Why vacuum arc cathode spots can appear larger than they are

The visual appearance of arc cathode spots in vacuum is studied experimentally and theoretically. Emission photographs of the spots taken with line radiation have a broad light profile with a rather flat slope (proportional to r^-β with β≈2, r being the distance from the spot center), while photographs taken in absorption are small, having a sharp edge with a steep slope of the profile (β⩾4). Emission photographs from the continuum are similar to absorption photographs. Theoretical analysis shows that the emitted line radiation cannot stem from the locus of excitation. As a consequence, the particles are excited at the edge of the dense spot core, but they radiate at a greater distance due to the finite lifetime of the excited levels and the plasma expansion. Thus, emission photographs from line radiation indicate a greater spot size (about 100 μm) than corresponding to the active spot radius which amounts to ⩽10 μm. This statement holds for discharge durations from 10 ns up to at least 100 μs. The spots exist not only at ignition but during the whole time of the discharge, the location varying due to the spot movement. Absorption photographs show a small size of 10-20 μm still 200 μs after ignition     

Plasma parameters within the cathode spot of laser-induced vacuumarcs: experimental and theoretical investigations

Cathode spot formation in laser-induced breakdown in vacuum was investigated by laser absorption photography with high spatial (0.5 μm) and temporal (100 ps) resolution. The discharge was initiated between Cu electrodes with a cathode-anode distance of 15-250 μm. The duration of pulsed discharges was 750 ns and dc discharges some milliseconds; the current was below 10 A. Picosecond momentary absorption photography yielded spatial-temporal density distributions in the ignition phase of the cathode spot. An absolute electron density >5×10²⁶ m^-3 in narrow plasma fragments with a diameter smaller than 5 μm was estimated. Mathematical modeling has satisfactorily explained the formation of the narrow plasma channel due to the bulk current self-focusing, as well as due to the generation of nonstationary emissive centers at the moving boundary of the expanding cathode spot plasma     

Microscopic high speed investigations of vacuum arc cathode spots

The main parameters and dimensions of cathode spots have been under discussion for years. To solve these current questions, a new system was specially designed. The image converting high speed framing camera (HSFC) combines a micrometer lateral resolution with a nanosecond time resolution and a very high optical sensitivity. This camera was used to study the microscopic behavior of vacuum arc cathode spots in a pulsed high current arc discharge on copper. The direct observation of these spots with high resolution revealed that one single cathode spot, as normally observed by optical means, consists of a number of simultaneously existing microscopic subspots, each with a diameter of about 10 μm and a mean distance of 30-50 μm between them. The mean existence time of these subspots on copper was found to be about 3 μs, where the position of a subspot remains unchanged (with an upper limit of about 5 μm) during its existence time. The lower limit of the current density in the cathode spots was estimated to be on the order of 10¹⁰-10¹¹ A/m². An upper limit of the crater surface temperature was estimated by a comparison between the brightnesses of a cathode spot and of a black body radiation lamp to about 3000 K     

Pulsed dye laser diagnostics of vacuum arc cathode spots

The ignition and arc phases of vacuum arcs were investigated using differential dye laser absorption photography with simultaneous high spatial (micrometer) and temporal (nanosecond) resolution. The discharge duration was 800 ns, the current 50-150 A, the electrode material copper, and the cathode-anode distance less than 50 μm. A 0.4 ns laser pulse (tunable, γ=480-530 nm) was used to obtain momentary absorption photographs of the cathode region. During ignition, an optically thick anode plasma expanded toward the cathode, decaying within 25 ns after bridging the electrode gap. In the arc phase, a fragmentary structure of the cathode spots was observed in situ for the first time. The microspots have a characteristic size of 5-10 μm. They appear and disappear on a nanosecond time scale. The plasma density of the microspots was estimated to be greater than (3-6)×10²⁶ m^-3     

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     

Plasma development in the early phase of vacuum surface flashover

The primary physical mechanism responsible for charge-carrier amplification, in a developing surface discharge, has eluded conclusive identification for decades. This paper describes the results of experiments to directly detect charge-carriers, above the dielectric surface, within the developing discharge. Free electrons are detected by measuring the deflection of a laser beam, focused to a 20 μm 1/e diameter, with an angular sensitivity of 0.18 mV/μrad and a risetime of 6 ns. The estimated detection threshold for electrons in the developing discharge is 10¹⁶ cm^-3 to 10¹⁷ cm^-3. A streak camera is used to gather spatial information regarding luminous processes with a maximum resolution of 25 μm and 0.6 ns. Current measurements have a sub-nanosecond response time and a detection threshold of 100 mA. Laser deflection measurements demonstrate the rapid development of a particle gradient, generally within 10 μm of the surface near the cathode and in the range of 75 to 175 μm from the surface near the anode, during the developing discharge. Streak camera measurements demonstrate the formation of an intense, visible emission, 25 to 50 μm in diameter, located near the insulator surface, during the formation of the discharge. These results imply that charge-carrier amplification occurs above the surface of the insulator, in a region of neutral particles desorbed or otherwise ejected from the insulator surface     

