a larger difference between the positive and floating electrodes potentials of the triple probe (V+ -Vfl), the latter being proportional to Te in the TPT. The larger the magnetic field, the greater this overestimation [7].…”

“…Thus, if the probe is swept, it can provide measurements of the plasma edge parameters -the floating potential, Vfl, the ion saturation current density, jsat, the electron temperature, Te, the electron density, ne, and the plasma potential, Vpl. When several pins are installed on one probe head, different regimes of operation can be used for different purposes, e.g., applying the triple probe technique [5][6][7] for fast turbulent measurements.…”

“…In some cases, the real EEDF is non-Maxwellian in fusion devices [11,12]. Generally, three probe techniques have been employed to determine plasma parameters -the four-parameter fit [8][9][10], the FDPT and the triple-probe techniques (TPT) [5][6][7]. A good agreement between the plasma parameters values is observed when using the four-parameter fit and the FDPT.…”