Deterministic chaos in Alcator C-Mod edge turbulence

Abstract

Understanding the underlying dynamics of turbulence in magnetic confinement fusion experiments is extremely important. Turbulence greatly reduces the confinement time of these devices and therefore greater knowledge of turbulent dynamics can help with its mitigation. Experiments from the Alcator C-Mod tokamak [18] provide support for a theory that edge turbulence in tokamak fusion plasmas is the result of deterministic chaos, rather than stochastic processes [15]. Using readily available reflectometer data from Alcator C-Mod (C-Mod), analysis of C-Mod edge turbulence in Ohmic plasmas and Ion Cyclotron Range of Frequencies (ICRF) heated L-Mode plasmas shows that density fluctuations just inside or at the Last Closed Flux Surface (LCFS) exhibit exponential power spectra. Theoretically, the characteristic slope of the data on a semi-log plot gives the full width of the underlying Lorentzian pulses, which give rise to the exponential power spectra due to the dynamics of deterministic chaos. Using a separate fitting routine, individual Lorentzian pulses in the reflectometer time series data are identified, and the widths of the Lorentzian pulses match the inverse characteristic frequency of the exponential power spectra. Analysis of the waiting times between pulses and the pulse amplitudes indicate these are randomly distributed yet the pulse widths have a narrow distribution. These characteristics are consistent with a chaotic process. There is also a preliminary comparison of GPI data and a discussion of limitations of the analysis presented here and plans for future work. Overall, the experimental results in this study are consistent with edge turbulence that is at least partially generated by chaotic dynamics.