AN EXPERIMENTAL-STUDY OF PLASMA-DENSITY DETERMINATION BY A CYLINDRICAL LANGMUIR PROBE AT DIFFERENT PRESSURES AND MAGNETIC-FIELDS IN A CYLINDRICAL MAGNETRON DISCHARGE IN HEAVY RARE-GASES

This article presents an experimental study that contributes to the pr oblem of interpretation of cylindrical Langmuir probe data obtained in a non-isothermal tow-temperature plasma in magnetic field. A discussi on on the influence of positive ion-neutral collisions on the charged particle density estimation is also given and the effect is demonstrat ed on the experimental data. A Maxwellian electron energy distribution is assumed throughout the present study. The Langmuir probe data are obtained in a cylindrical magnetron discharge in argon at pressures fr om 1.5 to 6 Pa and magnetic fields between 100 and 500 G. The radially movable Langmuir probe was made of either 100 mu m or 41 mu m diamete r tungsten wire in order to investigate the effect of the probe dimens ions on the estimated plasma density. The electron density is calculat ed from the electron current at the space potential (used as a referen ce) and from the OML collisionless theory. The ion density is calculat ed by using ABR-Chen theory without and with the correction due to the collisions of positive ions in the probe sheath. Also, the recent col lisional positive-ion-collection-theory is used for comparison. The re sulting numerical values of plasma density are compared over more than one order of magnitude change in the plasma density given by its radi al dependence in the cylindrical magnetron discharge. Optical measurem ents were made in order to quantitatively assess the neutral gas tempe rature in the discharge and the density of particles in excited states that could induce secondary electron emission from the probe surface and thus apparently enhance the positive-ion density estimated from th e probe positive-ion current. The effect of secondary electron emissio n from the probe surface on the probe data interpretation has been fou nd small compared to the experimental error limits and consequently no t substantial for our experimental conditions. In the range of our exp erimental conditions the ABR-Chen theory with the collisional correcti on gives the best agreement of the estimated numerical values of ion a nd electron densities in the whole range of its investigated change. A lso from our results it follows that the effect of the magnetic field on the thinner-probe-electron-current at the space potential and hence on the reference-electron-density-estimation is negligible within the experimental uncertainties up to a magnetic field strength of 500 G w hich was the maximum used in our experimental study.