Simulation study of a capacitively coupled plasma torch array

Using capacitively coupled electrical discharges, an array of three plasma torches powered by a single 60-Hz source are lit up simultaneously to produce a dense plasma in the open air. The discharge voltage and current of each torch is measured for three cases of one to three torches being lit up in the array. The results determine the ν-i characteristic of the discharge which indicates that the torch is operating in a diffuse are mode. The torch array is modeled by an equivalent circuit for simulating its operation. The simulation results of the discharge voltage and current of a torch are shown to agree well with those from the experimental measurements for the three cases. The lump circuit model is then used to carry out numerical simulations of the discharge for a broad parameter space of plasma species. By fitting the simulation results, a function giving the parametric dependence of the consumed average power density 〈P〉 on the normalized average electron density 〈n_e〉 maintained in the plasma is determined to be 〈P〉 48 〈n_e〉 ^1.9α_^0.4(W/cm³), where 〈n_e〉 is normalized to 10¹³cm^-3 and α_, the electron-ion recombination coefficient normalized to 10^-7 cm³·s^-1, is used as a variable parameter in the simulation     

Production of high-current large-area H- beams by abucket-type ion source equipped with a magnetic filter

A negative-ion-based neutral beam injection (NBI) system is planned for plasma heating of the Large Helical Device (LHD). We have developed a negative ion source, which is 1/3 the scale of the source for the NBI. A magnetic filter held was generated by external permanent magnets to lower the electron temperature in a large-area bucket plasma source (35 cm×62 cm) for efficient H^- production. We investigated the magnetic field configuration and found a low electron temperature high density plasma (<1 eV, 10¹²/cm³) could be achieved with an optimized configuration, The filter strength (B_max=70 G, line-integral flux=780 G cm at the center axis of the source) was proved to be enough to lower the electron temperature below 1 eV at high arc discharge power (100 kW) and low pressure (0.4 Pa). We injected cesium vapor into the plasma source to enhance H^- production efficiency and obtained a 16.2-A H^- beam current (31 mA/cm², 47 kV) using a large-area, four-grid electrostatic extraction system (25 cm×50 cm). This satisfied the development target (>15 A: 1/3 current of LHD ion source). Based on the results, we are designing a negative ion source for the LHD     

A multiple-cathode arc-discharge plasma source

A plasma source for obtaining a 1-cm length of high-fractional ionized plasmas with electron densities in the range of 10¹⁵-10¹⁷ cm^-3 is described. The source consists of a capacitive discharge between a metal anode and an array of small metal cathodes embedded in epoxy, each with a current-limiting resistor. The effect is to have an array of sources which merge into each other, producing the plasma as a whole. The design and performance of the plasma source are presented. The entire system consists of the main electrode and shielding structures, the preionization circuit, and the main discharge circuit. Each of these components is described and the operating parameters and results are given     

Highly flexible and excellent performance continuous carbon nanotube fibrous thermoelectric modules for diversified applications

A highly flexible and continuous fibrous thermoelectric (TE) module with high-performance has been fabricated based on an ultra-long single-walled carbon nanotube fiber, which effectively avoids the drawbacks of traditional inorganic TE based modules. The maximum output power density of a 1-cm long fibrous TE module with 8 p-n pairs can reach to 3436 μW·cm^-2, the power per unit weight to 2034 μW·g^-1, at a steady-state temperature difference of 50 K. The continuous fibrous TE module is used to detect temperature change of a single point, which exhibits a good responsiveness and excellent stability. Because of its adjustability in length, the flexible fibrous TE module can satisfy the transformation of the temperature difference between two distant heat sources into electrical energy. Based on the signal of the as-fabricated TE module, a multi-region recognizer has been designed and demonstrated. The highly flexible and continuous fibrous TE module with excellent performance shows a great potential in diversified applications of TE generation, temperature detection, and position identification.     

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Using a one-dimensional (1D) neutronics model, the neutronics performance in the China fusion engineering test reactor (CFETR) with latest design dimensions of vacuum vessel is calculated under the 2GW fusion power. The shielding effect of neutron reflecting material ZrH₂ on neutrons is calculated, and it is found that the 20cm reflector can shield 94.3% neutron fluence and 94.9% neutron nuclear heat. Meanwhile, the minimum shield blanket thickness corresponding to different neutron wall loads is calculated when CFETR is operated at 10FPY (full power year) and 20FPY. The results show that the minimum shield blanket thickness are 44cm, 53cm, and 65cm corresponding to the neutron wall loads with 1.0MW·m⁻², 1.5MW·m⁻², and 2.5MW·m⁻² respectively after the device is operated at 10 FPY; whereas the shielding blanket needs to be thicker in the radial direction to meet the neutron shielding requirements after the device is operated at 20FPY. The optimized size of the shielding blanket provides a significant reference for the design of CFETR advanced blanket.     

