Spontaneous Excitation of Intermittent Electron Currents in an ECR Discharge Plasma

Laboratory plasmas are intrinsically non-equilibrium open system in which energy and particles are being injected and exhausted continuously; such systems exhibit various intermittent behaviors. Recently, the spontaneous excitation of 1-cycle magnetic pulses has been observed in an electron cyclotron resonance (ECR) discharge plasma produced in the HYPER-I device at the National Institute for Fusion Science. Simultaneous measurement using two magnetic probes and a directional Langmuir probe revealed that the magnetic pulses were excited by intermittent electron currents, or high-energy electron fluxes, along the magnetic field. The energy distribution of those electrons was examined by a retarding field analyzer. We also developed a wire-grid probe which consists of 16 electrically floated electrodes to measure the two-dimensional profile of the high-energy electrons. Since the occurrence of high-energy electron fluxes appeared to be random in time and space, we applied several statistical analyses to the intermittent events. The waiting time analysis demonstrated an exponential-like distribution, which implies the stochastic nature of the events.