Self-consistent Kinetic Description of the Low-Pressure Solenoidal Inductively Coupled Argon Discharge

Using an one-dimensional slab model, we have studied the electron energy distribution, the anomalous skin effect, and power absorption in the solenoidal-inductively-coupled argon discharge under low pressures (≤1.33 Pa). The electron energy distribution function and rf electromagnetic field in the plasma are determined self-consistently by the linearized Bolztmann equation incorporating with the Maxwell equations. The numerical results show that, at low pressures, the electron energy distribution function exhibits a non-Maxwellian distribution with a long high-energy tail. The anomalous skin effect is greatly enhanced under low pressures and the negative power absorption is also obtained.     

On peak current in atmospheric pulse-modulated microwave discharges by the PIC-MCC model

Pulse modulation provides a new way to tailor the electron density,electron energy and gas temperature in atmospheric radio-frequency (rf) discharges.In this paper,by increasing the rf frequency to several hundreds of MHz,or even much higher to the range of GHz,a very strong peak current in the first period (PCFP) with much larger electron energy can be formed during cthe power-on phase,which is not observed in the common pulse modulation discharges at a rf frequency of 13.56 MHz.The PIC-MCC model is explored to unveil the generation mechanism of PCFP,and based on the simulation data a larger voltage increasing rate over a quarter of a period and the distribution of electron density just before the power-on phase are believed to play key roles;the PCFP is usually produced in the rnicroplasma regime driven by the pulsed power supply.The effects of duty cycle and pulse modulation frequency on the evolution of PCFP are also discussed from the computational data.Therefore,the duty cycle and pulse modulation frequency can be used to optimize the generation of PCFP and high-energy electrons.     

Effect of Internal Antenna Coil Power on the Plasma Parameters in13.56 MHz/60 MHz Dual-Frequency Sputtering

The plasma property of a hybrid ICP/sputtering discharge driven by 13.56 MHz/60 MHz power sources was investigated by Langmuir probe measurement. For the pure sputtering discharge, the low electron density and ion flux, the rise of floating potential and plasma potential with increasing power, as well as the bi-Maxwellian distribution of electron en- ergy distributions （EEDFs） were obtained. The assistance of ICP discharge led to the effective increases of electron density and ion flux, the suppression of rise of floating potential and plasma potential, as well as the change of EEDFs from bi-Maxwellian distribution into Maxwellian dis- tribution. The increase of electron density and ion flux, and the EEDFs evolution were related to the effective electron heating by the induced electric field.     

Numerical simulation of atmospheric pulse-modulated radio-frequency glow discharge ignition characteristics assisted by a pulsed discharge

A one-dimensional self-consistent fluid numerical model was developed to study the ignition characteristics of a pulse-modulated(PM) radio-frequency(RF) glow discharge in atmospheric helium assisted by a sub-microsecond voltage excited pulsed discharge. The temporal evolution of discharge current density and electron density during PM RF discharge burst was investigated to demonstrate the discharge ignition characteristics with or without the pulsed discharge. Under the assistance of pulsed discharge, the electron density in RF discharge burst reaches the magnitude of 1.87 × 10~(17) m~(-3) within 10 RF cycles, accompanied by the formation of sheath structure. It proposes that the pulsed discharge plays an important role in the ignition of PM RF discharge burst. Furthermore, the dynamics of PM RF glow discharge are demonstrated by the spatiotemporal evolution of the electron density with and without pulsed discharge. The spatial profiles of electron density, electron energy and electric field at specific time instants are given to explain the assistive role of the pulsed discharge on PM RF discharge ignition.     

An RF Technique for Plasma Number Density Measurement

It is well-known that a cylindrical plasma column in a transverse rf electric field is subject to a dipole resonance. The frequency at which this occurs can be used to determine the average electron density across the plasma section. This paper describes the experimental application of the technique to plasmas with and without a confining axial static magnetic field, and gives theoretical curves to assist in rapid reduction of the data. Higher order multipole resonances can also be excited. The feasibility of applying these to determination of the number density profile is discussed.     