2GW功率下CFETR中子屏蔽的初步中子学研究

在中国聚变工程实验堆(CFETR)真空室最新设计尺寸下,利用蒙特卡洛中子输运程序(MCNP)建立一维中子学模型,在2GW 的聚变功率下进行了计算。分析了中子反射材料ZrH₂ 对中子的屏蔽效果,发现200mm 的反射层可以屏蔽94.3%的中子通量和94.9%的中子核热。研究CFETR 在运行10 个满功率年(FPY)和20FPY 后,对应不同中子壁载荷的最小屏蔽包层厚度。结果显示,装置运行10FPY 后中子壁载荷在1.0MW·m⁻²、1.5MW·m⁻²、 2.5MW·m⁻² 时所对应的最小屏蔽包层厚度分别为44cm、53cm、65cm;而在装置运行20FPY 后,则需要在径向方向更厚的屏蔽包层才能满足中子屏蔽要求。屏蔽包层的尺寸优化将为目前阶段的CFETR 先进包层设计提供参考。     

Inelastic light scattering in highly excited GaAs

The coupled plasmon-LO-phonon and single-particle excitations of a photocreated electron-hole plasma in high purity GaAs are studied by means of Raman scattering measurements. The experimental results are described by the random-phase approximation with respect to the multicomponent character of optical plasma. From single-particle spectra the plasma distribution function and drift velocities up to 10⁷ cm/s have been determined. Using the coupled modes as probe for the plasma density the temperature dependence of the recombination in the EHP is studied.     

高功率微波窗内外表面闪络击穿流体模拟研究

建立理论模型,将电磁场时域有限差分方法与等离子体流体模型结合,编制一维电磁场与等离子流体耦合程序,数值研究了3 GHz高功率微波窗内外表面闪络击穿的不同物理过程.研究结果表明:外表面闪络击穿中,输出微波脉宽缩短(未完全截止),窗体前均方根场强呈驻波分布,波节与波腹位置不变,窗体外表面形成有一层高密(约10~(21)·m~(-3)量级)极薄(约mm量级)等离子体(扩散缓慢),入射波可部分透过该薄层等离子体,脉宽缩短主要源于等离子体吸收效应;降低初始等离子体密度、厚度、入射波场强及缩短入射波脉宽等方式,可不同程度地改善输出脉宽缩短效应.内表面闪络击穿中,窗体前均方根场强亦出现驻波分布f但波节与波腹位置随时间变化),等离子体向波源方向运动;强释气下,输出脉宽缩短(未完全截止),形成多丝状高密(约10~(21)·m~(-3)量级)极薄(约mm量级)等离子体区域(扩散缓慢),间距1/4微波波长,脉宽缩短主要源于等离子体吸收效应;弱释气、低场强下,脉宽缩短有所改善(但最终截止),形成多带状致密(约10~(18)·m~(-3)量级)略厚(mm-cm量级)等离子体区域(扩散较快),间距1/4波长,脉宽缩短主要源于等离子体吸收效应;弱释气、高场强下,脉宽缩短严重(很快截止),形成块状高密(约10~(21)·m~(-3)量级)较厚(约cm量级)等离子体区域(扩散迅速),脉宽缩短主要源于等离子体反射效应.     

高气压空气环状感性耦合等离子体实验研究和参数诊断

等离子体的电子密度分布, 电子碰撞频率分布, 覆盖面积, 厚度是影响其覆盖目标电磁散射特征的关键属性. 对此, 本文开展了在20 cm×20 cm×7 cm石英腔内感性耦合等离子体(ICP)的放电实验, 观察了在高气压条件下, 空气ICP的环形放电形态, E-H模式跳变现象和分层结构, 测量了其电负性核心区和电正性边缘区宽度和厚度随功率、气压的变化趋势, 并通过COMSOL Multiphysics对平板线圈磁场强度分布的分析和电负性气体扩散理论给予上述现象合理的解释, 同时, 利用微波透射干涉法测量了核心区域的电子密度随功率和气压的变化曲线, 利用理论模型计算了边缘区域的电子密度分布, 最后通过辅助气体Ar发射谱线的玻尔兹曼图形法得到了核心区和边缘区的电子激发温度.     

