Evolution of the plasma bubble in a narrow gap

We investigate the evolution of the plasma bubble in a narrow gap. According to the morphological changes, we further show that there are three phases during the evolution for spherical fluctuating, radial fingering, and dense branching plasma bubbles, which are similar to the radial fingering pattern in a Hele-Shaw cell. The dependences of the wavelength of the fingering boundary are experimentally discussed. The dense branching plasma bubble is found with a fractal dimension of D(f)=1.74. The reduced surface tension pressure from the local heatings due to the filamentary discharges is suspected of being responsible for the growth of the radial fingering and the dense branching plasma bubbles.     

HL-2A 装置的边缘参数测量

HL-2A 装置中平面边缘的等离子体特性通过磁力传动的马赫/ 雷诺协强/ 朗缪尔10 探针组进行了研究。10 探针组安装在可径向向里和向外移动, 并可绕轴旋转360^o 的传动杆上, 用于测量主等离子体边缘的温度、密度、悬浮电位、空间电位、径向和极向电场、湍流的雷诺协强、径向和极向等离子体流速及其径向分布。HL- 2A 装置的实验结果表明, 边缘等离子体扰动诱发的雷诺协强产生了边缘极向流; 雷诺协强的径向梯度驱动带状流抑制了湍流输运。     

The Influence of Stochastic Layers of Magnetic Field Lines on Transport Barrier Formation in a Stellarator System

The results of local measurements of RF discharge plasma parameters in the process of internal transport barriers (ITB) formation in the vicinity of rational magnetic surfaces in the Uragan-3M torsatron are presented. The following phenomena were observed in the process of ITB formation: widening of the radial density distribution, formation of plateaus on radial density and electron temperature distributions, formation of regions with high shear of poloidal plasma rotation velocity and radial electric field in the vicinity of stochastic layers of magnetic field lines, decrease of density fluctuations and their radial correlation length, decorrelation of density fluctuations, and increase of the bootstrap current.After the ITB formation, the transition to the improved plasma confinement regime takes place. The transition moves to the beginning of the discharge with the increase of heating power. The possible mechanism of ITB formation near rational surfaces is discussed.     

激光等离子体细丝的场分布和共振吸收

本文对激光等离子体细丝的场分布和共振吸收进行了理论研究.在冷等离子体的条件下,首先求得场方程以及电场的径向分量和轴向分量的解析表示式;然后,通过数值计算发现:在等离子体细丝的径向场存在一个隧道效应,即在径向共振点r_o附近,场出现极大值.     

二极管爆炸发射阴极等离子体的膨胀扩展

分析了二极管中爆炸发射产生阴极等离子体的演化特征,在考虑了阴极等离子体朝阳极膨胀运动使二极管阴阳极间距缩短这一效应的同时,还计入了阴极等离子体沿发射表面径向扩展运动对二极管有效发射面积的影响。基于Child-Langmuir定律,利用在一个四脉冲强流电子束源装置上得到的电流、电压等实验数据,假定阴极等离子体轴向膨胀和径向扩展速度近似相等,研究了阴极等离子体的膨胀扩展动力学行为。计算结果表明,阴极等离子体朝阳极的膨胀和沿径向的扩展速度为0.9～2.8 cm/s。     

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.     

Multislit streak camera investigation of plasma focus in thesteady-state rundown phase

The technique of multislit streak photography was applied to investigate the plasma focus in the steady-state rundown phase. This enabled the axial and radial positions, and the rundown velocity of the current-carrying plasma sheath to be measured simultaneously. From these measurements, the radial profile of the plasma sheath in the plasma focus tube could be reconstructed and the plasma current was calculated. These calculated currents represented about 70% of the total circuit current flowing out of the capacitor bank, and these agreed with earlier results which were obtained via computer simulations of the measured voltage and current waveforms     

Langmuir单探针诊断电子回旋共振等离子体参数分布特性

本文采用Langmuir单静电探针法,分析诊断了电子回旋共振等离子体的参数分布特性,并分析了微波功率、气压对轴向、径向等离子体空间分布的影响。在微波功率400W~650W范围内等离子体具有较高的密度及良好的径向均匀性。     

