How plasma parameters fluctuations influence emissive probe measurements

Abstract

Équipe 107 : Physique des plasmas chaudsInternational audienceRelationship between the floating potential of an emissive probe and plasma potential oscillations is studied in the case of controlled oscillations of plasma parameters. This relationship is compared to a quasi-static model for floating potential oscillations that assumes a constant emission current and includes the fluctuations of plasma parameters (density and electron temperature). Two different plasma regimes are considered. In the first one, the model is coherent with experimental results. In the second, the model does not fulfill one of the assumption due to the evidence of emission current oscillations when the mean emission current exceeds a given threshold. This second regime highlights the importance of taking into account emission current oscillations in the interpretation of emissive probe measurements. Nevertheless, discrepancies are still observed between emissive probe floating potential and plasma potential oscillations

Strongly emissive plasma-facing material under space-charge limited regime: Application to emissive probes

“…However, the present comparison demonstrate the basic features of the space-charge limited emission model and the validity of the hypothesis used. [4] and two experiments performed on the von-Karman plasma experiment [5]. Notice that the mean density n0 for the Mirabelle's case is a rough estimation as a current leak in the electronic was affecting the measurement.…”

“…The first set of experiment was conducted in the Mirabelle device [4] and the two others on the von-Kármán plasma machine [5,16] -these latter sets will be referred to as VKP1 and VKP2. The mean plasma parameters of these experiments are recalled in Tab.…”

“…Moreover, the experimental investigation reported in [5] demonstrated that the fluctuation level of the floating potential of emissive probes follows a thermoemission limited current model only for low emission currents. Above a critical value of the emission current, fluctuations of the floating potential go clearly off the model curve.…”

“…While it is now widely accepted that the time-averaged value of the floating potential of emissive probes can equal the surrounding plasma potential in cold plasmas [3], it has been recently shown that the fluctuations of the floating potential can differ significantly from the fluctuations of the plasma potential, both in phase and amplitude [4,5]. Moreover, the experimental investigation reported in [5] demonstrated that the fluctuation level of the floating potential of emissive probes follows a thermoemission limited current model only for low emission currents.…”

“…More specifically, we derive current-voltage characteristics of emissive probes in the space-charge limited regime, as the classical model for thermoemission limited current regime did not account for experimental measurements described in Ref. [4,5]. The thermoemission regime exists as long as the potential profile in the sheath is monotonic (see Fig.…”