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

Abstract

The emissive Langmuir probes are made up of thin metallic wires exposed to the plasma and heated up to the electron thermoionic emission by a DC current <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1,2,3</sup> . They are essentially used to determine the local plasma potential V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sp</inf> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1,2,3</sup> and the electron temperature T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</inf> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> . The weak and strong electron operation emission regimes of the probe are determined by means of the probe temperature T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">w</inf> . For probe bias voltages V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</inf> &#60; V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sp</inf> the experimental evidence points out that a fraction of the emitted electron population returns to the probe after colliding. The probe temperature dependent voltage current curves were found in agreement with a simple model, which accounts for this returned electron current. Therefore for low positive probe polarization voltages V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</inf> ⌷V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sp</inf> two electron populations also coexist close to the probe; the electrons from the plasma and also the local emitted electrons of temperature TW that slightly alter the local potential around the probe. Thus, the electron saturation current results from the sum of two drained electron currents, and one of them relies on the wire temperature. The determination of the plasma potential and the electron temperature for polarization potentials V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</inf> &#60; V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sp</inf> by using the floating potential of the probe is also investigated. The plasma potential was obtained from the measurements of the floating potential as a function of the probe temperature. A sharp transition between two different regimes is observed at the temperature where floating potential equals the plasma potential. However, while on theoretical grounds the determination of the electron temperature T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">e</inf> is also possible <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> , the results obtained when using emissive and collecting probes differ. The different electron populations close to the probe when V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</inf> &#60; V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sp</inf> would be responsible of this disparity between the experimental results found with both probes.