Plasma Structures in Texas Helimak

Intermittent structures with extreme events (bursts) have been detected in the turbulence of the scrape of layer (SOL) of Tokamaks and these structures have a major role in plasma confinement. The Texas Helimak is a toroidal plasma device with one-dimensional equilibrium, magnetic curvature and shear, thus resembling closely a SOL of a Tokamak. The Texas Helimak vacuum vessel has a rectangular cross section with 0.6 m of internal radius, 1.6 m of external radius, and 2 m of height and the plasma is generated by electron cyclotron resonance heating. The Helimak has 16 bias plates, where a large set of Langmuir probes is mounted and from where is possible to impose an external electrical bias. This electrical bias can change the turbulence properties and even suppress the appearance of the intermittent structures. In this work, we study the statistical characteristics of bursts in the ion saturation current, and we show that a shot noise model can reproduce the signal statistics. We also characterize both the spacial and temporal profiles of the bursts and their propagation, as a function of the external imposed bias and the radial position inside the Helimak. Both profiles and the bursts propagation are analyzed by conditional analysis and the cross correlation between the probes signals. We acknowledge the financial support of FAPESP (grants 2014/07043-0 and 2011/19296-1).