A plasma source driven predator-prey like mechanism as a potential cause of spiraling intermittencies in linear plasma devices

Abstract

Three-dimensional global drift fluid simulations are carried out to analyze coherent plasma structures appearing in the NAGDIS-II linear device (nagoya divertor plasma Simulator-II). The numerical simulations reproduce several features of the intermittent spiraling structures observed, for instance, statistical properties, rotation frequency, and the frequency of plasma expulsion. The detailed inspection of the three-dimensional plasma dynamics allows to identify the key mechanism behind the formation of these intermittent events. The resistive coupling between electron pressure and parallel electric field in the plasma source region gives rise to a quasilinear predator-prey like dynamics where the axisymmetric mode represents the prey and the spiraling structure with low azimuthal mode number represents the predator. This interpretation is confirmed by a reduced onedimensional quasilinear model derived on the basis of the findings in the full three-dimensional simulations. The dominant dynamics reveals certain similarities to the classical Lotka-Volterra cycle.

Mode Structure Analysis of Detached Plasmas with 2D Images

“…Reiser et al discussed based on three-dimensional global drift fluid dynamics simulation that spiral structures, like the one observed in the NAGDIS-II device, is dominated by m = 1 and m = 2 modes with a time oscillation of m = 0 mode [12]. Thus, the present mode analysis are qualitatively agree well with the global drift fluid dynamic simulation.…”

“…In this study, the characteristic time scale of the phenomena was in the range of 10 -20 kHz. Tanaka et al showed under a different condition in NAGDIS-II that the ejection frequency was approximately 3.2 kHz [10], and it was 2.0 kHz in simulation [12]. Although the rotation frequency of blob-like plasma has been revealed to depend on the magnetic field strength and density profile, no detailed investigation has yet to be conducted for the ejection frequency; the results in this study suggested that the ejection frequency also altered by the plasma condition.…”

“…In this study, m = 0 and m = 1 modes are almost in phase. One of the reasons is attributed to the fact that the analysis in [12] included the peripheral region, while we focused on inside the plasma column in this study. However, even if we included the peripheral region for the analysis, the in phase tendency would not be reduced.…”

“…Recent three dimensional simulation aiming at linear plasma devices predicted that resistive drift instability leads to generation of blob-like structures [11]. From the simulation, it was also suggested that the instability with m = 0 mode in the core region of the cylindrical plasma was important for the spiral structure formation in linear devices in addition to the instability with m = 1 author's e-mail: kajita.shin@nagoya-u.jp mode [12]. However, the dynamics in the plasma column especially in the core region has yet to be investigated, because the intensity in the core region was saturated in previous works [4,10].…”

Localized Density Fluctuation in the Downstream of Detached Plasma

“…When the ion flux dramatically decreases due to an increase of the neutral gas pressure, a large amplitude ion flux fluctuation with ≤ 10 kHz components was detected by using a circular end-target plate with a diameter of 50 mm [4]. Threedimensional simulation suggested that such axisymmetricmode fluctuation generates intermittent plasma structures in the periphery [5], however, there were a few measurements of this fluctuation. In addition, it is not known where the large amplitude fluctuation occurs in the detached plasma.…”