M Tanaka

Pr obability Density Functions of Floating Potential Fluctuations Due to Local Electron Flux Intermittency in a Linear ECR Plasma

Plasmas in nature and laboratories often exhibit intermittent transport of energy and particles; for instance, a solar flare explosively releases massive energy from the Sun, and a blob filament [1] causes bursty convective transport in the edge region of magnetic confinement devices. Recently, a new intermittent behavior of local electron flux, which is attributable to spontaneous and probabilistic transition of electron energy with a finite spatial size on plasma cross-section, has been observed in a linear electron cyclotron resonance (ECR) plasma produced in the HYPER-I device [2]. The intermittent changes in electron flux may result in sporadic large-amplitude fluctuations (negative spikes) on the time series of floating potential measured by a Langmuir probe. A statistical analysis based on probability density functions (PDFs), which gives occurrence frequency distributions, is a powerful tool for investigating such intermittent time series. Here, the basic statistical features of the floating potential fluctuations associated with the electron flux intermittency are reported in terms of the PDFs of fluctuation amplitude, time interval between two consecutive events (waiting time), and characteristic time of each intermittent event (duration). Preliminary results are as follows: (i) The amplitude PDF has a form of negatively skewed non-Gaussian distribution, which reflects the intermittency seen in the time series. (ii) The waiting time PDF is found to be an exponential distribution, which describes the waiting time for a Poisson process [3] in which events are independent and the probability per unit time is constant. (iii) The duration PDF shows an inverse power-law dependence in the long duration side.