短管螺旋波放电中等离子体参数测量和模式转化研究

对长度为45 cm的短放电管螺旋波放电等离子体进行了Langmuir探针、原子发射光谱以及集成电荷耦合检测器(ICCD)检测诊断,研究螺旋波等离子体的放电特性.Langmuir探针数据显示电子密度在射频功率增加过程中出现两次大幅增长,由此确认了放电模式的转换及螺旋波放电模式的出现.发射光谱测量结果与Langmuir探针测量的电子密度数据一致,发现Ar原子和Ar离子的谱线强度与放电模式变化有着密切相关性.而通过对不同放电模式的ICCD测量,获得射频功率吸收因放电模式转变而变化的方式,认为放电模式转换时电子行为和能量传递方式也发生着变化.     

磁场对螺旋波等离子体波和能量吸收影响的数值研究

本文考察在径向电子数密度呈抛物形分布的情况下,外加稳恒磁场,射频通过螺旋波天线在等离子体中激发电磁波的传播性质.采用线性扰动波假设,数值求解Maxwell方程组,得到80—800 G(1 G=10-4T)磁场条件下等离子体中径向电、磁场强度及能量沉积密度的分布情形.计算结果表明,磁场增大(80→800G)时,螺旋波受到的阻尼较小,可深入等离子体传播;Trivelpiece-Gould(TG)波受到的阻尼增大,在等离子体-真空边界处衰减增强;整体的能量吸收向边界集中.磁感应强度小于100 G时,TG波可深入主等离子体区传播,等离子体径向能量吸收相对均匀.     

氩气压力对螺旋波放电影响的发射光谱诊断及仿真研究

螺旋波等离子体源以其高电离效率与高密度优势受到多个领域的青睐。螺旋波放电高电离效率的机理或者功率耦合模式,一直是困扰该领域学者的难点之一,对于放电过程与特性的诊断则是揭示其物理机制的重要途径。光谱诊断能够克服介入式诊断手段对等离子体的干扰同时受等离子体烧蚀等弊端,且响应速度快、操作灵活。为研究螺旋波等离子体的放电特性以及气体压力的影响,开展了以氩气为工质气体的光谱实验研究,并针对实验开展了Helic程序数值模拟。通过改变光纤探头焦距调整径向诊断位置,得到谱线强度的径向分布。由氩原子4p-4s能级跃迁产生的谱线主要集中在740～920 nm区间,谱线相对强度较离子激发谱线较强。实验研究发现,在较低氩气压力范围（0.2 Pa<P_Ar<1.0 Pa）,随着压力增加,放电光强迅速增加,但是当压力增加到大于1.0 Pa之后,光强增长的趋势变缓,甚至部分谱线的相对强度不再增长,达到类饱和状态,朗缪尔探针测量得到离子密度变化趋势与其相似。光强分布在靠近径向边界处（r≈4 cm）存在凸起,且随压力增加,该凸起分布更为明显。通过对电子温度的计算发现,压力增加到一定程度将影响放电均匀性。仿真结果显示,增大压力,功率沉积密度的径向分布逐渐向径向边界处积累,与实验观察到的谱线强度径向凸起相一致,螺旋波与TG波的耦合效率增加。随着气体压力的增加,E_r的径向边界峰值降低,原因是波所受阻尼增强,TG波被有效地局限于径向较窄的边界处。电流密度轴向分量J_z在等离子体内部和边界处的峰值呈显著的减小趋势,可见,虽然压力增加一定程度上提高了等离子体密度,但却相应的减小了电离率,导致轴向电流密度受限。但是径向电流密度J_r却呈现先减小后增大的趋势,且增长幅度明显,综合来看,放电效率有所提高。可见适当增加气体压力,有助于提高放电的功率耦合效率和强度,增加等离子体密度。光强比值法是针对线性谱线参数计算的典型方法,Helic程序亦是专业领域内认可度很高的计算工具,结果可靠,分析方法具有可借鉴性。实验及仿真结果对于提高氩气工质下的螺旋波放电强度提供了一定的参考价值。     

