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Statistical properties of intermittent bursts in the Texas Helimak

We present investigations of intermittent turbulence in the Texas Helimak, a simple toroidal plasma device in which the turbulence properties are modified by applying a bias voltage. The analyzed turbulence presents high density bursts, detected by Langmuir probes measuring ion saturation current fluctuations. The turbulent time series is reproduced by a synthetic signal model that has bursts with the same temporal profile and random amplitudes, plus a low amplitude fluctuating stochastic background. Using this model, we identify two burst regimes, observed according to the radial position and external bias: in the first regime, the bursts occur in random instants, leading to a Poisson distribution of the time interval between bursts, while in the second regime, the time interval between large bursts is correlated and modeled by a Gamma distribution. Furthermore, we use the shape parameter k that measures the correlation between occurrence times of successive bursts to characterize the burst regime in most of the low field side. We find that in the region described by the second regime, the k values increase with positive applied bias.