Numerical experiment to estimate the validity of negative ion diagnostic using photo-detachment combined with Langmuir probing
- Laboratoire des plasmas de décharges, Centre de Développement des Technologies Avancées, Cité du 20 Aout BP 17 Baba Hassen, 16081 Algiers (Algeria)
- Plasma Research Laboratory, School of Physical Sciences and NCPST, Dublin City University, Dublin 9 (Ireland)
- Unité de Recherche Matériaux et Energies Renouvelables, BP 119, Université Abou Bekr Belkaïd, Tlemcen 13000 (Algeria)
- Istituto di Metodologie Inorganiche e di Plasmi, CNR, via Amendola 122/D, 70126 Bari (Italy)
- Laboratoire de Physique des Plasmas, (CNRS, Ecole Polytechnique, Sorbonne Universités, UPMC Univ Paris 06, Univ Paris-Sud), École Polytechnique, 91128 Palaiseau Cedex (France)
- Laboratoire d'Electronique Quantique, Faculté de Physique, USTHB, El Alia BP 32, Bab Ezzouar, 16111 Algiers (Algeria)
This paper presents a critical assessment of the theory of photo-detachment diagnostic method used to probe the negative ion density and electronegativity α = n{sub -}/n{sub e}. In this method, a laser pulse is used to photo-detach all negative ions located within the electropositive channel (laser spot region). The negative ion density is estimated based on the assumption that the increase of the current collected by an electrostatic probe biased positively to the plasma is a result of only the creation of photo-detached electrons. In parallel, the background electron density and temperature are considered as constants during this diagnostics. While the numerical experiments performed here show that the background electron density and temperature increase due to the formation of an electrostatic potential barrier around the electropositive channel. The time scale of potential barrier rise is about 2 ns, which is comparable to the time required to completely photo-detach the negative ions in the electropositive channel (∼3 ns). We find that neglecting the effect of the potential barrier on the background plasma leads to an erroneous determination of the negative ion density. Moreover, the background electron velocity distribution function within the electropositive channel is not Maxwellian. This is due to the acceleration of these electrons through the electrostatic potential barrier. In this work, the validity of the photo-detachment diagnostic assumptions is questioned and our results illustrate the weakness of these assumptions.
- OSTI ID:
- 22490993
- Journal Information:
- Physics of Plasmas, Vol. 22, Issue 7; Other Information: (c) 2015 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA); ISSN 1070-664X
- Country of Publication:
- United States
- Language:
- English
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