Supercontinuum (SC) is a drastic spectrum broadening of the initial pump wave caused by nonlinear interaction between the seed wave with the optical medium It is widely observed in various fields such as optics hydrodynamics plasmas and so forth There exist industrial applications of this phenomenon such as a source of optical coherence tomography etc It is well known that finite amplitude Langmuir waves (LWs) in plasmas are described by the nonlinear Schro ?dinger equation which is considered as a governing equation of optical SC phenomenon Therefore possibility of SC generation of LWs is highly expected This study aims at demonstration of generation of Langmuir wave supercontinuum (LWSC) in laboratory plasmas To this end we have designed and conducted experiments in which external longitudinal electric fields are applied to plasmas for LWSC generation in the MPX device at ISAPS/NCKU A quasi one-dimensional configuration was realized by application of the background magnetic fields From the experiments we found the followings: 1 Excitation of electrostatic waves whose frequencies and wave-numbers agreed with the linear dispersion relation of the LWs was observed The wave generation can be explained by excitation of LWs 2 The correlation length (~ propagation length) of the excited waves was approximately 5000 λ_De which is much longer than the linear Landau damping length 3 ~ 10λ_De indicating that the excited waves were nonlinear Langmuir waves 3 The excited waves showed a wide power spectrum whose frequency broadening ?f/f_pe ~0 1 and coherence ? 1 0 at the propagation distance of 5500λ_De where f_pe is the plasma frequency These satisfy our definition of LWSC 4 The broadening of the power spectra was enhanced as the injection power of the external driver was increased These observations indicate a possibility of LWSC generation in the MPX plasmas Although further confirmation including its reproducibility existence of nonlinear couplings of the waves and so on are needed
Experimental Verification of Langmuir Wave Supercontinuum Generation in Laboratory Plasma
皓, 張. (Author). 2018 Aug 20
Student thesis: Master's Thesis