Balling phenomenon, as a typical selective laser melting (SLM) defect, is detrimental to the forming quality. In this work, a detailed investigation into the balling behavior of selective laser melting of stainless steel and pure nickel powder was conducted. It was found that the SLM balling phenomenon can be divided into two types generally: the ellipsoidal balls with dimension of about 500 μm and the spherical balls with dimension of about 10 μm. The former is caused by worsened wetting ability and detrimental to SLM quality; the latter has no obvious detriment to SLM quality. The oxygen content plays an important role in determining the balling initiation, which can be considerably lessened by decreasing the oxygen content of atmosphere to 0.1%. A high laser line energy density, which can be obtained by applying high laser power and low scan speed, could enable a well-wetting characteristic. The effect of scan interval on balling initiation is not obvious as long as the scan track is continuous. The surface remelting procedure can also alleviate the balling effect in a certain extent, due to the melting and wetting of metal balls.Moreover, the balling phenomenon of pure nickel was also studied, and the results implied that the balling discipline had a universality.

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We present the experimental evidence of the collisionless electron bounce resonance heating (BRH) in low-pressure dual-frequency capacitively coupled plasmas. In capacitively coupled plasmas at low pressures when the discharge frequency and gap satisfy a certain resonant condition, the high energy beamlike electrons can be generated by fast sheath expansion, and heated by the two sheaths coherently, thus the BRH occurs. By using a combined measurement of a floating double probe and optical emission spectroscopy, we demonstrate the effect of BRH on plasma properties, such as plasma density and light emission, especially in dual-frequency discharges.

We developed an implicit Particle-in-cell/Monte Carlo model in two-dimensional and axisymmetric geometry for the simulations of the radio-frequency discharges, by introducing several numerical schemes which include variable weights, multigrid field solver, etc. Compared to the standard explicit models, we found that the computational efficiency is significantly increased and the accuracy is still kept. Numerical schemes are discussed and benchmark results are shown. The code can be used to simulate practical reactors.

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.

Most plasma sources have to undergo a breakdown process, during which, energy is injected, and particles are ionized. However, we still know little about this fast evolution process. In this work, a one-dimensional direct implicit particle-in-cell/Monte-Carlo collision (PIC/MCC) program is used to study the breakdown process of a capacitively coupled plasma (CCP) driven by dual radio frequencies. The results show that the breakdown process can be divided into three phases: the pre-breakdown, transition, and post-breakdown phases. In the pre-breakdown phase, the plasma density and heating power grow exponentially. The electric field can penetrate the whole discharge region without any shielding, resulting in a higher-than-average electron energy. Secondary electron emission is critical to grow the electron numbers under these discharge conditions. During the transition phase, the formation of sheaths maximizes the electron generation rate and heating power. The formation of sheaths also causes a drastic change in the electrical characteristics of CCP devices. In the post-breakdown phase, the plasma parameters gradually evolve until a steady state is reached. The decreasing rate of generation and the increasing rate of particle loss gradually equalize. The trends of the power gain and plasma loss are similar to the curves for the particle generation and loss rates, and a dynamic equilibrium is finally reached in the last steady state.

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.

Recently a so-called electrical asymmetry effect (EAE), which could achieve high-degree separate control of ion flux and energy in dual-frequency capacitively coupled radio-frequency (CCRF) discharges, was discovered theoretically by Heil et al. [J. Phys. D: Appl. Phys. 41, 165202 (2008)] and was confirmed by experiments and theory/numerical simulations later on for electropositive argon discharges. In this work simulations based on particle-in-cell/Monte Carlo collision are performed to study the EAE on electronegative oxygen plasmas in geometrically symmetric CCRF discharges. Dual frequency discharges operating at 13.56 and 27.12 MHz are simulated for different pressures and the results are compared with those of electropositive argon discharges at the same conditions. It is found that in general the EAE on oxygen discharges has similar behavior as on argon discharge: The self-bias voltage η increases monotonically and almost linearly with the increase in the phase angle θ between the two driving voltages in the range 0<θ<90°, and the maximum ion energy varies by a factor of 3 by adjusting θ. However, the ion flux varies with θ by ±12% for low pressure and by ±15% for higher pressure, due primarily to an enhanced plasma series resonance, which then leads to dramatic changes in plasma density, power absorption and consequently the electronegativity. This may place a limitation for achieving separate control of ion energy and flux for electronegative plasma via the EAE.