HL-1M托卡马克边缘参数和雷诺胁强的径向分布

利用多组马赫/郎缪尔探针测量了HL-1M装置刮离层和边缘静电雷诺胁强，等离子体极向旋转，径向和极向电场的径向分布，在低杂波电流驱动，超声分子束注入，多发弹丸注入和中性束注入实验中，给出了雷诺胁强和极向流的关系。结果表明，由于雷诺胁强的径向变化，托卡马克等离子体可以自发地产生剪切极向流。     

Plasma density evolution in plasma opening switch obtained by a time-resolved sensitive He-Ne interferometer

To understand the formation process of vacuum gap in coaxial microsecond conduction time plasma opening switch (POS), we have made measurements of the line-integrated plasma density during switch operation using a time-resolved sensitive He-Ne interferometer. The conduction current and conduction time in experiments are about 120 kA and 1 μs, respectively. As a result, more than 85% of conduction current has been transferred to an inductive load with rise time of 130 ns. The radial dependence of the density is measured by changing the radial location of the line-of-sight for shots with the same nominal POS parameters. During the conduction phase, the line-integrated plasma density in POS increases at all radial locations over the gun-only case by further ionization of material injected from the guns. The current conduction is observed to cause a radial redistribution of the switch plasma. A vacuum gap forms rapidly in the plasma at 5.5 mm from the center conductor, which is consistent with the location where magnetic pressure is the largest, allowing current to be transferred from the POS to the load.     

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通过中平面活动10探针组、往复快速扫描4探针和低杂波天线口的固定4探针测量了主等离子体边缘的温度、密度、悬浮电位、径向和极向电场、雷诺协强、径向和极向等离子体流速及其径向分布。用偏滤器靶板上的14组嵌入式静电3探针阵列测量了同一环向截面的内外中性化板上的电子温度、密度、悬浮电位及其分布。比较了在孔栏位形和偏滤器位形下边缘等离子体特性的差异,特别是两种位形下边缘温度和密度衰减长度的变化。分析了在多脉冲超声分子束加料和低杂波注入条件下的边界等离子体特性,以及雷诺协强的径向梯度与极向流和径向电场梯度与湍流损失的相互关系。     

Characterisation of the plasma density with two artificial neural network models

This paper establishes two artificial neural network models by using a multi layer perceptron algorithm and radial based function algorithm in order to predict the plasma density in a plasma system.In this model,the input layer is composed of five neurons:the radial position,the axial position,the gas pressure,the microwave power and the magnet coil current.The output layer is the target output neuron:the plasma density.The accuracy of prediction is tested with the experimental data obtained by the Langmuir probe.The effectiveness of two artificial neural network models are demonstrated,the results show good agreements with corresponding experimental data.The ability of the artificial neural network model to predict the plasma density accurately in an electron cyclotron resonance-plasma enhanced chemical vapour deposition system can be concluded,and the radial based function is more suitable than the multi layer perceptron in this work.     

Relativistic thermal equilibrium of non-neutral plasmas

Finite temperature effects on the relativistic radial equilibrium of a non-neutral plasma are analyzed. An equation for an effective potential governing the self-consistent radial density profile is derived in the case of global thermal equilibrium. The effect of a finite temperature turns out to be particularly strong for the fast mode of rigid plasma rotation, where the density profile can extend beyond the cold limiting radius.     

Observation of the coherence between the plasma density fluctuations in the core and at the edge of the plasma column in the L-2M stellarator

Results are presented from the measurements of the radial coherence between the plasma density fluctuations in the core and at the edge of the plasma column in the L-2M stellarator. It is found that the radial coherence between these fluctuations is fairly high: the coherence coefficient attains a value of higher than 0.5 for the spectral wavelet components with frequencies lying in the range from several kilohertz to 30 kHz. The statistical characteristics of density fluctuations are also studied. It is shown that the power distribution function of the plasma density fluctuations in the plasma core differs from Gaussian.     

