Importance of high local cathode spot pressure on the attachment ofthermal arcs on cold cathodes

The importance of having high local cathode spot pressures for the self-sustaining operation of a thermal arc plasma on a cold cathode is theoretically investigated. Applying a cathode sheath model to a Cu cathode, it is shown that cathode spot plasma pressures ranging 7.4-9.2 atm and 34.2-50 atm for electron temperatures of ~1 eV are needed to account for current densities of 10⁹ and 10¹⁰ A·m^-2, respectively. The study of the different contributions from the ions, the emission electrons, and the back-diffusing plasma electrons to the total current and heat transfer to the cathode spot has allowed us to show the following. 1) Due to the high metallic plasma densities, a strong heating of the cathode occurs and an important surface electric field is established at the cathode surface causing strong thermo-field emission of electrons. 2) Due to the presence of a high density of ions in the cathode vicinity, an important fraction of the total current is carried by the ions and the electron emission is enhanced. 3) The total current is only slightly reduced by the presence of back-diffusing plasma electrons in the cathode sheath. For a current density j_tot=10⁹ A·m^-2 , the current to the cathode surface is mainly transported by the ions (76-91% of j_tot while for a current density j_tot = 10¹⁰ A·m^-2, the thermo-field electrons become the main current carriers (61-72% of j_tot). It is shown that the cathode spot plasma parameters are those of a high pressure metallic gas where deviations from the ideal gas law and important lowering of the ionization potentials are observed     

Measurements of the behaviour of neutral atom density in a diffusevacuum arc by laser-induced fluorescence (LIF)

The method of laser-induced fluorescence was used to study the behavior of the absolute neutral vapor density of a diffuse vacuum arc on FeCu contacts. The local and temporal resolutions were 1 mm³ and 10 μs, respectively. The arc current had a sinusoidal shape of 5.8-ms duration with peak values of 90 and 510 A. It was found that the maximum densities of the iron and copper atoms are 1.2×10 ¹⁷ m^-3 and 7.5×10¹⁷ m^-3, respectively. During the arc the density was correlated with the current. At current zero the measured densities decreased to 10 ¹⁶ m^-3. After current zero, an exponential density decay with a time constant of about 100 μs was observed, indicating the recovery of dielectric strength after current zero. Measurements of the neutron iron vapor density at different spatial positions in the electrode gap reveal a nonisotropic distribution. From the measurements of the population distribution of the iron ground-state multiplet a ⁵D, the excitation temperature was derived. This temperature was low compared with the cathode spot temperature 2500-4000 K and decreased from 1600 K at the current maximum to 1000 K at current zero. The results indicate that the generation of neutrals is caused by flying evaporating metal droplets rather than by molten surface areas     

Metal vapor densities and excitation temperatures in a pseudosparkswitch

The densities of iron, tungsten, copper, and nickel vapors produced by pseudosparks in a switch-like configuration are measured by laser-induced fluorescence. The cathode is made of a composite material essentially consisting of tungsten, but also containing the other metals mentioned. Total vapor densities are calculated from ground state densities using the excitation temperature of iron, which decays from 1900 K at 9 μs after initiation of the discharge to 600 K about 150 μs later. With maximum copper and tungsten vapor densities of 1.5×10¹⁸ m^-3 and 2×10¹⁷ m ^-3, respectively, the composition of metal vapor differs considerably from that of the cathode material. Iron and nickel vapors are present with densities in the range of 10¹⁶ m^-3. By comparison of vapor density ratios with vapor pressure ratios it is found that regions with temperatures in excess of 5000 K exist on the cathode. These are attributable to emission sites providing the electrons for current conduction. The vapor densities are roughly proportional to the current amplitude, while the gas pressure has practically no influence between 15 and 30 Pa     

温稠密钛电导率计算

温稠密物质是惯性约束核聚变、重离子聚变、Z箍缩动作过程中物质发展和存在的重要阶段. 其热力学性质和辐射输运参数在聚变实验和内爆驱动力学模拟过程中有至关重要的作用. 本文通过建立非理想Saha方程, 结合线性混合规则的理论方法模拟了温稠密钛从10^-5-10 g·cm^-3, 10⁴ K到3×10⁴ K区间的粒子组分分布和电导率随温度密度的变化, 其中粒子组分分布由非理想Saha方程求解得到. 线性混合规则模型计算温稠密钛的电导率时考虑了包括电子、原子和离子之间的多种相互作用. 钛的电导率的计算结果与已有的爆炸丝实验数据相符. 通过电导率随温度密度变化趋势判断, 钛在整个温度区间, 密度0.56 g·cm^-3时发生非金属相到金属相相变. 对于简并系数和耦合系数的计算分析, 钛等离子体在整个温度和密度区间逐渐从弱耦合、非简并状态过渡到强耦合部分简并态.     

