Kinetic Electrostatic Electron Nonlinear (KEEN) Waves and their Interactions Driven by the Ponderomotive Force of Crossing Laser Beams

TL;DR

This paper reports the discovery of stable, long-lived nonlinear KEEN waves in plasma, formed by crossing laser beams, which interact with electron plasma waves and challenge existing assumptions about wave interactions in unmagnetized plasmas.

Contribution

It introduces the existence and properties of KEEN waves driven by ponderomotive forces, revealing their formation, stability, and interactions in plasma, a novel insight beyond traditional wave models.

Findings

KEEN waves form inside the band gap between plasma and acoustic waves.

KEEN waves consist of multiple phase-locked harmonic modes.

They interact strongly with electron plasma waves, especially near resonance.

Abstract

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.