Kinetic study of instability growth rate in a helicon plasma discharge source

In this work, a kinetic model is developed to study the effects of the radio frequency antenna wavenumber, helicon plasma electron density, as well as their drift velocity and temperature on the instability increment rate of the helicon wave in both longitudinal and transverse directions. The ion acoustic (IA) wave frequencies and wavenumbers of the helicon waves are obtained when the maximum wave energy is deposited on the plasma ions. Moreover, it is shown that, at the IA wavenumber and frequencies, while the longitudinal instability increment rates for both the helicon and IA waves are ignorable, the transverse instability increment rate for both the helicon and IA wave increases. Besides, the longitudinal instability increment rate for the helicon or IA wave has non‐zero resonant frequencies. On the other hand, the transverse instability increment rate of helicon or IA wave can be neglected. Furthermore, it is observed that, while both the imaginary part of longitudinal permittivity and longitudinal instability increment rate are not influenced by the electron temperature, their transverse component increases linearly with the electron temperature. In addition, the imaginary part of transverse permittivity increases almost linearly with the drift velocity of the plasma electrons.     

The role of second‐order radial density gradient for helicon power absorption

To demonstrate the mysterious and still controversial mechanism of high ionization efficiency during helicon discharges, this work focuses particularly on the role of second‐order radial density gradient (SRDG) in helicon power absorption, both analytical and numerical. It was found that the positive or negative sign of SRDG and radial location of vanishing SRDG determine the radial profile of power absorption remarkably. First, by measuring SRDG at two radial locations (near plasma core and edge) where power absorption usually peaks, and varying it as a function of free parameter, we see that: (a) the power absorption from the antenna to plasma increases for positive SRDG and decreases for negative SRDG when viewed in the same x‐coordinate direction of SRDG and (b) the power absorption is maximized near the position where this local SRDG vanishes, consistent with the theory of radially localized helicon mode (B. N. Breizman and A. V. Arefiev, Phys. Rev. Lett., 84:3863, 2000). Second, by choosing the whole radial profiles of typical plasma distribution that have zero‐crossing SRDG, we find that the power absorption redistributes significantly when the location of vanishing SRDG moves radially outwards, and specifically when the radial locations of maximum power absorption and vanishing SRDG move in the same direction near the plasma core but noticeably in the opposite direction near the plasma edge. These findings are very interesting for helicon plasma applications that require certain power distribution or heat flux configuration, for example, material processing, which can be controlled by adjusting the radial profile of SRDG, especially the zero‐crossing SRDG.     

高通鸟笼线圈激发螺旋波等离子体特性研究

研制了高通鸟笼线圈,使用氩气作为工质气体对射频天线的工作性能进行了初步评估。利用COMSOL5.4模拟出了鸟笼天线在13.56MHz的工作频率下,电场和磁场呈线性极化分布。对鸟笼线圈进行了电路结构解剖,推导出了其谐振频率计算公式。利用热耦合红外测温仪测试了正常工作状态下的鸟笼线圈外表温度明显低于传统射频天线,电容器最高温度仅65.8°。使用光谱仪对螺旋波等离子体放电光谱特性进行诊断。通过朗缪尔探针诊断了不同压强和磁场强度下螺旋波等离子体密度,在1.0Pa、600Gs、射频功率700W条件下等离子体密度达到1.62×10¹⁸m^-3。诊断了正向功率和反向功率对应的等离子体密度,其与磁滞现象变化趋势雷同。测试了螺旋波等离子体的径向密度分布,其在轴心处密度达到最高。探究了无磁场条件下等离子体特性,其密度值不会发生大幅度跃迁,纵向磁场是引发螺旋波等离子体放电的关键因素,低压条件下有利于得到更低的电子温度,最低达到2.67eV。表明鸟笼线圈低热耗、高馈入的特性使其在激发大体积的高密度螺旋波等离子体方面具有明显优势,可以投入到下一阶段氢气螺旋波等离子体的激...     

Low-Power RF plasma sources for technological applications: III. helicon plasma sources

The third part of the paper is devoted to an investigation of the so-called helicon plasma sources. These are RF devices operating with a relatively weak external magnetic field, which is, at the same time, strong enough for the resonant electron gyrofrequency to be much higher than the industrial frequency (ω=8.52×10⁷ s⁻¹). As in [1, 2], a study is made of elongated cylindrical plasma sources in a longitudinal (directed along the cylinder axis) magnetic field and planar disk-shaped plasma sources in a transverse (perpendicular to the plane of the disk) magnetic field. A comparison of the present results with the results that were obtained in [3] without using the helicon approximation leads to the conclusion that elongated helicon sources hold great promise for plasma technologies.     

