Shaoyan Cui

Frequency effects on the electron density and α-γ mode transition in atmospheric radio frequency discharges

In this paper, a one-dimensional model is explored to investigate the frequency effects on the characteristics of atmospheric radio frequency discharges at a given power. The simulation data and analytical results show that the improvement of electron density can be observed with better discharge stability by increasing excitation frequency in an appropriate range. Using the analytical equations deduced from the model, the mean electron density could be inferred by means of the measured parameters. The α-γ mode transition especially in high frequency discharges is also analytically discussed based on the theoretical equations.

Combined effects of trapped energetic ions and resistive layer damping on the stability of the resistive wall mode

A dispersion relation is derived for the stability of the resistive wall mode (RWM), which includes both the resistive layer damping physics and the toroidal precession drift resonance damping from energetic ions in tokamak plasmas. The dispersion relation is numerically solved for a model plasma, for the purpose of systematic investigation of the RWM stability in multi-dimensional plasma parameter space including the plasma resistivity, the radial location of the resistive wall, as well as the toroidal flow velocity. It is found that the toroidal favorable average curvature in the resistive layer contributes a significant stabilization of the RWM. This stabilization is further enhanced by adding the drift kinetic contribution from energetic ions. Furthermore, two traditionally assumed inner layer models are considered and compared in the dispersion relation, resulting in different predictions for the stability of the RWM.

Numerical studies for the linear growth of resistive wall modes generated by plasma flows in a slab model

The resistive wall mode generated by plasma-wall relative rotations is studied numerically in a slab model with a compressible plasma flow parallel to the magnetic field. The linear growth of the mode is investigated with different parameters in numerical simulations. The critical plasma flow velocities for the instability are calculated as the wave number of the mode and other parameters vary. It is found that in the long wavelength regime, the critical velocity is in the range of the sound speed cs, as predicted in theory. In the short wavelength regime however, the critical velocity increases to a level of Alfven velocity VA and a second stable region is found. This region eventually merges with the first stable region as the wave number increases and stabilizes the mode. The growth rate of the mode decreases with the wave number of the mode and the plasma viscosity. The critical wave number for the instability is also calculated as the plasma velocity changes.

Kinetic shear Alfvén instability in the presence of impurity ions in tokamak plasmas

The effects of impurity ions on the kinetic shear Alfven (KSA) instability in tokamak plasmas are investigated by numerically solving the integral equations for the KSA eigenmode in the toroidal geometry. The kinetic effects of hydrogen and impurity ions, including transit motion, finite ion Larmor radius, and finite-orbit-width, are taken into account. Toroidicity induced linear mode coupling is included through the ballooning-mode representation. Here, the effects of carbon, oxygen, and tungsten ions on the KSA instability in toroidal plasmas are investigated. It is found that, depending on the concentration and density profile of the impurity ions, the latter can be either stabilizing or destabilizing for the KSA modes. The results here confirm the importance of impurity ions in tokamak experiments and should be useful for analyzing experimental data as well as for understanding anomalous transport and control of tokamak plasmas.

Finite toroidal flow generated by unstable tearing mode in a toroidal plasma

The neoclassical toroidal plasma viscosity torque and electromagnetic torque, generated by tearing mode (TM) in a toroidal plasma, are numerically investigated using the MARS-Q code [Liu et al., Phys. Plasmas 20, 042503 (2013)]. It is found that an initially unstable tearing mode can intrinsically drive a toroidal plasma flow resulting in a steady state solution, in the absence of the external momentum input and external magnetic field perturbation. The saturated flow is in the order of 0.5% ωA at the q=2 rational surface in the considered case, with q and ωA being the safety factor and the Alfven frequency at the magnetic axis, respectively. The generation of the toroidal flow is robust, being insensitive to the given amplitude of the perturbation at initial state. On the other hand, the flow amplitude increases with increasing the plasma resistivity. Furthermore, the initially unstable tearing mode is fully stabilized by non-linear interaction with the self-generated toroidal flow.

Instability and energy cascade in isotropic and anisotropic media

The evolution of isotropic and anisotropic systems governed by the complex nonlinear Schrodinger equation is investigated numerically. It is found that modulational instability, collapse, mode cascade and strong turbulence can occur in different forms.

Numerical Studies on Resistive Wall Instabilities in Cylindrical Plasma Confined by Surface Current

The stability of resistive wall mode is studied in cylindrical plasma confined by surface current, which is Dirac -function distribution. For Dirac -function distribution case, it is shown that the perturbations oscillate and even decline wherever the initial perturbation seed is placed. The whole system is stable and the plasma flow has little effect on it.