Nonlinear evolution of kinetic energy E_k for multiple-n and single-n cases with β_h = 1.81% and beam ion injection speed υ_h = 2.55υ_A. Multiple-n (n_multiple = 1, 2, 3, 4) stands for n = 1, 2, 3, 4 in the multiple-n cases, respectively. Single-n (n_single = 1, 2, 3, 4) stands for n = 1, 2, 3, 4 in the single-n cases, respectively. The growth rates of n_multiple = 1, 2, 3, 4 modes are γ_{n = 1} = 0.0167ω_A, γ_{n = 2} = 0.0525ω_A, γ_{n = 3} = 0.0656ω_A, γ_{n = 4} = 0.0937ω_A, respectively.