Floating potential method using a thermionic emissive probe including an ionizing and collisional presheath

Abstract

This article studies the interaction of an argon plasma with an emissive probe considering the effect of both ionization and ion-neutral collisions. The floating potential is determined from the I-V characteristic curves as a function of the probe temperature, neutral gas pressure and plasma electron temperature. This potential increases with increasing probe temperature until reaching the plasma potential, exceeding the saturation value previously indicated by other authors. Finally, a relationship between the plasma electron temperature and the probe temperature at which the floating potential reaches the plasma potential is shown, demonstrating that these probes can be used for diagnosis of the plasma electron temperature.

Analysis of the virtual cathode and floating potential of a thermionic emissive probe operating in the space-charge-limited regime

“…One important result of this paper is that the behaviour of the floating potential will depend on the probe radius. As shown in a previous work [38] for the same conditions studied in this paper, the floating potential does not saturate, and can even reach the plasma potential before the emissive probe reaches the SCL limit. However, this work shows that when a larger probe radius is considered, the probe can operate in the S-region at floating conditions.…”

“…The middle and bottom parts of this figure depict in more detail this characteristic curve for each region, and the formula used to fit the numerical data. In the T-region, there is no reflection of emitted electrons and this formula is [38] J w (ϕ w ) = a 0 + a 1 e eϕw/kBTe + c…”

“…This saturation value is considered an approximation of the plasma potential [10,29]. Some authors have shown that this saturation value is caused by the formation of the virtual cathode [8,11,[22][23][24] and is on the order of k B T e /e below the plasma potential [16,23,30] Others have found that the floating potential can not only reach the plasma potential, but it can even exceed it [31][32][33][34][35][36][37][38].…”

Determination of Plasma Potential Using an Emissive Probe with Floating Potential Method

“…The inflection-point method adopts one emissive probe and determines the plasma potential as the peak in the first derivatives of the measured current–voltage curve [ 23 ]. The floating potential method uses one emissive probe that is electrically floated with plasma [ 24 , 25 ]. With strong electron emission equal to the incoming electron flux from plasma, its floating potential approximates to the plasma potential; thus, this method determines the plasma potential as the floating potential.…”

“…Compared with other methods, the floating potential method is regarded to be more effective and convenient for measuring plasma potential due to its relatively simple system elements and capability for time-transient measurements [ 25 , 26 , 27 , 28 , 29 , 30 ]. Despite considering its long history, its underlying physics has yet to be fully understood.…”