Three-dimensional time-resolved H pellet trajectory reconstruction in RFX by position sensitive detector H α diagnostic

Hydrogen pellets injected in reversed field pinches (RFPs) experience large poloidal and toroidal accelerations along magnetic field lines due to the rocket effect caused by asymmetric ablation processes. The 3D pellet trajectory and the global ablation rate is diagnosed in RFX by an absolutely calibrated multiple position sensitive detector (PSD) system looking at the H α radiation coming from the pellet ablation cloud. The design of the diagnostic is presented. Then the performance is highlighted by cross-comparison between different PSD measurements and with time-integrated images from a CCD camera. Overall accuracy of the order of a few millimeters is achieved, which makes the instrument well-suited to assist transport studies (particle deposition), to estimate the radial profile of fast electron tails (derived from the modulus of pellet acceleration) and to obtain information on internal magnetic field pitch (from the direction of pellet acceleration).