The plasma behavior in a parallel-plate dielectric barrier discharge (DBD) is simulated by a twodimensional particle-in-cell/Monte Carlo collision model, comparing for the first time an unpacked (empty) DBD with a packed bed DBD, i.e., a DBD filled with dielectric spheres in the gas gap. The calculations are performed in air, at atmospheric pressure. The discharge is powered by a pulse with a voltage amplitude of −20 kV. When comparing the packed and unpacked DBD reactors with the same dielectric barriers, it is clear that the presence of the dielectric packing leads to a transition in discharge behavior from a combination of negative streamers and unlimited surface streamers on the bottom dielectric surface to a combination of predominant positive streamers and limited surface discharges on the dielectric surfaces of the beads and plates. Furthermore, in the packed bed DBD, the electric field is locally enhanced inside the dielectric material, near the contact points between the beads and the plates, and therefore also in the plasma between the packing beads and between a bead and the dielectric wall, leading to values of 4 10 8 V m −1 , which is much higher than the electric field in the empty DBD reactor, i.e., in the order of 2 10 7 V m −1 , thus resulting in stronger and faster development of the plasma, and also in a higher electron density. The locally enhanced electric field and the electron density in the case of a packed bed DBD are also examined and discussed for three different dielectric constants, i.e., 22 r  = (ZrO 2 ), 9 r  = (Al 2 O 3 ) and 4 r  = (SiO 2 ). The enhanced electric field is stronger and the electron density is higher for a larger dielectric constant, because the dielectric material is more effectively polarized. These simulations are very important, because of the increasing interest in packed bed DBDs for environmental applications.

We have simulated tokamak breakdown phase driven by pure Ohmic heating with implicit particle in cell/Monte Carlo collision (PIC/MCC) method. We have found two modes can be differentiated. When performing breakdown at low initial gas pressure, we find that it works at lower density and current, but higher temperature, and requires lower heating power, compared to when having a high initial pressure. Further, two stages can be distinguished during the avalanche process. One is the fast avalanche stage, in which the plasma is heated by induced toroidal electric field. The other is the slow avalanche stage, which begins when the plasma density reaches 1015 m−3. It has been shown that ions are mainly heated by ambipolar field and become stochastic in the velocity distribution. However, when the induced electric field is low, there exists a transition phase between the two stages. Our model simulates the breakdown and early hydrogen burn-through under ideal conditions during tokamak start-up. It adopted fewer assumptions, and can give an idealized range of operative parameters for Ohmic start-up. Qualitatively, the results agree well with certain experimental observations.

Magnetical asymmetric effect (MAE) in a geometrically and electrically symmetric capacitively coupled plasma is investigated by a one‐dimensional implicit Particle‐in‐cell/Monte Carlo collision simulation. We applied four types of asymmetric magnetic field parallel to the electrodes and the discharge operates at a single‐frequency rf source of 13.56 MHz and 150 V in argon with the pressure of 30 mTorr. The simulation results show that the asymmetric magnetic field can generate a significant dc self‐bias, which is the result of a particle‐flux balance applied to each electrode. The asymmetric magnetic field with variable gradient can produce controllable asymmetry in the plasma density and ion flux profiles to each electrode, together with a significant change on IEDF shape and width on the powered electrode. It has demonstrated that the MAE is a promising approach to increase the ion flux and still make the ion energy be adjusted in a certain range, that is, independent control of ion flux and energy to the electrode. The results suggest that the MAE can be an effective means to control the plasma properties as an augmentation to conventional measures.

We investigate the band and transport features of Dirac electrons on the surface of a three-dimensional topological insulator under the modulation of a periodic exchange field provided by an array of ferromagnetic insulating (FI) stripes. The Dirac point is shifted (unchanged) by the superlattice when the magnetizations of adjacent FI stripes are parallel (antiparallel). For a finite magnetic superlattice, there exists a full transmission gap for both the parallel and antiparallel configurations. When the two kinds of transmission gaps have no overlap, a large magnetoresistance ratio with a tunable sign can be achieved.