Absolute instability in finite length electron cyclotron maser systems

The temporal growth rate of the absolute instability in finite length electron cyclotron maser systems has been theoretically calculated and confirmed by particle simulations. The non-linear saturation mechanism is due to that in the unstable system, the non-linear electron phase shift induced by the wave field gives rise to a virtual end plane which tends to shorten the effective interaction length and brings the system into the marginal stable state. In the system far above threshold, the virtual plane first oscillates and eventually approaches a steady state location. This early oscillating behaviour strongly correlates to the initial pronounced spiky temporal structure in the output power.