Investigation of suprathermal ion transport in TORPEX

Basic aspects of suprathermal ion transport in interchange-mode unstable plasmas are investigated in the simple toroidal plasma device TORPEX. Suprathermal ions are generated with a miniaturized lithium 6+ ion source with energies up to 1keV, and are detected using a double-gridded energy analyzer mounted on a two-dimensional movable system in the poloidal cross-section. The signal-to-noise ratio is enhanced by applying a modulated biasing voltage to the suprathermal ion source and using a synchronous detection scheme. An analog lock-in amplifier has been developed, which allows removal of capacitive noise associated with the voltage modulation. We characterize vertical and radial transport of the suprathermal ions, which is associated with the plasma turbulence. Experimental measurements of the 2D poloidal distribution of the fast ion beam show a good agreement with numerical simulations of the suprathermal ion transport in a global fluid simulation of the TORPEX plasma. The simulations have revealed the presence of a ballistic phase followed by an interaction phase where the dispersion regime can be subdiffusive, diffusive or superdiffusive depending on the ion energy and on the amplitude of the fluctuations. To investigate these mechanisms in detail, a movable system for the ion source has been developed allowing the source to move continuously in the toroidal direction. This system permits us to reconstruct the 3D profile of the suprathermal ion beam. First measurements in the ballistic and interaction phase are shown.

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