合肥光源横向束流反馈系统中矢量运算单元和光纤陷波滤波器的研制

 合肥光源横向束流反馈系统已经建成,着重介绍了系统中矢量运算单元和光纤陷波滤波器的研制。矢量运算单元中使用混频器控制信号的衰减,调节控制电压的大小以控制反馈信号的相位;光纤陷波滤波器创新性地提出用光纤延时制作陷波滤波器,很好地滤除了信号中的回旋频率分量,节省了反馈功率。     

High current electron beam generation in a vacuum transient hollowcathode discharge

Experimental observations are presented of prebreakdown electron beam generation in a transient hollow cathode discharge (THCD) in a vacuum. The discharge driver consists of a 400-kV maximum voltage, 25-nF Marx operated at 450-J stored energy coupled to a 120-ns, 1.5-Ω coaxial line. Electron beams with peak currents up to tens of kA are observed when a pulsed Nd:YAG laser is used to produce a plasma at the back of the cathode surface, inside the hollow cathode region (HCR). It is found that a plasma density of a few 10¹⁸ cm^-3 in a volume of a few mm³ is required to generate intense electron beams. Optimal conditions are determined by varying the position of the laser focal spot inside the HCR and the time delay between the laser and the applied voltage. The main features of the electron beams are similar to those observed in conventional THCD at pressures in the 10-200 mtorr range     

S—Ka频段电磁波在等离子体中传输特性的实验研究

在感应耦合等离子体风洞上开展了等离子体中电磁波传输特性实验研究,获得了不同频率电磁波在等离子体中的传输衰减.通过微波诊断技术,获得了等离子体射流的电子数密度和碰撞频率.通过矢量网络分析仪和标准增益天线组成的电磁波传输特性测试系统,获得了电磁波经过等离子体之后的衰减,研究了电子数密度范围7.0×10~(10)-1.0×10~(13)cm~(-3)、等离子体碰撞频率在109 Hz量级的等离子体对2.6—40 GHz不同频率电磁波传输特性的影响,分析了经典传输理论和薄层理论预测结果与实验结果的差异.该实验工作为等离子体中电磁波传输特性的理论研究和数值仿真提供了基础数据.     

放射性废物玻璃固化专用等离子体炬的数值模拟与实验研究

采用了数值模拟与实验结合的方法研究了用于模拟放射性固体废物玻璃固化的非转移弧型等离子体炬的电、热特性。基于包括电弧室和开放空间在内的3D 模型得到了电弧等离子体和等离子体射流的温度场。根据计算结果,电弧室内的最高温度位于第一阳极内,达到1.77×10⁴ K;弧电压的计算值高于实测值,二者之间的差异随着电流强度的增大而逐渐减小。采用该等离子体炬熔融模拟废物的实验发现,所确定的等离子体炬到炉底的距离能够满足废物熔融的要求,与计算的结果相符合。上述结果表明,数值模拟的结果可以作为等离子体炉工程设计的依据,并可以用作进一步分析等离子体炉炉膛内工艺过程的输入条件。     

Gd靶激光等离子体6.7nm光源的实验研究

本文对Gd靶激光等离子体极紫外光源进行了实验研究, 在 6.7 nm附近获得了较强的辐射, 并研究了6.7 nm 附近光辐射随打靶激光功率密度变化的规律以及收集角度对极紫外辐射的影响. 同时, 对平面Gd靶激光等离子光源的离子碎屑角分布进行了测量, 发现从靶面的法线到沿着靶面平行方向上Gd离子数量依次减少. 进一步研究结果表明采用0.9 T外加磁场的条件下可取得较好的Gd 离子碎屑阻挡效果.     

电磁波在非磁化等离子体中衰减效应的实验研究

针对等离子体隐身技术在航空航天领域的良好应用前景, 开展垂直入射到具有金属衬底的非磁化等离子体中电磁波衰减特性的理论与实验研究. 利用WKB方法对电磁波衰减随等离子体参数的变化规律进行了理论分析. 利用射频电感耦合放电方式产生稳定的大面积等离子体层, 搭建了等离子体反射率弓形测试系统, 进行了电磁波在非磁化等离子体中衰减效应的实验研究. 利用微波相位法和光谱诊断法, 得到不同放电功率下的等离子体电子密度, 其范围为8.17×10⁹–7.61× 10¹⁰ cm^-3. 本实验获得的等离子体可以使2.7 GHz 和10.1 GHz电磁波分别得到一定的衰减, 且电磁波衰减的理论与实验结果符合较好. 结果表明, 提高等离子体电子密度和覆盖均匀性有利于增强等离子体对电磁波的衰减效果.     

