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
E. Passoth et al., 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, Journal of physics. D, Applied physics, 30(12), 1997, pp. 1763-1777
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.