Thomson Scattering Detection of Ponderomotively Driven KEEN Waves in a Laser Produced Plasma

Kinetic simulations using ponderomotively driven excitations have~discovered the existence of stable, nonlinear, multimode coherent~ structures in plasmas named Kinetic Electrostatic Electron Nonlinear~ (KEEN) waves.$^{1}$ For a given wave number drive, they form and persist~ for drive frequencies in a broad range of ($\omega $,k) space lower in~ frequency that electron plasma waves and much higher than ion~ acoustic waves. Experiments were conducted on TRIDENT by~Polymath Research Inc., in collaboration with LANL, to detect these~waves driven by the beating of two laser beams.~The two lasers used~had 527 and 600 nm wavelengths which is predicted to drive waves in~the proper KEEN wave excitation band.$^{1}$ A nitrogen gas Raman cell was~used to convert a 527 nm laser beam to~600 nm. We drove KEEN waves and detected them with~263 and 351 nm Thomson scattering in nitrogen/hydrogen and helium gas~jet plasmas. This presentation will cover experimental conditions and~diagnostic attributes associated with the detection of KEEN waves.~We will discuss the properties of KEEN waves as a function of probe~beam intensity and as a function of pump-probe overlap time. Supported by DOE Academic Alliance Grant DE-FG03-03NA00059 and LANL~under Contract No. W-7405-ENG-36. $^{1}$ B. Afeyan et al., Proc. IFSA (2003, Monterey, CA), 213, Amer Nucl Soc, 2004.