Effects of drive current rise-time and initial load density distribution on Z-pinch characteristics

A two-dimensional, three-temperature radiation magneto-hydrodynamics model is applied to the investigation of evolutional trends in x-ray radiation power, energy, peak plasma temperature and density as functions of drive current rise-time and initial load density distribution by using the typical experimental parameters of tungsten wire-array Z-pinch on the Qiangguang-I generator. The numerical results show that as the drive current rise-time is shortened, x-ray radiation peak power, energy, peak plasma density and peak ion temperature increase approximately linearly, but among them the x-ray radiation peak power increases more quickly. As the initial plasma density distribution in the radial direction becomes gradually flattened, the peak radiation power and the peak ion-temperature almost exponentially increase, while the radiation energy and the peak plasma density change only a little. The main effect of shortening drive current rise-time is to enhance compression of plasma, and the effect of flattening initial load density distribution in the radial direction is to raise the plasma temperature. Both of the approaches elevate the x-ray peak radiation power.     

螺旋波等离子体推进器中双层形成的PIC模拟

建立了螺旋波等离子体推进器中双层等离子体形成过程的一维全粒子PIC模型,讨论了放电过程中电子加热模型和等离子体膨胀模型,计算了不同条件下螺旋波等离子体推进器中双层等离子体的分布。结果表明:采用氩气工质,在射频电流90A/m2和工作压强0.05~0.15Pa的条件下,可产生双层等离子体,其密度为1.3×1015~2.8×1015 m-3,双层电势降为10~30V,可加速氩离子束流,喷射速度为3~12km/s,双层加速效应对推进器的性能具有一定影响。     

Electrical characteristics of helicon wave plasmas

The external electrical characteristics of helicon wave plasmas have been studied over a wide range of magnetic fields, radio frequency (RF) power, frequencies, and Ar gas pressures. External parameters, such as antenna voltage, current, and phase shifts, and internal parameters, such as electron density, are measured. The equivalent discharge resistance, reactance, and power transfer efficiency are calculated through these measurements. The characteristics of helicon mode is compared with inductively coupled plasma (ICP) and low mode. The power efficiency of the helicon mode is better than that of other modes. Consequently, electron density of helicon mode is much higher than that of other modes. This means the existence of a mechanism where electrons are very efficiently accelerated by the electric field of the antenna in the helicon mode. The power efficiency of helicon mode is higher at lower RF frequency and at optimum gas pressure than that of other modes     

托卡马克等离子体不同运行模式下的电子回旋波电流驱动

在给定等离子体密度分布下，从电子、离子的能量方程出发，根据不同运行模式下等离子体的热传导率不同，分别求出了中心负剪切模式，常规剪切H模式和L模式下的等离子体温度分布，然后通过求解波迹方程与相对论情况下的Fokker-Planck方程，分别计算了这些模式下的电子回旋波电流驱动和波功率沉积.得到在中心负剪切下，驱动电流最大，驱动效率最高，功率沉积和电流分布区间跨度大；在常规剪切H模式下，驱动电流较小，分布区间跨度比较窄，驱动效率相对较低；在常规剪切L模式，驱动电流效率最低，分布区间跨度也非常集中. 关键词： 托卡马克 电子回旋波电流驱动 中心负剪切 常规剪切     

On helicon wave induced radial plasma transport

Estimates of helicon wave induced radial plasma transport are presented. The wave induced transport grows or decreases depending on the sign of the azimuthal wave number, these changes in transport may be important for helicon wave plasma sources.Dedicated to the memory of Professor Jozef Kvasnica.     

Electron transport properties of stationary plasma clouds generatedwith hollow cathode discharges

The electron transport properties of plasma clouds generated by a hollow cathode discharge are investigated. The voltage-current characteristic curves indicate that the electron-emitted electron current depends on both bias voltage and discharge current. The spatial dependence of the electronic density, plasma potential, and electronic temperature is measured. The energy loss rate of a high energy group of electrons with the radial distance is also presented. These experimental results are compared with a nonisothermal transport model which reproduces the spatial dependence of plasma properties     

Nonlinear collisional monte carlo simulations for high-temperature SOL plasma

A nonlinear Monte Carlo collisional model is applied to to investigate scrape-off layer (SOL) plasmas with high temperatures. In the proposed SOL modeling, A steady state SOL plasma, which satisfies the particle and energy balances and neutrality constraint, is determined in terms of total particle and heat fluxes across the separatrix, the edge plasma temperature, the secondary electron emission coefficient, and the SOL size. A conductive heat flux into the SOL is effectively modeled via random exchange of source particles and the SOL plasma particles. It is found that the potential drop and the electron transmission factor in the collisional SOL plasma are in good agreement with the theoretical prediction. The cooling effect of secondary electrons in the high temperature divertor operation is investigated. In such a collisionless plasma, the present nonlinear collision model is useful because the electron distribution function deviates far from a Maxwellian distribution. In the presence of strong secondary electron emission, the electron sheath energy transmission factor in the collisionless regime is found to be significantly smaller than that in the collisional regime. This fact suggests that a high-temperature divertor operation can be possible.     

