Transition regime of the one-dimensional two-stream instability
  The transition between kinetic and hydrodynamic regimes of the one-dimensional two-stream instability is numerically analyzed, and the correction coefficients to the well-known textbook formulae are calculated. The approximate expressions are shown to overestimate the growth rate several times in a wide parameter area. 
K.V. Lotov and I.V. Timofeev
physics.plasm-ph