Pressure dependence of plasma parameters in medium-vacuum nitrogenarc discharge with the titanium cathode

The plasma properties of a medium-vacuum nitrogen arc discharge from a titanium cathode were studied. The arc chamber use was 400 mm in diameter and 600 mm in length. The cathode diameter and thickness were 64 and 25 mm, respectively. The experimental conditions are given as follows: pressure range=1×10^-3~2×10^-1 torr; N₂ gas flow rate=6 ml/min; arc current=50 A. Electric probe characteristics are measured as a function of pressure and distance from the cathode surface. The analytical results obtained show that the electron energy distribution takes 1-Mx at pressures above 1×10^-2 torr but 2-Mx at pressures under 4×10^-2 torr and that the electron density has a maximum value at a certain pressure. The Ti⁺, Ti⁺⁺, and N ⁺₂ ion spectral intensities are measured as a function of pressure and distance from the cathode surface. On comparison of these results and the electron density, the Ti⁺ spectral intensity turns out to be proportional to that of the electron density. This suggests that the major ion in the plasma volume is of the Ti⁺ species     

A cold thermionically emitting dusty air plasma formed by radiativeheating of graphite particulates

Formation of an atmospheric pressure dusty air plasma is explored experimentally in this paper. The plasma is created by seeding an air flow with graphite particles and irradiating the particulates with a focused CO₂ laser beam. The graphite particles are, thus, heated to thermionically emitting temperatures, and average particle temperatures and average particle number densities are measured. The presence of charges is inferred both from these measured quantities using a simple theoretical transient model, and experimentally by applying a dc bias across the irradiated region. It is found that an electron density of ~6.7 × 10⁵ cm^-3 (6.7 × 10¹¹ m^-3) can be produced at steady state in the presence of O₂. This value can be increased to 3.6 × 10⁷ cm^-3 (3.6 × 10¹³ m ^-1) in the ideal case where an electron attachment to O₂ is suppressed and where a lower work function particulate is used     

A two-component model for the electron distribution function in ahigh-current pseudospark or back-lighted thyratron

Temperature, energy, and densities of two electron distribution function components, including an isotropic bulk part and an anisotropic beam, are analyzed for a hydrogen pseudospark and/or back-lighted thyratron switch plasma with a peak electron density of 1-3×10¹⁵ cm^-3 and peak current density of ≈10⁴ A/cm². Estimates of a very small cathode-fall width during the conduction phase and high electric field strengths lead to the injection of an electron beam with energies ⩾100 eV and density of 10¹³-10¹⁴ cm^-3 into a Maxwellian bulk plasma. Collisional and radiative processes of monoenergetic beam electrons, bulk plasma electrons and ions, and atomic hydrogen are modeled by a set of rate equations, and line intensity ratios are compared with measurements. Under these high-current conditions, for an initial density n_H2=10¹⁶ cm^-3 and electron temperature of 0.8-1 eV, the estimated beam density is ≈10¹³ -10¹⁴ cm^-3. These results suggest the possibility of producing in a simple way a very high-density electron beam     

Experimental study of CF4 conical theta pinch plasmaexpanding into vacuum

Langmuir probe, photodiode, and optical multichannel analyzer (OMA) measurements have been made on a pulsed CF₄ conical theta-pinch plasma. A cloud of CF₄ was puffed into a conical theta-pinch coil, converted to plasma, and propelled into the vacuum region ahead of the expanding gas cloud. At a position 67 cm away from the conical theta-pinch coil, the plasma arrived in separate packets that were about 20 μs in duration. The average drift velocity of these packets corresponded to an energy of about 3 eV. The OMA measurements showed that the second packet contained neutral atomic fluorine as well as charged particles. The electron temperature and ion density in the second packet were 2.0 eV and 1.5×10¹³ cm^-3, respectively. The electron temperature and ion density in the wake plasma were 8.3 eV and 4×10¹¹ cm^-3 , respectively. This device can be used for plasma processing or as a laboratory test of numerical and analytical models of the expansion of plasma into vacuum     

热退火对SiO2：Er薄膜发光特性的影响

利用离子注入法制备SiO₂:Er样品,并在不同温度下进行退火处理。通过微区拉曼光谱、吸收光谱、X射线光电子能谱等手段对其进行结构表征,并进行了室温和变温的光致发光特性研究,得到了可见区和红外区的光致发光。其中,⁴S_3/2→⁴I_15/2的发光强度随温度的升高,先增强后减弱,呈现出反常的温度淬灭效应,此现象是由Er³⁺与SiO₂的缺陷之间的能量传递造成的。     

