Nonlinear Dynamical Analysis of Two Current-Driven Low-Frequency Instabilities in a Magnetised Plasma Column

By drawing an electron current along the field lines of a magnetised plasma column, several low-frequency instabilities can be excited. Many such experiments have been performed in Q-machines which produce an alkaline plasma of about 109 cm-3 density, confined by a B-field of about 0.2 T. If the collector, which draws the current, has a radius smaller than that of the plasma column but larger than a few ion gyroradii, the electrostatic ion-cyclotron instability (EICI) is predominant, but also the potential relaxation instability (PRI) can appear and there can be a strong interaction between the two. The frequency of the EICI fEICI is slightly above the ion gyrofrequency Ωi = eB/mi, whereas that of the PRI fPRI is given by the axial ion transit time through the system. We have investigated this interaction in the Innsbruck Q-machine in a potassium plasma in the case when the amplitudes of the two instabilities have similar values. In our case, for B = 0.13 T the interaction leads not only to a strong modulation of the amplitude but also of the frequency of the EICI by the PRI. Our results have indicated that the EICI frequency as such decreases with increasing strength of the exciting electron current, which effect can be explained by electron-neutral impact ionisation processes in front of the collector. In order to clarify the strong interaction between the two instabilities, the experimental results have been subject to a nonlinear dynamical analysis, which delivered the autocorrelation function and enabled us to reconstruct the state space of the phenomenon and to simulate the signal in a special case. These results are presented in this contribution