Measurements of Plasma Properties Using the Floating Potential of Emissive Langmuir Probes

Abstract

The measurement of the plasma potential of unmagnetized Maxwellian plasmas by using the floating potential of emissive Langmuir probes is discussed. The temperature of the probe was monitored in order to estimate the emitted thermoionic electron current and to determine the limits of the strong electron emission regime. Under these ideal conditions, the measurements of the plasma potential of the emissive probe are cross checked against those of collecting Langmuir probes. In agreement with previous works, the current voltage curves of emissive probes show the temperature dependent electron saturation currents. An empirical expression is suggested for the dependence of this saturation current with the probe temperature which recovers the response of collecting probes when the probe is cold. The experimental data indicate that the floating potential of the emissive probe is close to the local plasma potential only when the emitted thermoionic electron current is similar to the electron saturation current. Larger electron emission currents lead to small increments the floating potential over the local plasma potential. Our results suggest that the electron saturation current of a hot emissive probe is composed by the thermal electrons from the plasma and a returned fraction of the thermoionic emitted electrons.

Emissive Langmuir probes in the strong emission regime for the determination of the plasma properties

“…1 has been previously reported by different authors but has not received a fully satisfactory explanation yet [14]. As proposed in [10], Fig. 5 shows the expression for I co (T p ) that could be approximated by…”

“…We have concluded that V p \ could be determined by using hot emissive Langmuir probes plotting V F (T P ) as a function of T p (or equivalently, probe heating current In) as discussed in [8]- [10] and [14]. The abrupt change in the slope of V F (T P ) that appears at temperature T ph where V F (T pl ) ~ V ph determines the value of the plasma potential.…”

“…This limit is equivalent to drop in (7) the additional electron group in (6) that is collected back by the probe. Finally, because of the larger values of jth(T p ) for the transition temperatures T p \, the ratio between the currents in (9) is closer to unity when R > 1 in (3) [10]. Finally, in order to compare the numerical calculations of the roots of (8) with the experimental data in Fig.…”

“…In these conditions, the floating potential I p (T p , V F ) = 0 of the probe in a Maxwellian plasma for V F (T P ) < V p \ is given by [8]- [10] …”

Propagator integral method for plasma physics kinetic equations with practical applications

“…Moreover, in plasmas is typical to analyse the electrical current density, defined as J γ = [q γ ] = q γ n γ u γ , which is a parameter related to the electrical current measured in laboratories by means of, for example, electrical probes [28,[83][84][85]. In addition, total heat flux can be computed by Q γ = [v 2 ]/2n γ .…”