Signals from the collector, modulation electrode, Langmuir and emissive probes are recorded by a digitizing oscilloscope and stored in the computer for analysis. Time series of the signals are recorded as 10235 words of 16 bit resolution. Analysis of the spectra and phase space reveals some typical nonlinear phenomena that are predicted by the van der Pol equation with an external force term. These are synchronization, periodic pulling and frequency pulling. Synchronization occurs when the internal plasma oscillation is completely suppressed by the external oscillation when the forcing frequency is close enough to the internal frequency and the amplitude of the external force is high enough.
Synchronization occurs in a certain frequency region that is called synchronization region.
From the dependence of the width of synchronization region on the external force amplitude the linear growth rate of the instability is measured. The dependence of the linear growth rate on the magnetic field density is measured in order to verify a possible connection between the linear growth rate and transport of charged particles across the magnetic field lines. Periodic pulling can be briefly described as periodically repeated incomplete synchronization. It is demonstrated experimentally from the spectra, time series and time dependence of the phase difference between the forcing signal and the system response. Frequency pulling can also be observed from the spectra.