For K > K c , chaotic unstable orbits are no longer constraints by invariant tori in the momentum direction and can explore the full phase space. For K ≫ ≫ K c , the particle after each kicks typically moved over a large distance, which strongly modifies the amplitude and sign of the following kick. At long time enough, the particle as thus been submitted to a series of kicks with quasi-random amplitudes. This quasi-random walk is responsible for a diffusion process in the momentum direction ⟨ ⟨ ( Δ Δ p n ) 2 ⟩ ⟩ = 2 D cl n (where the average runs over different initial conditions).