Note on quantitatively correct simulations of the kinetic beam-plasma   instability
  A large number of model particles is shown necessary for quantitatively correct simulations of the kinetic beam-plasma instability with the clouds-in-cells method. The required number of particles scales inversely with the expected growth rate, as in the kinetic regime only a narrow interval of beam velocities is resonant with the wave. 
K.V.Lotov, I.V.Timofeev, E.A.Mesyats, A.V.Snytnikov, V.A.Vshivkov
physics.plasm-ph physics.comp-ph