Characterization of fast ion confinement in the NSTX based on FIDA diagnostic measurements

Confinement of energetic ions is a critical issue for fusion devices. In last years, much interest has been devoted to losses or redistribution, in real and velocity space, caused by Alfv\'enic modes. More recently it emerged that, in absence of energetic particles modes, fast ions transport can differ from what expected from classical predictions. This work addresses experimentally the confinement of fast ions in NSTX. The radial fast ion density profile $n_f$, measured by the Fast Ion D$_\alpha$ (FIDA) diagnostic from the high energy tails of D$_\alpha$ emission, is characterized over an extended database of plasma discharges including L-mode, H-mode, and with different types of MHD and Alfv\'enic modes. Radial gradient of $n_f$, the drive of energetic particle instabilities, is observed to vary substantially in different plasma conditions. Dependence of $n_f$ peaking on plasma parameters (e.g. $I_p$, $n_e$, $B_t$) will be reported and compared with predictions based on classical fast ion transport.