Dynamic Control of Defective Gap Mode Through Defect Location
A one dimensional model is developed for defective gap mode (DGM) with two types of boundary conditions: conducting mesh and conducting sleeve. For a periodically modulated system without defect, the normalized width of spectral gaps equals to the modulation factor, which is consistent with previous studies. For a periodic system with local defects introduced by the boundary conditions, it shows that the conducting-mesh-induced DGM is always well confined by spectral gaps while the conducting-sleeve-induced DGM is not. The defect location can be a useful tool to dynamically control the frequency and spatial periodicity of DGM inside spectral gaps. This controllability can be potentially applied to the interaction between gap eigenmodes and energetic particles in fusion plasmas, and optical microcavities and waveguides in photonic crystals.
Parametric Study on Arc Behavior of Magnetically Diffused Arc
A model coupling the plasma with a cathode body is applied in the simulation of the diffuse state of a magnetically rotating arc. Four parametric studies are performed: on the external axial magnetic field (AMF), on the cathode shape, on the total current and on the inlet gas velocity. The numerical results show that: the cathode attachment focuses in the center of the cathode tip with zero AMF and gradually shifts off the axis with the increase of AMF; a larger cathode conical angle corresponds to a cathode arc attachment farther away off axis; the maximum values of plasma temperature increase with the total current; the plasma column in front of the cathode tip expands more severely in the axial direction, with a higher inlet speed; the cathode arc attachment shrinks towards the tip as the inlet speed increases. The various results are supposed to be explained by the joint e?ect of coupled cathode surface heating and plasma rotating flow.
Electron-Vibrational Energy Exchange in Nitrogen-Containing Plasma: a Comparison Between an Analytical Approach and a Kinetic Model
This paper investigates the electron-vibrational (e-V) energy exchange in nitrogen-containing plasma, which is very efficient in the case of gas discharge and high speed flow. Based on Harmonic oscillator approximation and the assumption of the e-V relaxation through a continuous series of Boltzmann distributions over the vibrational states, an analytic approach is derived from the proposed scaling relation of e-V transition rates. A full kinetic model is then investigated by numerically solving the state-to-state master equation for all vibrational levels. The analytical approach leads to a Landau-Teller (LT)-type equation for relaxation of vibrational energy, and predicts the relaxation time on the right order of magnitude. By comparison with the kinetic model, the LT-type equation is valid in typical electron temperatures in gas discharge. However, the analytical approach is not capable of describing the vibrational distribution function during the e-V process in which a full kinetic model is required.
Non-Thermal Equilibrium Atmospheric Pressure Glow-Like Discharge Plasma Jet
Non-thermal equilibrium atmospheric pressure plasma jet (APPJ) is a cold plasma source that promises various innovative applications, and the uniform APPJ is more favored. Glow discharge is one of the most effective methods to obtain the uniform discharge. Compared with the glow dielectric barrier discharge (DBD) in atmospheric pressure, pure helium APPJ shows partial characteristics of both the glow discharge and the streamer. In this paper, considering the influence of the Penning effect, the electrical and optical properties of He APPJ and Ar/NH3 APPJ were researched. A word “Glow-like APPJ” is used to characterize the uniformity of APPJ, and it was obtained that the basic characteristics of the glow-like APPJ are driven by the kHz AC high voltage. The results can provide a support for generating uniform APPJ, and lay a foundation for its applications.
Application of a BC501A Liquid Scintillation Detector with a Gain Stabilization System on the EAST Tokamak
A 2”×2” BC501A liquid scintillation detector with a gain stabilization system is developed and applied to neutron and γ-ray measurement on the EAST tokamak. Energy cali?bration of a liquid scintillator using a fast coincidence method is presented and compared with the Monte Carlo simulation. Determination of the proton light output function of the BC501A is presented. Results from dedicated experiments with an Am-Be neutron source, γ source and quasi-monoenergetic neutron beams, and from measurements on EAST tokamak are presented and discussed.
Evolution of Striation in Pulsed Glow Discharges
In this work, striations in pulsed glow discharges are studied by experiments and Particle-In-Cell/Monte Carlo Collision (PIC/MCC) simulation. The spatio-temporal evolution of the potential and the electron energy during the discharge are analyzed. The processes of striation formation in pulsed glow discharges and dielectric barrier discharges (DBD) are compared. The results show that the mechanisms of striation in pulsed DC discharge and DBD are similar to each other. The evolution of electron energy distribution function before and after the striation formation indicates that the striation results from the potential well of the space charge. During a pulsed breakdown, the striations are formed one by one towards the anode in a weak field channel. This indicates that the formation of striations in a pulsed discharge depends on the flow of modulated electrons.
