Understanding Langmuir probe characteristics of a magnetized plasma column in partial contact with grounded probe reference

Abstract

A: The article presents an experimental investigation on Langmuir probe measurements in a magnetized plasma column which exhibits two-temperature electron populations. It is a known fact that probe I(U) traces follow the usual exponential law if the measurements are performed with a reference electrode in good contact with plasma; which is usually a grounded discharge electrode. However in the present case, as the grounded probe reference is not a part of the discharge circuit the resulting I(U) analysis is not straightforward. It is found that owing to the high impedance between bulk plasma and probe reference, the probe measurement results in lower values of electron saturation current as compared to the ideal scenario. An appropriate correction is thus required to account actual electron saturation current and thereby to extract subsequent plasma parameters. Therefore, a simple analysis technique has been proposed to interpret probe I(U) traces resulting from such magneto-plasma devices, where reference to the probe is in partial/ poor contact with the bulk plasma. K: Plasma diagnostics -probes; Data analysis; Nuclear instruments and methods for hot plasma diagnostics; Plasma generation (laser-produced, RF, x ray-produced) 1Corresponding author.

Hairpin probe assisted saturation current ratio method to determine plasma electronegativity

“…method using LP (V P,LP ) and HP (V P,HP ), respectively. The hot (n e,h ) and total (n e,b + n e,h ) electron densities are evaluated by using their respective saturation currents, by extrapolating the slopes up to the plasma potential [56]. And, the bulk electron density is evaluated by subtracting hot electron density (n e,h ) from total electron density (n e,b + n e,h ).…”

“…Next, the corrected value of electron saturation current (I e,sat ) is obtained by extrapolating the linear line in the natural log plot of electron current with the probe bias [56], provided the exact value of plasma potential (V p ) is known. Nonetheless, the exact value of plasma potential can also be obtained using a DC biased HP.…”

“…The underestimation of the electron saturation current and/or the overestimation of the ion saturation current are the likely causes of this decrease in the SCR. The reasons for the underestimation of the electron saturation current are generally attributed to several factors, such as, probe surface contamination [54,55], an increase in plasma ground sheath resistance [55,56], the presence of an external magnetic field [55,[57][58][59][60], and non-Maxwellian [19,55,61,62] or drifting plasmas [63]. This leads to an underestimation of the plasma potential, either determined from the knee in the amperevoltage characteristic or using the first derivative method [56].…”

Equilibrium properties of inhomogeneous partially-magnetized plasma containing negative ions

“…The probe characteristics are obtained by applying a train of triangular voltage generated by a signal generator and amplified using a linear amplifier (WMA-300 Falco Systems). The details of the Langmuir probe circuit can be found in our previous paper [55]. The data acquisition from the oscilloscope (Tektronix 3054C) is carried out using Lab-VIEW software [56].…”

Addressing the anomalies in determining negative ion parameters using electrostatic probes

“…As the probe collects electrons, an equivalent amount of positive ion current must reach the reference grounded electrode. If the reference electrode is far away from the plasma or it is improperly grounded, it could lead to a virtual resistance in the current path [14], which limits the electron saturation current to attain its maximum value. Furthermore, external magnetic fields can also reduce the electron saturation current.…”