Observation of electron plasma waves inside large amplitude electromagnetic pulses in a temporally growing plasma

Abstract

Observation of electron plasma waves excited inside high power ($10 kW) short pulse ($20 ls) electromagnetic (em) waves interacting with a gaseous medium (argon) in the pressure range 0.2-2.5 mTorr is reported. The waves have long wavelength ($13 cm) and get damped at time scales slower ($3 ls) than the plasma period (0.1-0.3 ls), the energy conveyed to the medium lead to intense ionization (ion density n i $ 10 11 cm À3 and electron temperature T e $ 6-8 eV) and rapid growth of the plasma ($10 5 s À1 ) beyond the waves. Time frequency analysis of the generated oscillations indicate the presence of two principal frequencies centered around 3.8 MHz and 13.0 MHz with a spread Df $ 4 MHz, representing primarily two population of electrons in the plasma wave. The experimental results are in reasonable agreement with a model that considers spatiotemporal forces of the em wave on the medium, space charges and diffusion.