Study on Glow Discharge Plasma Used in Polyester Surface Modification
To achieve an atmospheric pressure glow discharge (APGD) in air and modify the surface of polyester thread using plasma, the electric field distribution and discharge characteristics under different conditions were studied. We found that the region with a strong electric field, which was formed in a tiny gap between two electrodes constituting a line-line contact electrode structure, provided the initial electron for the entire discharge process. Thus, the discharge voltage was reduced. The dielectric barrier of the line-line contact electrodes can inhibit the generation of secondary electrons. Thus, the transient current pulse discharge was reduced significantly, and an APGD in air was achieved. We designed double layer line-line contact electrodes, which can generate the APGD on the surface of a material under treatment directly. A noticeable change in the surface morphology of polyester fiber was visualized with the aid of a scanning electron microscope (SEM). Two electrode structures – the multi-row line-line and double-helix line-line contact electrodes - were designed. A large area of the APGD plasma with flat and curved surfaces can be formed in air using these contact electrodes. This can improve the efficiency of surface treatment and is significant for the application of the APGD plasma in industries.
Water Content Effect on Oxides Yield in Gas and Liquid Phase Using DBD Arrays in Mist Spray Hot!
Electric discharge in and in contact with water can accompany ultraviolet (UV) radiation and electron impact, which can generate a large number of active species such as hydroxyl radicals (OH), oxygen radical (O), ozone (O3) and hydrogen peroxide (H2O2). In this paper, a non-thermal plasma processing system was established by means of dielectric barrier discharge (DBD) arrays in water mist spray. The relationship between droplet size and water content was examined, and the effects of the concentrations of oxides in both treated water and gas were investigated under different water content and discharge time. The relative intensity of UV spectra from DBD in water mist was a function of water content. The concentrations of both O3 and nitrogen dioxide (NO2) in DBD room decreased with increasing water content. Moreover, the concentrations of H2O2,O3 and nitrogen oxides (NOx) in treated water decreased with increasing water content, and all the ones enhanced after discharge. The experimental results were further analyzed by chemical reaction equations and commented by physical principles as much as possible. At last, the water containing phenol was tested in this system for the concentration from 100 mg/L to 9.8 mg/L in a period of 35 min.
Effects of Boundary Current on Electromagnetic Dispersion Characteristics for a Relativistic Electron Beam
With betatron oscillation characteristics of the electron beam and ion channel effect taken into account, dispersion characteristics of electrostatic modes and TM modes for a relativistic electron beam guided by ion channel are studied. Dispersion relations are derived and solved numerically to investigate the dependence of the dispersion characteristics for electrostatic modes and TM modes on the betatron oscillation frequency and the ratio of the relativistic electron beam radius to the waveguide radius. The effects of the boundary current on the dispersion characteristic of the TM modes and the interaction between the betatron modes and TM modes are analyzed. When considering the boundary current, for a strong ion channel, a new low-frequency branch of the TM modes arises and the interaction frequency between the betatron modes and the TM01 modes is increased with the same parameters.
Improvement of the Harmonic Technique of Probe for Measurements of Electron Temperature and Ion Density
Conventional Langmuir probe techniques usually face the difficulty of being used in processing plasmas where dielectric compounds form, due to rapid failure by surface insulation. A solution to the problem, the so-called harmonic probe technique, had been proposed and shown effectiveness. In this study, the technique was investigated in detail by changing bias signal amplitudes V0, and evaluated its accuracy by comparing with the conventional Langmuir probe. It was found that the measured electron temperature Te increased with V0, but showing a relatively stable region when V0 >Te/e in which it was close to the true Te value. This is contrary to the general consideration that V0 should be smaller than Te/e for accurate measurement of Te. The phenomenon is interpreted by the non-negligible change of the ion current with V0 at low V0 values. On the other hand, the measured ni also increased with V0 due to the sheath expansion, and to improve the accuracy of ni it needs to linearly extrapolate the ni-V0 trend to V0=0. The results were applied to a diagnosis of the plasmas for chemical vapor deposition of diamond-like carbon thin films and the relationship between plasma parameters and films deposition rates was obtained.
Experimental Investigation on Electromagnetic Attenuation by Low Pressure Radio-Frequency Plasma for Cavity Structure
This paper reports on an experiment designed to test electromagnetic (EM) atten?uation by radio-frequency (RF) plasma for cavity structures. A plasma reactor, in the shape of a hollow cylinder, filled with argon gas at low pressure, driven by a RF power source, was produced by wave-transmitting material. The detailed attenuations of EM waves were investigated under different conditions: the incident frequency is 1-4 GHz, the RF power supply is 13.56 MHz and 1.6-3 kW, and the argon pressure is 75-200 Pa. The experimental results indicate that 5-15 dB return loss can be obtained. From a first estimation, the electron density in the experiment is approximately (1.5-2.2)×1016 m-3 and the collision frequency is about 11-30 GHz. The return loss of EM waves was calculated using a finite-difference time-domain (FDTD) method and it was found that it has a similar development with measurement. It can be confirmed that RF plasma is useful in the stealth of cavity structures such as jet-engine inlet.
