LANGMUIR PROBE DIAGNOSTICS OF A LOW-TEMPERATURE NONISOTHERMAL PLASMA IN A WEAK MAGNETIC-FIELD

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%).