Two-dimensional simulation of polysilicon etching with chlorine ina high density plasma reactor

A two-dimensional fluid simulation of polysilicon etching with chlorine in an inductively-coupled high density plasma source is presented. A modular approach was used to couple in a self-consistent manner the disparate time scales of plasma and neutral species transport. This way, complex plasma chemical reactions (involving electrons, ions and neutrals) as well as surface chemistry can be included in the simulation, The power deposited into the plasma was calculated by an electromagnetics module which solves Maxwell's equations. The power deposition was used in the electron energy module to find the electron temperature and the rate coefficients of electron-impact reactions. These were in turn used as source terms in separate neutral and charged species transport modules. By iterating among the modules, a self-consistent solution was obtained. Quantities of interest, such as power deposition, species density and flux, and etch rate and uniformity were thus calculated, As power deposition was increased, the electron density increased linearly, the plasma became less electronegative, the degree of gas dissociation increased, and the plasma potential remained constant. The radial uniformity of the Cl atom flux was better than that of the ion flux. The reactivity of the wafer as compared to that of the surrounding electrode surface significantly affected the etch uniformity, despite the low pressure of 10 mtorr     

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Based on the design of the 2015 version of China Fusion Engineering Test Reactor (CFETR) water cooled ceramic breeder (WCCB) blanket modules surrounding the plasma, a tritium transport model has been developed. Tritium transport analysis has been carried out for each blanket module with different breeding zones, purge gas loop, coolant loop and steam generator. The results indicate that the concentration, permeability and retention of tritium among blanket modules are different. For all of the WCCB blanket modules in CFETR, the tritium retention inside the breeder is 6.62×10^-2g, the tritium retention inside the structural materials is 2.01g, the tritium retention inside purge gas and coolant loop are 4.03×10^-4g and 0.19g respectively, the tritium permeation through the steam generator tube walls is 20mg•y^-1, the tritium permeation from the coolant pipes is 0.1mg•y^-1.     

Modeling plasma equipment using neural networks

Equipment plasma has been modeled semi-empirically using neural networks in conjunction with statistical experimental design. A 3³ factorial design was employed to characterize the plasma, in which the variables that were varied include a source power, pressure, and Ar flow rate. As a test data for model validation, 16 experiments were additionally conducted. A total of six plasma attributes were modeled, which include electron density, electron temperature, and plasma potential as well as their spatial uniformities. A planar, inductively coupled plasma was generated in a multipole plasma etch equipment and Langmuir probe was utilized for data collection. Root mean-squared prediction errors measured on the test data are 0.323 (10 ¹¹/cm³), 0.267 (eV) and 1.141 (V) for electron density, electron temperature, and plasma potential, respectively. Comparisons with a statistical response surface model (RSM) revealed that neural network models are more accurate by an improvement of more than 25% in prediction performance. A similar level of prediction accuracy was also achieved in modeling spatial uniformity data. Consequently, neural networks demonstrated much better prediction capabilities over RSM in modeling complex equipment plasma     

Electrothermal efficiency, temperature and thermal conductivity of plasma jet in a DC plasma spray torch

A study was made to evaluate the electrothermal efficiency of a DC arc plasma torch and temperature and thermal conductivity of plasma jet in the torch. The torch was operated at power levels from 4 to 20 kW in non-transferred arc mode. The effect of nitrogen in combination with argon as plasma gas on the above properties was investigated. Calculations were made from experimental data. The electrothermal efficiency increased significantly with increase in nitrogen content. The plasma jet temperature and thermal conductivity exhibited a decrease with increase in nitrogen content. The experiment was done at different total gas flow rates. The results are explained on the basis of dissociation energy of nitrogen molecules and plasma jet energy loss to the cathode, anode and the walls of the torch     

等离子体密度对激光拉曼放大机理的影响

等离子体中的背向拉曼散射机理可以用来产生超短超强的激光脉冲. 本文采用粒子模拟方法模拟研究了等离子体密度对激光拉曼放大过程的影响. 研究发现, 过低的等离子体密度会导致等离子体波提前波破而降低能量转换效率; 而过高的等离子体密度又会导致其他不稳定性的快速增长, 限制作用距离和输出能量. 因此, 拉曼放大机理的最佳等离子体密度应处于等离子体波破的密度阈值附近, 可以获得最高的能量转换效率和能量输出. 另外, 空间频谱分析显示放大激光的强度饱和主要来自于自相位调制不稳定性的发展. 利用10¹³ W·cm^-2的抽运激光脉冲, 模拟证实拉曼放大机理可有效地将种子激光的强度从10¹³ W·cm^-2 放大到10¹⁷ W·cm^-2, 脉宽压缩到40 fs, 且能量转换效率达到58%.