Description
Understanding fast ion (FI) transport is crucial in tokamaks, to ensure core plasma heating and avoid losses and potential damages to the vessel [1]. Despite recent progresses in diagnostics and simulations in tokamaks [2-4], performing fundamental studies of FI dynamics is a challenging endeavor in fusion plasmas. Basic plasma physics devices such as TORPEX (major radius 1 m, minor radius 0.2 m) can provide essential data to develop transport models and validate simulations [5]. In this work, we study the propagation of FI in TORPEX in the specific case of an X-point magnetic field configuration, which is crucial for the mitigation of heat fluxes in tokamaks.
First, the general features of hydrogen plasmas generated close to an X-point in TORPEX [6] are presented. The magnetic shear s is varied and is shown to significantly affect relevant plasma properties. FI transport is then experimentally studied with the reconstruction of three-dimensional FI profiles, obtained by scanning the poloidal plane with a gridded energy analyzer and by moving a FI source in the toroidal direction from one discharge to the other [7]. FI are injected at two positions of interest, being (1) in the region of the plasma where the dominant modes are propagating and (2) below the X-point, to observe the effect of the magnetic null on FI dynamics.
We show that, for injection position (1), FI transport is non-diffusive and depends on the FI injection energy, while the impact of s on the transport is observed to be negligible. In contrast, when injected below the X-point, FI exhibit strongly different behaviors with different
background plasma profiles, from a deviation along the separatrix (low s) to a sharp increase in diffusion as they cross the X-point (high s). These studies will serve as a benchmark to the development of models and validation of FI transport simulations in an X-point magnetic field.
[1] Fasoli A., et al 2013 Nucl. Fusion 53, 063013.
[2] Poley-Sanjuán J., et al 2025 Nucl. Fusion 65, 092006.
[3] Geiger B., et al 2017 Plasma Phys. Control. Fusion 59, 115002.
[4] Podestà, M., et al 2014 Plasma Phys. Control. Fusion, 56(5), 055003.
[5] Fasoli A., et al 2019 Nat. Phys. 15, 872-875.
[6] Sepulchre C., et al 2025 Phys. Plasmas, 32(8), 082101.
[7] Bovet A., et al 2012 Nucl. Fusion 52, 094017.