A model for a uniform steady-state vacuum arc with a hot anode

A model is formulated and evaluated for a Uniform electrical discharge sustained in vapor evaporated from an arc-heated anode. The plasma potential is positive with respect to both the cathode and anode. For a Cu anode, the anodic vapor dominates the plasma for current densities exceeding 8 kA/m². The anode heating potential is approximately 6.5 V, and the dominant cooling mechanism is evaporation for current densities exceeding 20 kA/m². Over the range 10 to 10000 kA/m², the electron density increases from 8×10¹⁷ to 5×10²³ m^-3, while the ionization fraction rises from 0.3% to 4%. At the lower end of this current range the electrical resistivity of 4 mΩ-m is determined primarily by electron-neutral collisions, while with increasing current the resistivity decreases to 0.7 mΩ-m, with electron-ion collisions contributing an equal share. This hot-anode vacuum arc may have potential for industrial application as a macroparticle-free high-deposition-rate coating source     

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采用TORAY代码对HL-2A装置ECRH系统在单零点偏滤器位形下的波与等离子体相互作用的情况进行了模拟计算,研究了等离子体和波参数对ECRH波迹和功率沉积以及电流驱动的影响。根据数值计算结果,HL-2A装置ECRH系统在等离子体线平均密度为3.0×1013cm-3、中心电子温度为1.19keV的情况下,以O模作为入射波垂直入射时的单次吸收系数为99.3%,最大电流驱动效率为0.005×1020A.W-1.m-2。     

Photo-ionized lithium source for plasma accelerator applications

A photo-ionized lithium source is developed for plasma acceleration applications. A homogeneous column of lithium neutral vapor with a density of 2×10^15-3 is confined by helium gas in a heat-pipe oven. A UV laser pulse ionizes the vapor. In this device, the length of the neutral vapor and plasma column is 25 cm. The plasma density was measured by laser interferometry in the visible on the lithium neutrals and by CO₂ laser interferometry on the plasma electrons. The maximum measured plasma density was 2.9×10 ¹⁴ cm^-3, limited by the available UV fluence (≈83 mJ/cm²), corresponding to a 15% ionization fraction. After ionization, the plasma density decreases by a factor of two in about 12 μs. These results show that such a plasma source is scaleable to lengths of the order of 1 m and should satisfy all the requirements for demonstrating the acceleration of electrons by 1 GeV in a 1-GeV/m amplitude plasma wake     

ZBLAN玻璃中Pr3+掺杂离子3P0和1D2能级的发光特性和寿命

对ZBLAN氟锆酸盐玻璃中Pr³⁺掺杂离子³P₀和¹D₂能级的寿命和发光特性进行了较详细的光谱学研究。首先测量了两种掺杂浓度(质量分数分别为1×10^-3,5×10^-3)的Pr³⁺:ZBLAN玻璃的吸收光谱,然后运用时间分辨激光光谱技术测量了³P₀和¹D₂能级在激光单光子共振激发下的荧光发射谱和能级寿命。将不同荧光发射谱带的强度和文献报道的Judd Ofelt理论计算辐射跃迁几率数值做了比较分析,证明了文献中理论计算结果的可靠性。由于浓度猝灭效应,在相同的激发条件下,掺杂浓度为1×10^-3样品的荧光发射强度明显大于5×10^-3样品的荧光发射强度。但是从我们的测量结果看,掺杂浓度对³P₀和¹D₂ 的能级寿命值无显著影响。掺杂浓度为1×10^-3时,Pr³⁺离子³P₀和¹D₂能级的寿命值分别为46,322μ_s。     

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The multichannel bremsstrahlung measurement system on HL-2A tokamak has been using to observe the intensity of visible (535.1nm) bremsstrahlung radiation. Combined the intensity with plasma electron density and electron temperature, the effective ion charge measurements can be obtained on HL-2A tokamak. The 1×10¹⁹m⁻³ to4×10¹⁹m⁻³ experimental results show that when the plasma electron density increases from 1×10 m to 4×10 m , the effective ion charge number changes from 5 down to around 2. The bremsstrahlung radiation signal intensity is proportional to the quadratic plasma density. When the electron density is higher than 3×10¹⁹m⁻³, the multichannel bremsstrahlung measurement system can collect sufficient signal through the plasma, which enables the measurement of effective ion charge radial profile.     

