  We propose a classification of bifurcations of Vlasov equations, based on the
strength of the resonance between the unstable mode and the continuous spectrum
on the imaginary axis. We then identify and characterize a new type of generic
bifurcation where this resonance is weak, but the unstable mode couples with
the Casimirs, which are constants of motion, to form a size 3 Jordan block. We
derive a three-dimensional reduced noncanonical Hamiltonian system describing
this bifurcation: coupling with the Casimirs controls the phase space portrait.
Comparison of the reduced dynamics with direct numerical simulations on a test
case gives excellent agreement. We finally discuss the relevance of this
bifurcation to specific physical situations.