Effect of Radial Density Configuration on Wave Field and Energy Flow in Axially Uniform Helicon Plasma

The effect of the radial density configuration in terms of width, edge gradient and volume gradient on the wave field and energy flow in an axially uniform helicon plasma is studied in detail. A three-parameter function is employed to describe the density, covering uniform,parabolic, linear and Gaussian profiles. It finds that the fraction of power deposition near the plasma edge increases with density width and edge gradient, and decays in exponential and "bumpon-tail" profiles, respectively, away from the surface. The existence of a positive second-order derivative in the volume density configuration promotes the power deposition near the plasma core, which to our best knowledge has not been pointed out before. The transverse structures of wave field and current density remain almost the same during the variation of density width and gradient, confirming the robustness of the m=1 mode observed previously. However, the structure of the electric wave field changes significantly from a uniform density configuration, for which the coupling between the Trivelpiece-Gould(TG) mode and the helicon mode is very strong, to non-uniform ones. The energy flow in the cross section of helicon plasma is presented for the first time, and behaves sensitive to the density width and edge gradient but insensitive to the volume gradient. Interestingly, the radial distribution of power deposition resembles the radial profile of the axial component of current density, suggesting the control of the power deposition profile in the experiment by particularly designing the antenna geometry to excite a required axial current distribution.     

Study of the fast electron behavior in electron cyclotron current driven plasma on J-TEXT

In J-TEXT tokamak, fast electron bremsstrahlung diagnostic with 9 chords equipped with multi-channel analyzer enables detailed studies of the generation and transport of fast electrons. The spatial profiles and energy spectrum of the fast electrons have been measured in two ECCD cases with either on-axis or off-axis injection, and the profiles processed by Abel-inversion are consistent with the calculated power deposition locations. Moreover, it is observed that the energy of fast electrons increases rapidly after turning off the ECCD, which may be attributed to the acceleration by the recovered loop voltage at low electron density.     

Effects of Sheared Flow on Microinstabilities and Transport in Plasmas

Theoretical and experimental studies associated with electric field effects on the stability and transport are briefly surveyed. The effects of radial electric field on the suppression and/or enhancement of various microinstabilities such as drift waves, flute mode and temperature gradient modes are discussed. The suppression of flow shear on the electron temperature gradient mode in plasmas with slightly hollow density profiles is investigated by solving the gyrokinetic integral eigenvalue equation. Comparison between theoretical predictions and experimental observations based on the HIBP measurements with high temporal and spatial resolutions is made in bumpy tori and heliotron (CHS) devices.     

Theoretical research on the transport and ionization rate coefficients in glow discharge dusty plasma

The electron kinetic model for investigating the transport and ionization rate coefficients of argon glow discharge dusty plasma is developed from the Boltzmann equation.Both of the electron-neutral and electron-dust collisions are considered as collision terms in the kinetic equation.The kinetic equation is simplified by employing the local approximation and nonlocal approach under the same discharge conditions,and the corresponding simplified kinetic equations are known as local and nonlocal kinetic equations respectively.Then the electron energy distribution function(EEDF)is obtained by numerically solving the local and nonlocal kinetic equations and the dust charging equations simultaneously.Based on the obtained EEDFs,the effective electron temperature,electron mobility,electron diffusion coefficient and ionization rate coefficient are calculated for different discharge conditions.It is shown that the EEDFs calculated from the local kinetic model clearly differ from the nonlocal EEDFs and both the local and nonlocal EEDFs are also clearly different with Maxwellian distributions.The appearance of dust particles results in an obvious decrease of high energy electrons and increase of low energy electrons when axial electric field is low.With the increase of axial electric field,the influence of dust particles on the EEDFs becomes smaller.The electron mobility and diffusion coefficients calculated on the basis of local and nonlocal EEDFs do not differ greatly to the dust-free ones.While,when dust density nd=10^6 cm^?3,the electron mobility increases obviously compared with the dust-free results at low axial electric field and decreases with the increasing axial electric field until they are close to the dust-free ones.Meanwhile,electron diffusion coefficients for dusty case become smaller and decrease with the increasing axial electric field.The ionization rate coefficients decrease when dust particles are introduced and they approach the dust-free results gradually with the increasing axial electric field.     