HL-2A 装置中利用可见光谱段轫致辐射测量有效离子电荷数

通过测量可见光谱段的轫致辐射(λ=535.1nm)强度,结合等离子体电子密度和电子温度,HL-2A     

高能电子辐射下聚四氟乙烯深层充电特性

介质深层充放电现象是诱发航天器异常故障的重要因素之一.分析了高能电子辐射下介质内部电荷沉积、能量沉积特性和电导特性,考虑了真空与介质界面电荷对电场分布的影响,建立了介质二维深层充电的物理模型,并基于有限元方法实现了数值计算.计算了高能电子辐射下聚四氟乙烯的深层充电特性.结果表明:真空环境下,介质的表面存在较弱的反向电场,随着介质深度增大,电场减小至零,随后逐渐增大,最大值出现在靠近接地附近,但在接地点,电场存在小幅降低.分析了不同辐射时间下(1 h,1 d,10 d和30 d),介质内部最大电位和最大电场的时空演变特性.随着辐射时间的增加,最大电位由-128V增加至-7.9×104V,最大电场由2.83×105V·m-1增加至1.76×108V·m-1.讨论了入射电子束流密度对最大电场的影响,典型空间电子环境(1×10-10A·m-2)下,电子辐照10 d时,介质内部最大电场为2.95×106V·m-1.而恶劣空间电子环境(2×10-8A·m-2)下,电子辐射42 h,介质内部最大电场即达到108V·m-1,超过材料击穿阈值(约为108V·m-1),极易发生放电现象.该物理模型和数值方法可以作为航天器复杂部件多维电场仿真的研究基础.     

Active species in microwave postdischarge for steel-surfacenitriding

Production of nitrogen atoms has been studied in a 2.45-GHz flowing postdischarge in N₂ and N₂-H₂ gas mixtures with Ar as a buffer gas in the high-pressure regime (5×10³ to 6.5×10⁴ Pa). N atom densities have been measured by NO titration in the 10¹⁴-10 ¹⁵ cm³ range and monitored by the first positive emission resulting from the N atom recombination. The rate coefficient of the N+N+N₂ recombination has been found to be _k=6×10^-33 cm⁶ atom^-2 s ^-1 at T₀=300 K, which agrees with previously published data. The N atom production (or degree of N/N₂ dissociation) in front of an Fe-0.1%C substrate correlates well with the thickness of a γ' Fe₄N layer produced by the postdischarge treatment. The H₂ gas was first introduced in the initial phase of treatment to remove surface oxidizing and then was cut off to keep high densities of N atoms. It is deduced that N atoms are more active nitriding species than NH-type radicals     

Measurement of cathode spot parameters with pulsed laserdiagnostics

The cathode spot formation in air within the first 170 ns was investigated by laser absorption photography and ps-pulse interferometry. The discharge was initiated between electrodes made from Ag or Pd with cathode-anode distance below 300 μm, the arc duration was some milliseconds, and the arc current 5-10 A. Picosecond holographic interferometry and momentary absorption photography yielded spatial-temporal density distributions in the ignition phase of the cathode spot. An absolute electron density value on the order of 4×10²⁶ m^-3 has been found. In contrast to vacuum, the cathode spot plasmas broaden little with increasing distance from the cathode, thus narrow plasma channels are observed in the vicinity of the cathode surface having diameters <20 μm     

快重离子辐照对非晶态SiO2薄膜光致发光谱的影响

用湿氧化法在单晶硅表面生长了非晶态SiO2薄膜,再用高能Pb和Xe离子对薄膜进行辐照,最后用荧光光谱分析了辐照参数(剂量、电子能损值)与发光特性改变的相关性.研究发现,快重离子辐照能显著影响薄膜的发光特性,进一步分析显示,辐照导致了SiO2薄膜内O-Si-O缺陷、缺氧缺陷和非桥式氧空位缺陷的产生,且缺氧缺陷和非桥式氧空...     

Validity conditions for complete and partial local thermodynamicequilibrium of nonhydrogenic level systems and their application tocopper vapor arcs in vacuum

Validity conditions for complete and partial local thermodynamic equilibrium (CLTE and PLTE) of homogeneous, time-dependent, and optically thin plasmas are derived. For Cu I, electron densities of n_e⩾(5×10²²-5×10²³ ) m^-3 are required for the establishment of CLTE. For Cu I and Cu II, n_e⩾(5×10²¹-5×10²¹ -5×10²²) m^-3 is necessary for PLTE (for electron temperatures of 1-2 eV). Application to low-current copper vapor arcs in vacuum shows that CLTE can be expected for r<200-600 μm (r=distance from the cathode spot). A further limitation follows for temperatures of 2 eV or higher if diffusion effects are taken into consideration. Consequently, the use of the LTE formulas in plasma spectroscopy of low-current vacuum arcs is very limited