Electron energy distribution functions in low pressure discharge plasmas have been investigated in helium, helium-neon and helium-nitrogen mixtures. The positive column of a discharge is examined by application of the probe analysis of Langmuir and Druyvesteyn. The second derivative of the probe current is obtained by measuring the second harmonic when an a.c. signal is superimposed on the d.c. bias applied to the probe. The electron energy distribution curves are analysed and are generally found to be markedly non-maxwellian. The high energy tail region is examined for variations in pressure, longitudinal discharge current, location of measuring probe and for different mixture ratios. The electron densities and average electron energies are determined from the summation of areas under the distribution curves. Electron drift velocities and mobilities are calculated from an equation for the longitudinal discharge current derived from the Schottky positive column theory. An asymmetrical double probe system is investigated and it is shown that the ratio of the reference probe area to that of the measuring probe must be 7000:1 to produce no distortion of the measurements. The necessary limit of probe radius to electron mean free path is also investigated. The relationship between the second harmonic of the probe current (i2w) and the amplitude (E) of the a.c. signal applied to the probe is investigated. A departure from the linear relationship of i2w v. E2 is observed at higher values of E, but there are differences in behaviour for the gases nitrogen and helium. A theoretical explanation is proposed for these differences.
|Date of Award||1977|