Huishan Cai

Hybrid simulation of energetic particle effects on tearing modes in tokamak plasmas

The effects of energetic ions on stability of tearing mode are investigated by global kinetic/MHD hybrid simulations in a low beta tokamak plasma. The kinetic effects of counter circulating energetic ions from the non-adiabatic response are found to be strongly destabilizing while the effects from the adiabatic response are stabilizing. The net effect with both adiabatic and non-adiabatic contributions is destabilizing. On the other hand, the kinetic effects of co-circulating energetic ions from the non-adiabatic response are calculated to be weakly stabilizing while the corresponding adiabatic contribution is destabilizing for small energetic ion beta. The net effect is weakly stabilizing. The dependence of kinetic effects on energetic ion beta, gyroradius, and speed is studied systematically and the results agree in large part with the previous analytic results for the kinetic effects of circulating particles. For trapped energetic ions, their effects on tearing mode stability are dominated by the adiab...

Effects of magnetic field on anisotropic temperature relaxation

In a strongly magnetized plasma, where the particles thermal gyro-radii are smaller than the Debye length, the magnetic field greatly affects the plasmas relaxation processes. The expressions for the time rates of change of the electron and ion parallel and perpendicular temperatures are obtained and calculated analytically for small anisotropies through considering binary collisions between charged particles in the presence of a uniform magnetic field by using perturbation theory. Based on these expressions, the effects of the magnetic field on the relaxation of anisotropic electron and ion temperatures due to electron-electron collisions, ion-ion collisions, and electron-ion collisions are investigated. Consequently, the relaxation times of anisotropic electron and ion temperatures to isotropy are calculated. It is shown that electron-ion collisions can affect the relaxation of an anisotropic ion distribution in the strong magnetic field.

Temperature relaxation in a magnetized plasma

A magnetic field greatly affects the relaxation phenomena in a plasma when the particles’ thermal gyro-radii are smaller than the Debye length. Its influence on the temperature relaxation (TR) is investigated through consideration of binary collisions between charged particles in the presence of a uniform magnetic field within a perturbation theory. The relaxation times are calculated. It is shown that the electron-electron (e-e) and ion-ion (i-i) TR rates first increase and then decrease as the magnetic field grows, and the doubly logarithmic term contained in the electron-ion (e-i) TR rate results from the exchange between the electron parallel and the ion perpendicular kinetic energies.

Tearing mode with guide field gradient in electron magnetohydrodynamics

The tearing mode instability with guide field gradient is investigated in electron magnetohydrodynamics. It is found that the guide field gradient has a significant influence in the inner region. The growth rates are derived analytically in small and large guide field gradient limits, respectively. When the guide field gradient is smaller than the magnetic field shear at the magnetic null plane, the growth rate of tearing mode instability is enhanced by the guide field gradient and has no oscillatory component. When the guide field gradient is larger than the magnetic field shear, the guide field gradient can destabilize tearing mode instability dramatically. In this case, the growth rate is proportional to the guide field gradient.

Magnetic reconnection with pressure gradient effect in compressible electron magnetohydrodynamics

The general dispersion relation of the tearing mode with charge separation and pressure gradient effects in the whistler frequency is analytically derived in the framework of electron magnetohydrodynamics (EMHD). It is shown that pressure gradient effect enhances the growth rate, and makes the EMHD tearing mode drift. The growth rate of the EMHD tearing mode is significantly affected by the pressure gradient effect in the large pressure gradient limit. Furthermore, in this limit, the growth rate in the compressible EMHD fluid is much different from that in the incompressible EMHD fluid.

Magnetic reconnection with electron viscosity in electron magnetohydrodynamics

The general dispersion relation of collisionless reconnection instability due to electron viscosity μe in the whistler frequency is derived. In the framework of electron magnetohydrodynamics (EMHD), the evolution of magnetic reconnection instability is studied, and the linear growth rates are obtained. The scaling laws of the reconnection instability growth rate with respect to the electron viscosity in constant-ψ (used in the tearing mode) and low-k regimes are obtained, respectively, and compare with those obtained in standard magnetohydrodynamic theory. In the constant-ψ regime for “tearing-mode-like” instability, the growth rate is proportional to μe1∕4, while in the low-k regime, it is proportional to μe1∕8.

Influence of ion orbit width on onset threshold of neoclassical tearing modes

The onset threshold of neoclassical tearing modes with finite ion orbit width is studied. The evolution of neoclassical tearing modes including the effect of ion orbit is derived analytically. When ion orbit width is comparable to island width, the effect of ion orbit is significant. It would increase the island width needed to flatten pressure in the island, and reduce the amplitude of ion perturbed bootstrap current. It is found that ion orbit effect tends to increase the onset threshold βθonset for a given seed island. It also would increase the lowest threshold βθ,min and the corresponding marginal island width. It has important implications for the onset of neoclassical tearing modes with comparable ion orbit width and island width in International Thermonuclear Experimental Reactor.

Magnetic reconnection with pressure tensor in electron magnetohydrodynamics

The dissipation mechanisms of reconnection and the pressure gradient effects on tearing mode with guide magnetic field are analyzed systematically by including the electron pressure tensor in electron magnetohydrodynamics. It is found that which dissipation mechanism dominates, either pressure-based dissipation or inertia-based dissipation, has a great relation with the relative scaling orders between the electron thermal Larmor radius and electron inertia skin depth. The effects of pressure gradient also depend on the relative magnitude between parallel and perpendicular equilibrium pressure gradients. When the pressure-based dissipation is dominant, the condition that pressure drives or suppresses tearing mode instability also depends on the relative magnitude between parallel and perpendicular equilibrium pressure gradients.

Influence of resistive internal kink on runaway current profile

An extended magnetohydrodynamic model including the effect of runaway current is developed and implemented in the M3D code. Based on this model, a simulation has been carried out to investigate the linear stability and the nonlinear evolution of the n = 1 resistive kink mode in the presence of runaway current. It is found that sawteeth oscillation is suppressed in a runaway plasma. The nonlinear evolution of the n = 1 mode only leads to a single sawtooth crash before reaching a new steady state axi-symmetric equilibrium with flattened current profile in the plasma core and q(0)>1.

The influence of energetic ions on resistive wall mode in reversed field pinch

A stability analysis of the circulating energetic ions (CEIs) on resistive wall mode is carried out in the reversed field pinch (RFP). In contrast to the minor resonant effects of CEI on resistive wall mode (RWM) in tokamak, the resonant interaction between RWM and CEI is important for high toroidal mode number in RFP with high beta value. The resonance provides an energy dissipation channel of free energy, and stabilizes RWM. As the fraction of CEI is large enough, the RWM is fully suppressed by CEI in the low plasma rotation, even vanishing rotation. Further, a possibility to suppress the RWM by CEI is suggested.