Alfvén eigenmode (AE) instabilities driven by α-particles have been observed in D-3He fusion experiments on the Joint European Torus (JET) with the ITER-like wall [Kiptily et al, submitted to Phys. Rev. Letters, 2020]. The impact of these type instabilities on a burning plasma is of crucial relevance for the performance of a future reactor. Fast ion studies in D-D plasmas were carried out in JET in order to assess how α-particles might impact D-T plasmas through AE activity. The 3-ion radio frequency (RF) scenario  has been used for this purpose, hence we can study effects of fusion-born α-particles in plasmas without tritium well before the sophisticated full-scale D-T experiments characterised by harsh radiation conditions for diagnostics. For the efficient generation of fusion α-particles from D-3He fusion reaction, we accelerated the neutral beam injection 100-keV deuterons by RF to higher energies in the core of mixed D-3He plasmas at high concentrations of 3He. A large variety of fast-ion driven magnetohydrodynamic modes were observed, including the elliptical Alfvén eigenmodes (EAEs) with mode numbers n = –1 and axisymmetric modes with n = 0 in the frequency range of EAEs. The simultaneous observation of these modes indicates the presence of rather strong α-particle population in the plasma with a “bump-on-tail” shaped velocity distribution . Experimental evidence of the AEs excitation by fusion-born α-particles in the D-3He plasma is provided by neutron and gamma-ray diagnostics as well as fast-ion loss measurements. Linear stability analysis and Fokker-Plank calculations support the observations. These studies of the AE instabilities driven by α-particles are continued in tritium plasmas. Indeed, the T(T,2n)4He fusion reaction gives rise to fast α-particles with a continuous energy spectrum (3 particles in the final state) in the energy range 0 – 3.8 MeV (~70% of the total reaction rate) as well as monoenergetic alphas (2 particles in the final state) at ~1.7 MeV (~30%). Also, an experimental proposal for the planned full-scale D-T plasma experiments on JET based on the physics insights gained from our D-3He and T-T experiments will be presented. Note that it may be possible for fusion α-particles in D-T experiments to excite AEs during transients, which could be induced in the plasma for example using neutral beam blips. It is important to be able to detect and study AEs excited directly by fusion-born α-particles in order to predict reliably the effects of these modes in future burning plasma devices such as ITER.