Experimental Investigation of Current-Driven Ion Wave Turbulence in Plasma

Abstract

An evolution of ion waves is experimentally examined from the marginally stable state up to the turbulent state. The threshold value of the electron drift velocity and the frequency at the onset of the instability are measured. In the unstable state, the spectrum of the ion wave depends on the electron drift. In the turbulent state, however, the shape of the spectrum is almost constant independent of the drift velocity. The wave energy piles up near the ion collision frequency and the wave potential φ ω decreases like φ ω ∝ω -2 ∼ω -2.5 . The energy of the ion wave grows with the drift velocity, but eventually saturates due to the electron trapping. We observe also a slow ion wave mode in the turbulent plasma. The results are qualitatively explained by the theories of ion wave turbulence.