Measurement of electron distribution function by means of direct differentiation of probe characteristic

Conclusion

Using the above described method we measured the electron distribution function in homogeneous positive column of a low-pressure discharge for different repetition frequencies and different time intervals of recording the probe characteristic. An example of the second derivative of the probe charecteristic measured at a repetition frequency of 1 kHz is shown in Fig. 3. The probe characteristic was recorded in 10 us intervals, i.e. 0.01 period.

The method can be easily applied for measurements in a steady state as well as in a periodically changing discharge. Substantial advantage of this method lies in the possibility to measure the electron distribution function and its changes during short time intervals (of the order of several us), which enables to apply the method for measurements in unstable and extinguishing plasmas.

Works on improving this method even further are presently under way; they are focussed in the first place on using this method for the measurements of the electron distribution function in an extinguishing plasma, where the maximum of electron distribution function lies at energies of the order of 0.1 eV. The method of superimposing a small harmonic signal on the probe bias is virtually impracticable for such low energies, because this method gives accurate results only for the probe voltages that are much higher than the amplitude of superimposed signal.

The procedure for testing the correct operation of the apparatus and its accuracy in determining the second derivative, as described above, is applicable also for other methods used in differentiating the probe characteristic, where only indirect methods were used so far.