Shadow photography of laser-driven shock waves in air

The interaction of a pulsed TE-CO₂-laser (10.6 μm wavelength, 7 μs pulse length, 0.7 J pulse energy, 10⁷ W/cm² power density, 100 kW mean power) with metals in air was investigated. Laser-supported absorption phenomena and material ablation processes are compared to those of conventional pulsed TEA-CO ₂-lasers. Of interest were the time-dependent plasma formation and the evolution of the shock waves. To achieve a time resolution better than 10 ns, a pulsed dye laser was used as a light source for the shadow photography     

Thermal characterization of longitudinal merging of turbulent spots

This study presents the thermal behavior of two young turbulent spots merging into a longitudinal direction on an isothermal flat plate for the local Reynolds number between 6.1 × 10⁴ and 1.3 × 10⁵ in a low freestream turbulence water tunnel having a turbulent intensity of 1.16%. The two turbulent spots are generated by water injection through a 1-mm-diameter hole in the perpendicular direction of the mainstream flow with a dimensionless separating time (Δτ) of 42.08, 84.16, and 126.24. Thermochromic liquid crystals are utilized mutually with an image processing technique to extract the spot characteristics qualitatively and quantitatively. The results demonstrate that the following turbulent spot directly causes an increase in the local Nusselt number and heat rate within the footprint of the merging spots. The relatively highest increase in this study occurs when Δτ = 84.16. The average Nusselt number and effectiveness characterize differently in the intersection zone, non-intersection zone of the leading spot, and non-intersection zone of the following spot. The results confirm that turbulent spots under the boundary layer transition augment the heat transfer rate to the level of full turbulence by not only their spot maturity but also the merging mechanism. Finally, the heat transfer mechanism is discussed and the predictive formulas for the Nusselt number and heat flux of the longitudinal merging of turbulent spots for Δτ from 0 to 126.24 are provided.     

On the Retrograde Motion of Arcs in Magnetic Fields

Experiments are reported which allow the determination of retrograde velocity of individual cathode spots and of the plasma flow in a pulsed discharge using various metals and carbon as cathode materials. For discharge currents from 10 to 40 amps, pressures of 3 mmHg and magnetic field strengths of 6.103G retrograde spot velocities from 30 to approximately 300 m/sec are observed and the corresponding plasma flow velocities are in the range from 4,400 to 8,600 m/sec. On cathode materials with low melting points, the splitting rate of spots and the motion of individual spots is small, whereas under identical conditions the spots on refractory materials are highly mobile, the splitting rate is large, and the lifetime of individual spots is short.     

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     

GaAs基高功率半导体激光器单管耦合研究

设计了一种高亮度、高功率半导体激光器单管耦合输出模块, 采用波长为975nm的10W的GaAs基半导体激光器, 将半导体激光器输出光束耦合进数值孔径0.18、纤芯直径105μm的光纤中, 获得10A电流下的输出功率为9.37W, 耦合效率为94.3%, 亮度为1.64MW/(cm²·str)。     

氘化物阴极真空弧放电光斑分布

氘化物真空弧放电在许多领域均有应用,如无损检测、石油探井、中子活化分析等。和金属阴极不同,氘化物阴极放电时会释放大量的气体分子,表现出许多不同性质。采用放大镜头和ICCD相机观察了氘化物阴极真空弧放电光斑分布。测量系统的空间分辨率约为5 μm,时间分辨率最小2 ns。放电脉冲半高全宽（FWHM）0.9 μs,弧流波形为半周期正弦波。实验结果表明,氘化物真空弧放电时,所有阴极斑聚集为一个群落,表现为一个大光斑;在液滴作用下,阴极斑群落偶尔也会分裂为两个或多个群落;光斑形状不受弧流影响,但面积和亮度会随弧流增加而增大。氘化物阴极放电斑点聚集有利于产生高密度等离子体,提高放电效率。     

