A New Understanding of Langmuir Plasma Turbulence

The Langmuir wave turbulence generated by a beam-plasma interaction has been studied since the early days of plasma physics research. In particular, mechanisms which lead to the quasi-power law spectrum for Langmuir waves have been investigated, since such a spectrum defines the turbulence characteristics. Meanwhile, the generation of harmonic Langmuir modes during the beam-plasma interaction has been known since the late 1960s through laboratory, spaceborne, and simulated computer experiments, and yet the phenomenon has not been satisfactorily accounted for thus far. In this talk, a generalized weak turbulence theory, recently formulated by the speaker, will be discussed which incorporates the multiple harmonics of Langmuir waves, and the resulting solution of the self-consistent equations shows that harmonic Langmui wave spectrum exhibits quasi power-law feature, implying multi-scale fractal-like structure in both frequency and wave number, spanning several decades. The theory is then compared against the Vlasov simulation, and excellent agreement is found. This indicates that the harmonic excitation process may be intimately related to the generation of high-frequency turbulence in plasmas.