Density disturbance of small-scale field- aligned irregularities in the ionosphere heating experiments

A theoretical model which describes the small-scale irregularities excited by powerful high frequency (3–30 MHz) electromagnetic wave in ionosphere heating is investigated quantitatively in this paper. The model is based on the transport equation in magnetic plasma and mode conversion from electromagnetic wave to electrostatic wave in ionospheric modification. Threshold electric field for exciting small-scale (meter scale) irregularities and spatial spectra of irregularities are analytically calculated by this model. The results indicate that background electron density and geomagnetic field play an important role for the threshold electric field and the spatial scale of the electron density irregularities. The results demonstrate that the electric field threshold increases with the decrease of the spatial scale of the irregularities. For exciting meter scale irregularities, the threshold electric field is about tens of mV m^-1 . The theoretical results are consistent with those of the experiments.     

Numerical Simulation of Modified Radial Electric Field by LHCD

Based on the electron‘s radial force equilibrium, the profiles of radial electric field in OH and LHCD phase are calculated by using a simulation code. The dependences of radial electron field on electron density and its profile and different current ratio, Irf/Ip, are given. The connections between the improvement of plasma confinement and the modified radial electric field by LHCD are discussed by comparing the calculated results with the experimental results.     

Effects of direct current discharge on the spatial distribution of cylindrical inductively-coupled plasma at different gas pressures

Stable operations of single direct current(DC) discharge, single radio frequency(RF) discharge and DC?+?RF hybrid discharge are achieved in a specially-designed DC enhanced inductivelycoupled plasma(DCE-ICP) source. Their plasma characteristics, such as electron density,electron temperature and the electron density spatial distribution profiles are investigated and compared experimentally at different gas pressures. It is found that under the condition of single RF discharge, the electron density distribution profiles show a ‘convex' shape and ‘saddle' shape at gas pressures of 3 m Torr and 150 m Torr respectively. This result can be attributed to the transition of electron kinetics from nonlocal to local kinetics with an increase in gas pressure.Moreover, in the operation of DC?+?RF hybrid discharge at different gas pressures, the DC discharge has different effects on plasma uniformity. The plasma uniformity can be improved by modulating DC power at a high pressure of 150 m Torr where local electron kinetics is dominant,whereas plasma uniformity deteriorates at a low pressure of 3 m Torr where nonlocal electron kinetics prevails. This phenomenon, as analyzed, is due to the obvious nonlinear enhancement effect of electron density at the chamber center, and the inherent radial distribution difference in the electron density with single RF discharge at different gas pressures.     

小型电视塔电磁环境影响特征分析

以某县级电视发射塔为研究对象,在系统分析其工作原理基础上,预测可能产生的电磁环境影响,确定重点评价因子为射频电场强度、功率密度。通过理论计算,与发射天线轴向距离20 m处即可满足电磁环境影响评价标准要求。考虑电视塔周边建构筑物分布复杂,对电磁波产生一系列反射、绕射,开展现场实测,结果表明周边环境保护目标电场强度满足、功率密度满足限值要求。在此基础上分析电视塔周围电场强度分布特征,明确电磁环境影响程度,并提出定期监测、划定限制区域、避免发射机故障等针对性防护措施及建议,为同类工作开展提供经验反馈。     

The Effect of Gas Flow Rate on Radio-Frequency Hollow Cathode Discharge Characteristics

It is known that gas flow rate is a key factor in controlling industrial plasma processing. In this paper, a 2D PIC/MCC model is developed for an rf hollow cathode discharge with an axial nitrogen gas flow. The effects of the gas flow rate on the plasma parameters are calculated and the results show that： with an increasing flow rate, the total ion（N＋2, N＋） density decreases, the mean sheath thickness becomes wider, the radial electric field in the sheath and the axial electric field show an increase, and the energies of both kinds of nitrogen ions increase;and, as the axial ion current density that is moving toward the ground electrode increases, the ion current density near the ground electrode increases. The simulation results will provide a useful reference for plasma jet technology involving rf hollow cathode discharges in N2.     