层流氩等离子体射流温度的测量

 采用给水冷管状静电探针施加负偏置电压、并使探针以一定速度垂直于射流轴线扫过层流氩等离子体射流的方法,测量探针所收集到的累积离子饱和电流随侧向位置的变化,利用Abel变换推导出了局部离子饱和电流密度沿射流径向的分布;采用自制的水冷动压探针,以动态扫描法测量了射流动压沿射流径向的分布;根据局部离子饱和电流密度和射流动压的测量数据,由理论关系式推导出了等离子体射流横截面上的温度分布,同时,采用谱线相对强度法测量了等离子体射流的激发温度。结果表明：两种方法得到的等离子体射流中心温度吻合较好,所得到的射流中心温度随弧电流加大而增大的变化趋势也一致。     

Time-resolved radial uniformity of pulse-modulated inductively coupled O_2/Ar plasmas

Time-resolved radial uniformity of pulse-modulated inductively coupled O_2/Ar plasma has been investigated by means of a Langmuir probe as well as an optical probe in this paper. The radial uniformity of plasma has been discussed through analyzing the nonuniformity factor β(calculated by the measured n_e, lower β means higher plasma radial uniformity). The results show that during the active-glow period, the radial distribution of ne exhibits an almost flat profile at the beginning phase, but it converts into a parabola-like profile during the steady state. The consequent evolution for β is that when the power is turned on, it declines to a minimum at first, and then it increases to a maximum, after that, it decays until it keeps constant. This phenomenon can be explained by the fact that the ionization gradually becomes stronger at the plasma center and meanwhile the rebuilt electric field(plasma potential and ambipolar potential) will confine the electrons at the plasma center as well. Besides, the mean electron energy( ε_(on)) at the pulse beginning decreases with the increasing duty cycle. This will postpone the plasma ignition after the power is turned on. This phenomenon has been verified by the emission intensity of Ar(λ = 750.4 nm). During the after-glow period, it is interesting to find that the electrons have a large depletion rate at the plasma center. Consequently, ne forms a hollow distribution in the radial direction at the late stage of after-glow. Therefore, β exhibits a maximum at the same time. This can be attributed to the formation of negative oxygen ion(O~-) at the plasma center when the power has been turned off.     

Internal transport barriers in plasmas with reversed plasma flow

Evidences of internal particle transport barriers have been observed in plasma discharges with reversed plasma flow. To investigate the influence of the radial electric field profile on these barriers, we apply a drift wave map that describe the plasma particle transport and allows the integration of particle drift in the presence of a given electrostatic turbulence spectrum. With this procedure we show that transport barriers due to the shearless flow invariant lines are created inside the plasma. Moreover, by varying the radial electric field profile, we observe the formation and destruction of internal transport barriers constituted by shearless invariant lines, as well as its effects on the transport in the map's phase space. Applicability of our results are discussed for the Texas Helimak, a toroidal plasma device in which the radial electric field can be changed by application of bias potential.     

不同气体组分的PS-PVD射流光谱诊断

采用发射光谱分析不同气体组分的等离子喷涂-物理气相沉积(PS-PVD)射流,通过Abel转换得出射流径向各点处的光谱强度,比较和分析射流中不同气体的成分分布。采用多谱线斜率法研究不同气体组分的射流在径向距离上电子温度的变化,通过H_β谱线的stark展宽计算径向上电子密度的分布。结果表明：Ar/H₂气体中,H₂在焰流中心区域(0～30 mm)分布较为均匀,但在焰流中心稍靠外的区域(30～60 mm)随着径向距离的增加而增加;加入He后,Ar和H₂在焰流中心处浓度较低并在一定范围内随着径向距离的增加而增加,He往焰流中心聚集;不同气体组分的电子温度和电子密度随着径向距离的增加而降低,同时受到H₂和He的影响。     

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.     

Optical diagnostics of a low frequency instability rotating around a magnetized plasma column

An argon magnetized plasma column is created with primary energetic electrons in the Mistral device. Low frequency instabilities regularly rotating around this column are observed with an ultra-fast camera and a spectroscopic device. Experimental results coupled to a coronal code show the presence of a few percents of fast (hot) electrons inside the ejected plasma. It also shows that ultra-fast camera analysis of the ejected plasma can only give information on the primary electron population. Finally, these results suggest that the radial decrease of the light emitted by the ejected plasma is essentially due to the radial decrease of the mean energy of the hot electrons.