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 investi gated experimentally. The target chamber of the triple plasma device i s positively biased with respect to the source chamber and the bias vo ltage mainly drops in a strong double layer formed in the central cham ber. At the low potential side of the double layer, a variable negativ e potential forms that gives rise to a region of negative resistance i n the static current-voltage characteristic of the device. In this reg ime, if a sufficiently high inductance is added to the external circui t, strong nonlinear low-frequency oscillations both in the plasma curr ent and the voltage drop over the plasma occur. These oscillations are interpreted as the periodically repeated exchange between magnetic en ergy and particle motion in the double layer. The dynamics of the curr ent circuit is described by a van der Pol-type equation where the nonl inearity is given by the derivative of the current-voltage characteris tic of the plasma. An additional periodic driver signal, added to the bias voltage, gives rise to a considerably enriched dynamical behaviou r as predicted by the theory of driven nonlinear oscillators, includin g frequency entrainment, quasiperiodicity, periodic pulling and period doubling bifurcations. The experimental observations are well explain ed by means of the known bifurcation structure of the the periodically driven van der Pol oscillator model.