Measurements of EEDF in Helium Flowwing Afterglow at Pressures 500 - 2000 PA

• Published in: Contributions to plasma physics (1988) Vol. 48; no. 5-7; pp. 461 - 466
• Main Authors: Korolov, I., Plašil, R., Kotrík, T., Dohnal, P., Novotný, O., Glosík, J.
• Format: Journal Article
• Language: English
• Published: Berlin WILEY-VCH Verlag 01-07-2008; WILEY‐VCH Verlag
• Subjects: afterglow; electron energy distribution function; Plasma; probe diagnostics
• ISSN: 0863-1042; 1521-3986
• DOI: 10.1002/ctpp.200810074

For measurements of recombination rate coefficients of slowly recombining ions with rate coefficients below 10–7cm3s–1 in Flowing Afterglow Langmuir Probe (FALP) experiment the plasma decay time has to be long and this requires buffer gas (helium) pressures in range 500 – 2000 Pa. Application of Langmuir probe for measurements of electron energy distribution function, EEDF (f (ε)), at these pressures is not trivial. The Druyvestein formula for calculating f (ε) from the second derivative of the electron current to the probe (I″e) cannot be used. More complex analyses of measured probe characteristic are required to obtain reliable EEDF. To characterize the extent of the required corrections, the probe characteristics in well‐defined flowing afterglow plasma were measured and two different methods were used to obtain EEDF. In the first method, the first and second derivative of probe characteristics were used to obtain EEDF [1]. In the second method, numerical correction was calculated [2] and applied to values obtained from Druyvesteyn formula. By comparison of obtained EEDF with Maxwell distribution we determine a region of electron energies where advanced analysis is necessary. It was found that in a helium afterglow at 250 K, with pressure 600 – 2000 Pa, where plasma is nearly Maxwellian with T e ∼ 250 – 600 K, the correction is necessary for electron energies ε ≤ 3.5 kBTe. At higher electron energies correction is negligible and is not necessary. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)