Floating potential calculation for a Langmuir probe in electronegative plasmas and experimental validation in a glow discharge

Abstract

A radial Langmuir probe sheath model is used to make a prediction of the floating potential of a Langmuir probe immersed in an electronegative plasma. The new electronegative plasma sheath model takes into account the positive ion and the negative ion thermal energies and is valid for any ion temperature value. The values predicted can be used for diagnosing and controlling an electronegative plasma, and we compare them with measurements of the floating potential in an Argon plasma and in a Neon plasma with with two distinct electron populations at different temperatures. We have found that the agreement is very good in a wide range of plasma pressure and discharge current values, and thus the model is applicable for electronegative plasmas. Moreover, the model can be used to measure the energy with which the ions will collide with the surface of the probe.

Low Electron Temperature Plasma Diagnosis: Revisiting Langmuir Electrostatic Probes

“…In this section, a method for measuring the IVCP, that is used throughout the article and that has been widely used by the authors in other works, is presented [14,15,17,18,22,23,36,37,49,51].…”

“…It assumes that, after each ion-neutral collision, which, as stated before, is the only type of collision to be considered, the ion loses all its kinetic energy; therefore, after the last collision, it falls towards the probe in a radial movement. Initially, this theory did not consider the thermal movement of the ions, that is, it assumed = 0, and was extended by the authors of this article for the case of ≠ 0 [15,[44][45][46][47][48][49][50].…”

Influence of the Ion Mass in the Radial to Orbital Transition in Weakly Collisional Low-Pressure Plasmas Using Cylindrical Langmuir Probes

“…The current-voltage characteristic of the cylindrical Langmuir probe can be used to obtain an indirect measurement of the parameters that characterize the plasma in the zone of the discharge where it is placed. The plasma potential, V plasma , the floating potential, V f loat , and the electron energy distribution function, EEDF, can be measured [1,2,4,6,26,[34][35][36][37][38][39][40][41]. Regarding the measured EEDF, it is checked that, in every measurement, the EEDF can be considered as following the distribution of Maxwell-Boltzmann [1,4,6,26,28], which is essential since the assumption of a Maxwellian EEDF is made in both radial and orbital theories.…”

“…For argon, neon and helium plasmas, the discharge DC current is always lower than 12.5 mA. The pressure range for the argon plasma is p(Pa) ∈ [2,10]; for the neon plasma, it is p(Pa) ∈ [10,35], and for the helium plasma, it is p(Pa) ∈ [13,37]. Given that the transition in the validity of the orbital and radial theories is found in the helium plasma, a total of 448 current-voltage characteristics were measured, while in argon and neon plasmas, for which no transition was found, a total of 171 and 111 current-voltage characteristics, respectively, were measured.…”

Accurate measurement of the ion saturation current collected by a cylindrical Langmuir probe in cold plasmas

“…[ 17 ] The original ABR theory, adapted by Chen [ 18 ] for cylindrical geometry, is valid only for positive ions having a null temperature (, with and T e being the positive ion and electron temperature, respectively), and it has been extended by the authors for . [ 2,19–24 ] It is important to be able to discriminate which theory is applicable in each plasma condition, as each one provides different results for the ion density in the plasma diagnosis. Moreover, this fact implies a paradox in the analysis of positive ion saturation zone in the Langmuir probe current–voltage characteristic when used in plasma diagnosis, as we do not know a priori which theory is valid before it is applied in the diagnosis.…”