P. Kudrna et E. Passoth, LANGMUIR PROBE DIAGNOSTICS OF A LOW-TEMPERATURE NONISOTHERMAL PLASMA IN A WEAK MAGNETIC-FIELD, Contributions to Plasma Physics, 37(5), 1997, pp. 417-429
This article presents measurements by a cylindrical Langmuir probe in
the plasma of a DC cylindrical magnetron discharge at the pressure 1.5
Pa that aim at the experimental assessment of the influence of a weak
magnetic field to the estimation of the electron density when using c
onventional methods of probe data interpretation, The probe data. was
obtained under the presence of a weak magnetic field in the range 1.10
(-2)-5.10(-2)T. The influence of the magnetic field on the electron pr
obe current is experimentally assessed for two cylindrical probes with
different radii, 50 mu m and 21 mu m. This assessment is based on com
parison of the values of the electron density estimated from the elect
ron current part with the values of the positive ion density estimated
from the positive ion current part of the probe characteristic respec
tively by assuming that at the magnetic field strengths used in the pr
esent study the probe positive ion currents are possible to be assumed
as uninfluenced by the magnetic field, For interpretation of the prob
e positive ion current two theories are used and compared to each othe
r: the radial motion model by Alien, Boyd and Reynolds [10] and Chen [
11] and the model that accounts for the collisions of positive ions wi
th neutrals in the probe space charge sheath that we call Chen-Talbot
model [8], At lower magnetic field 3.10(-2) T the positive ion density
values interpreted by using the Chen-Talbot model [8] are in better a
greement with tile values of electron density compared to those obtain
ed by using the theory [10,11]; therefore the model [8] is used for ca
lculation of the positive ion density from the probe data at higher ma
gnetic fields. The comparison of the positive ion and electron density
values calculated from the same probe data at higher magnetic fields
shows that up to the magnetic field strength 4.10(-2) T with the probe
100 mu m and up to 5.10(-2) T with the probe 42 mu m in diameter resp
ectively the decrease of the magnitude of the electron current at the
space potential due to the magnetic field does not exceed the error li
mits that are usual for Langmuir probe measurements (absolute error +/
- 20%).