The residual zonal flow in tokamak plasmas with a poloidal electric field

In a tokamak plasma with auxiliary heating by cyclotron waves, a poloidal electric field will be produced, and as a consequence influence the residual zonal flow(RZF) level. The poloidal electric field can also be induced through biasing electrodes at the edge region of tokamaks.Numerical evaluation for a large aspect ratio circular cross section tokamak for the electron cyclotron wave heating indicates that the RZF level decreases significantly when the poloidal electric field increases. Qualitatively, the ion cyclotron wave heating is able to increase the RZF level. It is difficult to apply the calculation to the real cyclotron wave heating experiments since we need to know factors such as the plasma profiles, the exact power deposition and the cross section geometry, etc. It is possible to use the cyclotron wave heating to control the zonal flow and then to control the turbulence level in tokamak experiments.     

Effect of radio-frequency substrate bias on ion properties and sputtering behavior of 2 MHz magnetron sputtering

The effect of radio-frequency substrate bias on ion properties and sputtering behavior of 2 MHz magnetron discharge was investigated. The ion velocity distribution function(IVDF), the maximum ion energy and ion flux density were measured at the substrate by a retarding field energy analyzer. The sputtering behavior was investigated by the electric characteristics of target and bias discharges using voltage–current probe technique. It was found that the substrate bias led to the decrease of sputtering power, voltage and current with the amplitude 7.5%. The substrate bias also led to the broadening of IVDFs and the increase of ion flux density, made the energy divergent of ions impacting the substrate. This effect was further enhanced by increasing bias power and reducing discharge pressure.     

理想长空间电荷透镜中的电子云

本文研究理想长空间电荷透镜中电子云的运动。对于在确定条件下运行的空间电荷透镜,其电子云的状态可用磁约束因子、电子温度和透镜中心的电子密度这三个参数来描述。这些参数确定了电子云的大小、形状和电子密度分布,从而也确定了空间电荷透镜的电场分布与聚焦特性。文中利用数值方法进行了电子密度和电位的计算。     

Magnet stage optimization of 5 kW multi-cusped field thruster

The multi-cusped field thruster is a unique electric thruster device, which has many advantages such as long lifetime, large-range thrust throttling ability, high thrust density, and low mass. The thruster employs several alternating polarity permanent magnets to create a periodic magnetic field with several cusps. Previous studies have indicated that the basic ionization and acceleration processes are directly related to the electron motion behavior, which mainly depends on the magnetic field characteristics. The magnet number and magnet stage length are two key magnetic field parameters that have important effects on the thruster performances. In this paper, both the magnet number and magnet stage length parameters are studied for the optimization of a 5 k W multi-cusped field thruster. The results indicate that the three-stage thruster has a better electron confinement than the two-stage thruster. It has lower ion energy loss at the wall, and shows a higher ionization rate. Therefore, the three-stage magnetic field is a superior magnetic field configuration. Besides, the three-stage magnetic field simulation results indicate that an optimal accelerating electric field distribution and ionization region distribution could be obtained when the magnet length ratio is 78:25:20.     

Kinetic simulation of the transition from a pulse-modulation microwave discharge to a continuous plasma

The generation of a very strong peak current in the first period(PCFP) in a pulse-modulated microwave discharge has been discussed in previous studies. In this paper we focus on the transition process from a pulsed discharge to a fully continuous one driven by the pulsed microwave power source by means of a kinetic model. The computational results show that by increasing the duty cycle or voltage modulation rate(VMR), the discharge eventually becomes fully continuous and PCFP can no longer be observed. In the transition process, the distributions of the electric field, electron energy probability function(EEPF) and plasma density are discussed according to the simulation data, showing different discharge structures. The simulations indicate that many high-energy electrons with electron energy larger than 20 eV and low-energy electrons with electron energy less than 3 eV could be generated in a pulsed microwave discharge, together with a reversal electric field formed in the anode sheath when PCFP occurs. However, only medium-energy electrons could be observed in a fully continuous discharge. Therefore, by investigating the transition process the pulse-modulated microwave discharges can be further optimized for plasma applications at atmospheric pressure.