Three-dimensional measurements of non-diffusive fast ion transport in TORPEX
In the basic plasma physics device TORPEX, progress in the fundamental understanding of supra-thermal ion transport is achieved by extensive sets of three-dimensional (3D) data, together with numerical simulations of supra-thermal ion tracers in fluid turbulent fields. A miniaturized lithium 6+ ion source injects fast ions with energies up to 1 KeV and a double-gridded energy analyzer is used as a detector. The source is mounted on a toroidally movable system and the detector can be moved in the poloidal cross-section, allowing one to reconstruct 3D fast ion current profiles. Synchronous detection is used to enhance the signal-to-noise ratio. A modulated biasing voltage is applied to the fast ion source and an analog lock-in amplifier is used to demodulate the detector signal. The analog lock-in amplifier is specially designed to remove the capacitive noise associated with the voltage modulation. Radial transport of the fast ions, associated with plasma turbulence, is characterized. A synthetic diagnostic allows comparing the experimental results with numerical simulations of the fast ion transport in a global fluid simulation of the TORPEX plasma. A good agreement is shown.