Duk In Choi

Drift wave test particle transport in reversed shear profile

Drift wave maps, area preserving maps that describe the motion of charged particles in drift waves, are derived. The maps allow the integration of particle orbits on the long time scale needed to describe transport. Calculations using the drift wave maps show that dramatic improvement in the particle confinement, in the presence of a given level and spectrum of E×B turbulence, can occur for q(r) profiles with reversed shear. A similar reduction in the transport, i.e., one that is independent of the turbulence, is observed in the presence of an equilibrium radial electric field with shear. The transport reduction, caused by the combined effects of radial electric field shear and both monotonic and reversed shear magnetic q profiles, is also investigated.

Electromagnetic particle simulation of electron cyclotron resonance microwave discharge

We present a numerical model to study the electron cyclotron resonance (ECR) microwave discharge using a one‐dimensional electromagnetic particle‐in‐cell Monte Carlo collision method [C. K. Birdsall, IEEE Trans. Plasma Sci. 19, 65 (1991)]. In our model, the electromagnetic wave is polarized circularly and propagates along an external static magnetic field and elastic, excitational, and ionizing electron‐neutral collisions and elastic and charge exchange ion‐neutral collisions are included. The discharge for helium gas is simulated and the simulation results explain well the physical properties of the ECR discharge which include the energy absorption of electrons through ECR coupling, the propagation of microwave, the transports of the charged particles, and the effect of divergent external magnetic field.

Influence of ion effects on high-current relativistic diodes

Properties of bipolar electron and ion flow in a relativistic planar diode are investigated within the framework of the cold‐fluid‐Poisson equations. Defining the parameter q by q=(Ji/Je) [(mi/2Zme)]1/2, the electron current density Je is expressed in terms of the diode voltage, the anode‐cathode gap, and the parameter q satisfying q≤qs=(γ0+1)1/2/2. Here me is the rest mass of electrons, Z and mi are the charge state and the rest mass, respectively, of ions, Ji is the ion current density, γ0 is the relativistic mass ratio of electrons at the anode, and the extreme value q=qs represents the space‐charge limited ion flow. From the analysis, it is found that presence of the ions in the diode significantly enhances the electron current, although the ion current is less than few percent of the electron current. In an appropriate physical parameter regime, the electron current with counterstreaming ions is more than twice of that without ions.

Test particle simulations for transport in toroidal plasmas

The authors derive the guiding center equations of motion from the phase space Euler–Lagrange formulation for the motion of a charged particle in toroidal magnetic confinement geometry. The guiding center equations are numerically solved together with the Monte Carlo Coulomb collisional pitch angle scattering. The numerically calculated microscopic diffusion coefficients for various values of collisionality ν* in the case of no electrostatic potential agree well with the results of neoclassical theory. The diffusion coefficient is then measured in the presence of a model electrostatic drift wave fluctuation and an equilibrium potential. The diffusion coefficient increases with increasing fluctuation amplitude while the equilibrium potential diminishes the diffusion processes through both the orbit squeezing and the Er×B shear flow of the poloidal velocity.

Global drift wave map test particle simulations

Global drift wave map equations that allow the integration of particle orbits on long time scales are implemented to describe transport. Ensembles of test particles are tracked to simulate the low-confinement mode/reversed shear/enhanced reversed shear plasmas in the Tokamak Fusion Test Reactor (TFTR) tokamak and the Optimized Shear plasma in the Joint European Torus (JET) tokamak. The simulations incorporate a radial electric field, Ēr, obtained from a neoclassical calculation [Zhu et al., Phys. Plasmas 6, 2503 (1999)] and a model for drift wave fluctuations that takes into account change in the mode structure due to Ēr [Taylor et al., Plasma Phys. Controlled Fusion 38, 1999 (1996)]. Steady state particle density profiles along with two different measures of transport, the diffusion coefficient based on a running time average of the particle displacement and that calculated from the mean exit time, are obtained. For either weak or reversed magnetic shear and highly sheared Ēr, particle transport barriers...

Influence of Hall current on the stability properties of a self‐pinched intense electron beam

Influences of Hall current on the stability properties of azimuthally symmetric perturbations (e.g., sausage and hollowing modes) of a self‐pinched electron or ion beam immersed in a resistive plasma are investigated. The perturbed space charge and plasma current with the Hall term are treated self‐consistently for any value of the plasma conductivity. It is assumed that ν/γb ≪1, where ν is Budker’s parameter and γbmc2 is the characteristic beam electron energy. The analysis is carried out within the framework of linearized Vlasov–Maxwell equations. In the high‐conductivity background plasma, the growth rates of the sausage (n=1) and hollowing (n=2) modes are significantly enhanced by the influence of Hall currents.

Drift Wave Test Particle Transport in Reversed Shear Profile

Drift wave maps, area preserving maps that describe the motion of charged particles in drift waves, are derived. The maps allow the integration of particle orbits on the long time scale needed to describe transport. Calculations using the drift wave maps show that dramatic improvement in the particle confinement, in the presence of a given level and spectrum of E x B turbulence, can occur for q(r)-profiles with reversed shear. A similar reduction in the transport, i.e. one that is independent of the turbulence, is observed in the presence of an equilibrium radial electric field with shear. The transport reduction, caused by the combined effects of radial electric field shear and both monotonic and reversed shear magnetic q-profiles, is also investigated.

Electron diamagnetic drift effect on the trapped hot particle-driven resistive interchange mode

The linearized flux-surface-averaged resistive magnetohydrodynamic equations, which include the electron diamagnetic drift and hot particle effects, are derived and studied numerically. The effect of electron diamagnetic drift on the trapped hot particle-driven resistive interchange mode is then investigated. It is shown that the effect can have a strong stabilization effect on the mode, similar to the conventional resistive interchange mode case.

Influence of the return current effects on the diocotron instability of a relativistic hollow electron beam

Influence of the return current effects on the diocotron instability of a relativistic hollow electron beam propagating through a background plasma is investigated within the framework of a cold fluid model. The return current density induced in the background plasma is taken to be steadily proportional to the axial electron beam current density. By making use of the linearized fluid‐Maxwell equations, a dispersion relation for the eigenfrequencies of the system is derived and used to examine instabilities. It is found that as the fraction of current neutralization increases, lower mode instabilities become dominant while higher mode perturbations are stabilized except in the case of very thin beams. It is also observed that for all values of the fraction of current neutralization, increasing the plasma density gradient or decreasing the beam thickness causes a destabilizing influence.