Cycloid Motions of Aggregates in a Dust Plasma
Hypocycloid and epicycloid motions of aggregates consisted of one large and one small grains are experimentally observed in an rf dust plasma. The cycloid motions are regarded as combination of a primary circle and a secondary circle. Measurements with high spatiotemporal resolution show that the secondary circle is determined by the initial angle velocity of the dropped aggregate. The primary circle originates from the asymmetry of the aggregate. The small grain in the aggregate always leads the large one as they travelling, which results from the difference of the natural frequency of the two grains. Comparative experiments with regular microspheres show that the cycloid motions are distinctive features of aggregates immersed in a plasma.
PIC/MCC Simulation of Radio Frequency Hollow Cathode Discharge in Nitrogen
A two-dimensional PIC/MCC model is developed to simulate the nitrogen radio frequency hollow cathode discharge (rf-HCD). It is found that both the sheath oscillation heating and the secondary electron heating together play a role to maintain the rf-HCD under the simulated conditions. The mean energy of ions (N+2 ,N+) in the negative glow region is greater than the thermal kinetic energy of the molecular gas (N2), which is an important characteristic of rf-HCD. During the negative portion of the hollow electrode voltage cycle, electrons mainly follow pendulum movement and produce a large number of ionization collisions in the plasma region. During the positive voltage of the rf cycle, the axial electric field becomes stronger and its direction is pointing to the anode (substrate), therefore the ions move toward the anode (substrate) via the axial electric field acceleration. Compared with dc-HCD, rf-HCD is more suitable for serving as a plasma jet nozzle at low pressure.
Discharge Modes Suggested by Emission Spectra of Nitrogen Dielectric Barrier Discharge with Wire-Cylinder Electrodes
In this paper, nitrogen dielectric barrier discharge (DBD) plasma was generated in a quartz tube with coaxial wire-cylinder electrodes at atmospheric pressure. By varying the nitrogen gas flow (FN ) in the range of 0-1m 3/h, the plasma optical emission spectra (OES) were measured and studied. The vibration (Tvib) and rotation temperature (Trot) of nitrogen were obtained, by fitting the rovibronic bands of N2(C3Πu -B3Π g, 0-1), and by the Boltzmann plot method for purposes of comparison. Tvib increased up to 2481 K with increasing nitrogen flow till 0.2 m3/h, and then decreased with further increasing FN , while Trot decreased monotonously and approached to ∼350 K for FN ≥0.6m3/h. The intensity of N2(C3Πu-B3Πg, 0-0, 1-0, 0-3) and N2+(B2Σu+ -X2Σ+g, 0-0) exhibited similar evolution with increasing FN to those of the Tvib and Trot, respectively. The discharge photos revealed that the discharge filaments gradually decreased with increasing FN , and eventually disappeared, which implied that a discharge mode transition emerged with increasing FN . The possible mechanism for the discharge mode transition is studied in detail according to the vibration (Tvib) and rotation temperature (Trot) of nitrogen.
The Effect of Ion Motion on Laser-Driven Plasma Wake in Capillary
The effect of ion motion in capillary-guided laser-driven plasma wake is investigated through rebuilding a two-dimensional analytical model. It is shown that laser pulse with the same power can excite more intense wakefield in the capillary of a smaller radius. When laser intensity exceeds a critical value, the effect of ion motion reducing the wakefield rises, which becomes significant with a decrease of capillary radius. This phenomenon can be attributed to plasma ions in smaller capillary obtaining more energy from the plasma wake. The dependence of the difference value between maximal scalar potential of wake for two cases of ion rest and ion motion on the radius of the capillary is discussed.
Capability Assessment of the Equilibrium Field System in KTX
Radial equilibrium of the KTX plasma column is maintained by the vertical field which is produced by the equilibrium field coils. The equilibrium is also affected by the eddy current, which is generated by the coupling of copper shell, plasma and poloidal field coils. An equivalent circuit model is developed to analyze the dynamic performance of equilibrium field coils, without auxiliary power input to equilibrium field coils and passive conductors. Considering the coupling of poloidal field coils, copper shell and plasma, the evolution of spatial distribution of the eddy current density on the copper shell is estimated by finite element to analyze the effect of shell to balance. The simulation results show that the copper shell and equilibrium field coils can provide enough vertical field to balance 1 MA plasma current in phase 1 of a KTX discharge. Auxiliary power supply on the EQ coils is necessary to control the horizontal displacement of KTX due to the finite resistance e?ect of the shell.