Physical processes related to the onset of low-frequency instabilities in magnetized plasmas

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Electron currents through magnetized plasma can excite nonlinear low-frequency instabilities. as e.g. the potential relaxation instability and the electrostatic ion-cyclotron instability. The onset of these instabilities is often related with the appearance of a negative differential resistance in the static current-voltage characteristic. The interpretation of this phenomenon was linked with the limitation of the current due to the high amplitude oscillations. We report new experimental results obtained into the Q-machine plasma which demonstrate that this negative differential resistance is the result of a nonlinear dynamics of a complex space charge structure in form of a double layer, which takes place immediately before the onset of instabilities. The periodic generation and disruption of the double layer determine an injection of particles and energy in the plasma column. In this way, the double layer triggers low-amplitude oscillations of the plasma parameters (potential, electron density and temperature, etc.). The plasma column acts as a resonator in which oscillations can be maintained. When a resonance condition is fulfilled the oscillations amplitude strongly increases and the instability is developed. After the onset of the instability the differential resistance of the static current-voltage characteristic is positive.