Measurements of Plasma Properties Using Fast Sweep Langmuir Probes in Unmagnetized Weakly Ionized Plasmas

The design and operation modes of a small, low power ion plasma thruster and the properties of the emitted plasma plumes are discussed. The ion beam is extracted from a primary plasma produced by a stationary low pressure electric discharge where the ion production rate is essentially determined by the discharge current. The experiments evidence that the electron neutralization current controls the space charge levels of the outgoing ion current and also influences the spatial properties of emitted plasmas. The electron plasma density increases with the discharge and the electron neutralization currents, while decreases as the plasma plume expands. However, the corresponding electron temperatures decrease when the electron neutralization currents increments. The collisional origin of this electron cooling effect is excluded because of the large collisional mean free paths involved. Then, these electron energy losses during the neutralization of the ion beam would be caused by more subtle physical mechanisms than collisions. The experimental results are compared with previous numerical simulations and similar phenomena found in other experiments.

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“…13. This equipment bias the probes with voltage ramp signals of frequencies over 2.0 KHz added to a time independent DC polarization voltage.…”

Section: Langmuir Probe Diagnosticsmentioning

confidence: 99%

Ion beam neutralization and properties of plasmas from low power ring cusp ion thrusters

et al. 2012

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“…[9]. The emissive probes were made of a thin tungsten wire loop of 0.08 mm in diameter and variable lengths between 4.3 and 8.3 mm.…”

Section: Methodsmentioning

confidence: 99%

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

Tierno

Roibás

Domenech-Garret

et al. 2013

Contrib. Plasma Phys.

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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.

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“…1. These I-V characteristic curves for both collecting I P (V P ) and emissive I P (T P ,V P ) probes were obtained by means of a fast sweep biasing circuit discussed in [13]. Briefly, the probes are biased with voltage ramp signals with a repetition pulse over 2.0 kHz, and the values of the probe current and bias voltage were later digitized by means of a Yokogawa DL9140 digital oscilloscope for the subsequent analysis.…”

Section: Methodsmentioning

confidence: 99%

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

Tierno

Doménech

Donoso

et al. 2012

2012 Abstracts IEEE International Conference on Plasma Science

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Abstract-The determination of the plasma potential V^i of unmagnetized plasmas by using the floating potential of emissive Langmuir probes operated in the strong emission regime is investigated. The experiments evidence that, for most cases, the electron thermionic emission is orders of magnitude larger than the plasma thermal electron current. The temperature-dependent floating potentials of negatively biased V p < V^i emissive probes are in agreement with the predictions of a simple phenomenological model that considers, in addition to the plasma electrons, an additional electron group that contributes to the probe current. The latter would be constituted by a fraction of the repelled electron thermionic current, which might return back to the probe with a different energy spectrum. Its origin would be a plasma potential well formed in the plasma sheath around the probe, acting as a virtual cathode or by collisions and electron thermalization processes. These results suggest that, for probe bias voltages close to the plasma potential V p ~ V^i, two electron populations coexist, i.e., the electrons from the plasma with temperature T e and a large group of returned thermionic electrons. These results question the theoretical possibility of measuring the electron temperature by using emissive probes biased to potentials V p < V^i.

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Measurements of Plasma Properties Using Fast Sweep Langmuir Probes in Unmagnetized Weakly Ionized Plasmas

Cited by 15 publications

References 11 publications

Ion beam neutralization and properties of plasmas from low power ring cusp ion thrusters

Ion beam neutralization and properties of plasmas from low power ring cusp ion thrusters

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

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

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