Parametrization of Laframboise's results for spherical and cylindrical Langmuir probes

Three new aspects regarding the analysis of Langmuir probe data are present ed. First, we demonstrate that the numerical results of Laframboise for sph erical probes can be parametrized easily for arbitrary ratios of the probe radius r(p) to the Debye length lambda(D). The ion current can be expressed in the form a(-X)(b), where a and b are parameters depending on r(p)/lambd a(D), and X is the dimensionless probe voltage. This functional form is the same as the one for cylindrical probes reported previously, but the values of a and b are different. Second, we use numerical simulations to show tha t unless the plasma potential V-s is known, it is in general difficult to d etermine accurately the form of the ion current characteristic I-i(V-p), an d thus the ion density N-i, from typical probe data. This is because I-i(V- p), N-i, and r(p)/lambda(D) are interdependent. Third, the simulations indi cate that the apparent electron energy distribution is very sensitive to th e exact form of I-i(V-p) and to the method by which I-i(V-p) is subtracted from the total probe current to obtain the electron current. A linear extra polation of I-i(V-p) is often adequate for determining the electron tempera ture, but assuming a constant ion current leads to electron energy distribu tions that appear to have two components with different electron temperatur es. Additional issues discussed include the consequences of a slightly coll isional probe sheath and the importance of end-effect corrections with a cy lindrical probe. (C) 1999 American Vacuum Society. [S0734-2101(99)07905-1].