Control of solitary-drift-wave formation by radial density gradient in laboratory magnetized cylindrical plasma

Solitary drift waves (SDWs) in magnetized plasmas were discovered and then first investigated by experiment and numerical simulation by the Kyushu University group [i.e., H. Arakawa et al., Plasma Phys. Controlled Fusion 52, 105009 (2010)]. However, the formation mechanisms of SDWs still await thorough examination. Our work experimentally identifies a clear transition from turbulent drift waves (DWs) to SDWs for varied radial gradients in background density, which is in agreement with the preceding numerical simulations [M. Sasaki et al., Phys. Plasmas 22, 032315 (2015)]. The formation of SDWs is accompanied by a significant growth in the total fluctuation level and three-wave phase coupling between the constitutive harmonic modes. A subsequent saturation in the total fluctuation level and intensity of three-wave coupling when further increasing the density gradient is witnessed for the first time. The transition from turbulent DWs to SDWs is also characterized by an increase in the radial wavelength of the DWs. The SDW is considered a meso- (or macro-) scale ordered structure nonlinearly generated by turbulent DWs. Our work on SDW generation indicates that this phenomenon in magnetized plasmas is a universal rather than a device-dependent phenomenon.