“…They showed that single-humped distributions are always stable (Berz 1956, Appendix by W. Newcomb in Bernstein 1958, Auer 1958, Penrose 1960, Noerdlinger 1960, Jackson 1960. Then, they considered double humped distributions modeling two contrastreaming beams (Haeff 1949, Nergaard 1948, Pierce & Hebenstreit 1949 and determined particular conditions under which such distributions are unstable (Twiss 1952, Buneman 1958, Auer 1958, Kahn 1959, Buneman 1959, Penrose 1960, Noerdlinger 1960, Jackson 1960. They found qualitatively that the plasma becomes unstable when the stream velocity becomes sensibly larger than the thermal velocity.…”