  We have found, using 1D periodic Vlasov-Poisson simulations, new nonlinear,
nonstationary, stable, long lived, coherent structures in phase space, called
kinetic electrostatic electron nonlinear (KEEN) waves. Ponderomotively driven
for a short period of time, at a particular frequency and wavenumber, well
inside the band gap that was thought to exist between electron plasma and
electron acoustic wave frequencies, KEEN waves are seen to self-consistently
form, and persist for thousands of plasma periods. KEEN waves are comprised of
4 or more significant phase-locked harmonic modes which persist only when
driven sufficiently strongly. They also merge when two or more at different
frequencies are driven sequentially. However, the final stable KEEN state that
emerges is highly sensitive to their relative order of excitation. KEEN waves
also interact quite strongly with electron plasma waves (EPW) especially when
their harmonics are close to being resonant with the EPW frequency at the same
k. The common assumption that whenever sufficiently large amplitude coherent
laser energy is present in an unmagnetized plasma, EPWs and IAWs are the only
waves with which the electromagnetic energy can interact coherently may require
reconsideration.
