At the onset of a solar flare, initiated by magnetic reconnection high in the corona, reconnection
outflow sets up warm proton beams (PBs), streaming down along just-reconnected field lines
through steady underlying plasma. Incorporating this scenario, we study excitation of kinetic Alfvén
waves (KAWs) by PBs, keeping the effects of a beam-induced electric field and thermal effects.
Taking into account the high growth rate (about 105 /s), short relaxation distance (about 106 cm), and
energy flux partition between the waves and the beam after relaxation (PKAW/PPB about 1), we conclude
that PB-driven KAW instability is an efficient energy conversion mechanism in flaring loops.
The quasilinear spectral energy concentration at the largest wavenumbers indicates the possibility of
nonlinear spectral modification.We suggest that the resulting turbulence of KAWs plays an important
role in the flare plasma energization.