Electrostatic Turbulence and Transport in the Field-Reversed Configuration
- Author(s): Lau, Calvin
- Advisor(s): Lin, Zhihong
- et al.
Recent drastic improvement of plasma stability and confinement in the high performance C-2 advanced beam-driven field-reversed configuration (FRC) experiment at Tri Alpha Energy, Inc. (TAE) has led to consistently reproducible stable plasmas suitable for transport study. To understand and support the ongoing efforts towards an FRC-based reactor, the sources of the fluctuations and the mechanisms of the transport must first be identified such that a suitable transport scaling may be found and applied toward predicting confinement performance in larger, hotter, and denser FRC plasmas.
In the first half of this thesis, a mature, well-benchmarked turbulence simulation code, the Gyrokinetic Toroidal Code (GTC), has been extended and applied to a system with experimentally realistic C-2 parameters. Using GTC, local electrostatic drift-wave stabilities in the core and scrape-off layer (SOL) regions of C-2 have been characterized and compared to experimental findings. The drift-wave is found to be stable in the core. On the other hand, in the SOL, a class of ion-to-electron scale instabilities is observed. These simulation results are consistent with density fluctuation measurements in C-2.
Since experimental measurements show that fluctuations exist in the core, the discovery of the robust stability of the core suggests that the fluctuations may originate from the SOL. In the second half of this thesis, a new turbulence simulation code, A New Code (ANC), has been developed to study nonlocal phenomenon. Using ANC, the linear propagation and nonlinear spreading of a single-mode instability from the SOL to the core has been characterized. It is shown that nonlocal effects, due to the ion finite Larmor radius (FLR) effect and ion polarization drift, allows for the coupling of different drift-surfaces and for the instability in the SOL to spread into the core. The phase velocities of the instability is consistent with experimental measurements of radial propagation. The nonlinear saturation of the instability shows that the saturated mode amplitude in the SOL is higher than in the core, again, consistent with experimental measurements.
In addition, the first turbulent transport simulation in the FRC SOL has also been performed using gyrokinetic ions and adiabatic electrons. Self-consistent ion heat flux is calculated from these simulations to be ~6[kW/m^2] while an upper bound for the electron heat flux is calculated to be ~4[MW/m^2] through electron test particles, though the self-consistent electron heat flux is expected to be lower. An inverse spectral cascade is also observed, with unstable shorter wavelength modes feeding into stable or damped long wavelength modes.