The Fast Ion Loss Detector (FILD), installed on TCV in 2020 was redesigned and upgraded several times throughout this PhD. The collimator apertures were reduced to increase the energy resolution, a faster CMOS camera was intalled, a novel fast camera (up to 4 MHz) was installed to access microsecond velocity-space mapping, two more slits were added on the bottom partof the FILD head accessing to all magnetic field configurations, a bandpass optical filter was installed to reduce residual light contamination, and a new rotatable head was implemented. All these upgrades make the TCV-FILD a unique detector with high versatility and capabilities that allow it to explore plasma discharges and physics phenomena that were not previously accessible. The FILD was used to measure part of the fast-ion losses in several plasma discharge configurations, and, together with complementary information of the confined fast-ion population from the fast-neutron detector, to diagnose the fast-ion confinement properties. We focused on two main types of plasma discharges: i) the so-called high-confinement regime (H-mode) exhibiting edge localised modes (ELMs) and ii) an alternative plasma scenario characterised by high plasma shaping, high plasma core confinement properties and without ELMs (ELM-free). The FILD measurements were analysed and interpreted using dedicated modelling. In theH-mode case, fast-ion acceleration was observed, while for the ELM-free case, the impact on the fast-ion confinement of changing the plasma triangularity was addressed. In addition, installing top and bottom slits on the FILD led to the first velocity-space resolved measurements of lost Runaway Electrons (REs) on any tokamak. A noticeable trapped, co-current propagating, population of lost REs was measured simultaneously with the normal passing counter-current component. Modelling allowed us to understand the multiple contributions to the FILD signal and the FILD’s sensitivity to escaping REs.