Self-consistent 2D electrostatic presheath and its effects on the turbulent transport in the scrape-off layer of a field-reversed configuration
- Wang, Wenhao
- Advisor(s): Lin, Zhihong
Abstract
Field-reversed configuration (FRC) is one of the approaches to realizing magnetic fusion due to its engineering simplicity. The recent C2-W experiment showed long-lasting FRC plasmas over 30 ms, with the help of neutral beam injection (NBI) and edge-biasing system. Therefore, understanding the edge-biasing effects on the FRC can be essential to improve the FRC designs and enable the potential active controls. The penetration of the edge biasing applied at the divertor ends can affect the equilibrium potential structure in the scrape-off layer (SOL) of FRC. The first part of the thesis is to establish a formulation that accurately captures both parallel and radial variations of the 2D potential in the SOL under the influence of edge-biasing. A full-f gyrokinetic ion model and a massless electron model are implemented in the GTC-X code to solve for the self-consistent equilibrium potential, given fixed radial potential profiles at the boundaries. Current simulation results implied that accurate electron density and temperature profiles are important in predicting the potential structure in the FRC SOL. The Debye sheath potential and the biasing profiles applied at the boundaries could introduce additional E×B rotation in the FRC SOL. With the equilibrium potential, the turbulent transport in the FRC can be modified due to the added E×B flow shear. Such shearing effects were first studied with an artificial 1D radial equilibrium potential for the ion temperature gradient (ITG) instability in the FRC SOL, which was identified as the dominant source for SOL fluctuations in the C2-W device. Linear δf simulations with adiabatic electrons found that the total shear from both the diamagnetic flow and the equilibrium E×B flow can reduce the linear growth rates and cause a radial tilting of the mode structures in the toroidal plane. Nonlinear simulations further confirmed that the flow shearing significantly decreases ITG saturation and ion heat transport levels. In addition, the self-generated zonal flow shears were also studied and played a similar role to reduce the ITG nonlinear saturation, unless the collisional damping limited the zonal flow level in the SOL. These results may suggest that using a proper edge-biasing profile and minimizing collisions could improve the confinement of the FRC. The full-f scheme developed in the 2D equilibrium simulation was aimed for coupled simulations of the turbulent transport with the self-consistent 2D equilibrium potential. As a starting point, this full-f algorithm had been compared with the δf simulations with periodic boundaries. Future research will involve the full-f simulations with the non-Maxwellian equilibrium distribution functions and open boundary flows in the FRC SOL.