Interchange-turbulence-based radial transport model for SOLPS-ITER: A COMPASS case study

Mean-field plasma edge transport codes such as SOLPS-ITER heavily rely on ad-hoc radial diffusion coefficients to approximately model anomalous transport. Such coefficients are experimentally determined and vary between different machines, and also depend on the operational regime and plasma location within the same device. Therefore, to match experimental data the modeller is required to manually tune several free parameters in expensive simulations, and the code's predictive capabilities are significantly downgraded. As a solution, a new model has been developed for SOLPS-ITER, solving an additional transport equation for the turbulent kinetic energy k, derived by consistently time-averaging the Braginskii equations, and including a diffusive closure for the anomalous particle flux. This closure model relates the anomalous diffusion coefficient to the local k value. The resulting equation structure and its closure are inspired by TOKAM2D isothermal interchange turbulence simulation results. Within this model, fewer and hopefully more universal free parameters are retained, thus improving the code's predictive capabilities. The new model has been tested on a COMPASS case for which upstream plasma profiles were available. Experimental data and a reference solution, obtained by matching the profiles through manual tuning of radial diffusivities, have been used to estimate the parameters of our new transport model. A ballooned particle diffusivity profile is retrieved by the new radial transport model, thanks to the proposed interchange drive. The obtained upstream profiles qualitatively agree with the experiment and prove the new model is a promising first attempt to be further refined.     

Long-pulse improved central electron confinement in the TCV tokamak with electron cyclotron heating and current drive

Current profile tailoring by electron cyclotron heating (ECH) and current drive (ECCD) is used to improve central electron energy confinement in the TCV tokamak. Counter-ECCD on axis alone achieves this goal in a transient manner only. A stable scenario is obtained by a two-step sequence of off-axis ECH, which stabilizes magnetohydrodynamics modes, and on-axis counter-ECCD, which generates a flat or inverted current profile. This high-confinement regime, with central temperatures up to 9 keV (at a normalized beta(N) approximately 0.6), has been sustained for the entire duration of the heating pulse, or over 200 electron energy confinement times and 5 current redistribution times.     

Simulation of Inelastic Electron Tunnelling Spectroscopy on Different Contact Structures in 4,4＇-Biphenyldithiol Molecular Junctions

A first-principles computational method is developed to study the inelastic electron tunnelling spectroscopy （IETS） of 4,4＇-biphenyldithiol molecular junction with three different contact structures between the molecule and electrodes in the nonresonant regime. The obtained distinct IETS can be used to resolve the geometrical structure of the molecular junction. The computational results demonstrate that the IETS has certain selection rule for vibrational modes, where the longitudinal modes with the same direction as the tunnelling current have greatest contribution to the IETS. The thermal effect on the IETS is also displayed.     

Magnetic-dipole vortex generation by propagation of ultraintense and ultrashort laser pulses in moderate-density plasmas

A magnetic-dipole vortex is generated in the behind of an ultraintense and ultrashort laser pulse in a near critical density plasma. The vortex is self-sustained by its magnetic field pressure which expels background electrons, and resulting sheath field accelerates electrons to drive high amplitude electric current inside the vortex. The electron energy spectra shows nonthermal distribution with relatively high energy. The vortex is stable for a long period since it is in the electromagnetic equilibrium, whose structure and characteristics are explained by a simple analytical model.     

轴向加速管—一种新的高功率微波源

 提出一种新的高功率微波源器件——轴向加速管,与径向加速管比较,有两点不同：在二极管/共振腔后加了一个加速区及将径向加速改为轴向加速。研究了这种器件中电子束的群聚和辐射。发现在二极管与共振腔结合的系统里,电子束可实现理想群聚,各次谐波电流在非相对论条件下可达到初始电流的5.6倍。通过电子束再加速和仔细设计提取腔,可望得到可调频率和高功率的微波脉冲。