The bifurcation structure of periodically forced current disruptions

The bifurcation structure of periodically driven current oscillations in the central chamber of a magnetized triple plasma device is investigated experimentally. The target chamber of the triple plasma device is positively biased with respect to the source chamber and the bias voltage mainly drops in a strong double layer formed in the central chamber. At the low potential side of the double layer, a variable negative potential forms that gives rise to a region of negative resistance in the static current-voltage characteristic of the device. In this regime, if a sufficiently high inductance is added to the external circuit, strong nonlinear low-frequency oscillations both in the plasma current and the voltage drop over the plasma occur. These oscillations are interpreted as the periodically repeated exchange between magnetic energy and particle motion in the double layer. The dynamics of the current circuit is described by a van der Pol-type equation where the nonlinearity is given by the derivative of the current-voltage characteristic of the plasma. An additional periodic driver signal, added to the bias voltage, gives rise to a considerably enriched dynamical behaviour as predicted by the theory of driven nonlinear oscillators, including frequency entrainment, quasiperiodicity, periodic pulling and period doubling bifurcations. The experimental observations are well explained by means of the known bifurcation structure of the the periodically driven van der Pol oscillator model.