Er3+掺杂硫系玻璃微球在980nm激光泵浦下的荧光特性

采用玻璃粉料高温漂浮熔融法制备了0.9%Er₂S₃（质量分数）：75%GeS₂-15%Ga₂S₃-10%CsI（摩尔分数）硫系玻璃微球,并用熔融拉锥法制备了锥腰直径为2.31 μm的石英光纤锥。将其与直径119 μm的硫系微球进行耦合,在980 nm 激光泵浦下获得了微球中与Er³⁺：⁴I_13/2→⁴I_15/2跃迁对应的1.54 μm处的荧光光谱。分析了微球和块状玻璃荧光光谱差异的原因,并用Mie散射理论公式对微球荧光光谱共振峰间隔进行了计算。共振峰间隔实验结果与理论计算误差最小仅为0.05%,验证了理论分析的正确性。最后,讨论了微球峰值间隔与泵浦功率的关系,排除了泵浦功率对共振峰间隔的影响。     

强光一号兆安电流钨丝X箍缩实验研究

为了获得更高亮度的X射线点源,在"强光一号"装置上开展钨丝X箍缩实验研究.基于能量平衡方程,估算了1 MA电流热斑等离子体平衡半径为1—10μm.实验中,测试钨丝直径为25—100μm,丝根数为2—48,负载线质量为0.18—6.9 mg/cm;负载电流峰值为1—1.4 MA,10%—90%前沿为60—70 ns."强光一号"装置上匹配的X箍缩负载为30或32根25μm钨丝X箍缩,这种负载一定概率产生单个脉冲X射线辐射,辐射时刻位于电流峰值附近,典型参数为keV X射线脉宽1 ns,辐射功率35 GW,产额40 J,热斑尺寸～30μm.然而,兆安电流X箍缩通常产生多个热斑及多个脉冲X射线辐射.keV能段首个脉冲X射线辐射时刻与负载线质量正相关,并受到负载丝直径的影响.多个X射线脉冲可能由二次箍缩和局部箍缩产生,多个热斑可能由交叉点处微Z箍缩的长波长扰动和短波长扰动引起.与百千安电流X箍缩相比,兆安电流X箍缩热斑亮度更高,但X射线辐射脉冲的单一稳定性还有待于进一步改善.     

激光位移传感器传感探头微小型光学系统设计

光斑质量直接影响激光位移传感器测量的精度。为了提高激光位移传感器传感探头光学系统的成像质量,设计了传感探头四片式微小型光学系统。本文在理想成像基础上,分析光束在光学系统中能量传递的变化规律,对比光电探测器的感光能力,利用光学设计软件(ZEMAX)实现了激光位移传感器传感探头微小型光学系统的设计。通过理论计算分析,严格控制传感探头孔径光阑的大小,对光学系统进行优化处理,成像最大弥散斑半径低于3. 3μm,空间分辨率120lp/mm以下的传递函数MTF(Modulation Transfer Function)值大于0. 5,光线扇形图的最大像差小于5μm,畸变量低于0. 1859%。该光学系统具有良好的成像效果,可以满足激光位移传感器探测系统对成像系统成像光斑质量的要求,以保证传感器的测量精确度优于5μm。     

Investigations of the Cathode Spot Dynamics in a Vacuum Arc Coating Process

Influence of cathode materials (Ti, Al, Cu, TiN), ambient gases (Ar, N₂, p = 0.1-1 Pa) and the arc current itself on the motion and the velocity of cathode spots in an arc coating process have been investigated with the help of a new high speed framing camera. It was found, that the cathode material causes different spot currents but in general the spot arrangement and the motion on the surface are similar. Surface contaminations due to ambient gases affect this dynamics in several ways. Insulating layers like AIN can drastically increase the instantaneous spot velocity, for example from <5 m/s on Al up to 170 m/s on AIN contaminated areas. TiN layers with a high conductivity increase the spot mobility at first. But at nearly completely contaminated surfaces (simulated by a TiN cathode), the mobility is strongly decreased. The values change from an average velocity of 6.3 m/s with a diffusion constant of 54 cm²/s (Ti, 0.01 Pa) to 2 m/s and 6.4 cm²/s at TiN. The course of the instantaneous spot velocity during the spot splitting phase was investigated too. The instantaneous spot velocity of each of the two new spots originated from the starting spot is relatively high (30–40 m/s) within the first 50 μs. The cathode material and the ambient gases are of slight influence in this phase. The movement is directed. In the further development the instantaneous spot velocity is decreasing to values under 5–10 m/s. The motion is now more and more random. Additionally it could be proved, that the lower stability limit for a stable discharge is strongly connected with the spot current, which depends on discharge conditions.     

Modelling the spot shape influence on high-speed transmission lap welding of thermoplastics films

Modelling high-speed laser lap welding of thermoplastic films has been accomplished and the influence of laser beam spot shape, dimensions, and position relative to sample displacement was analysed.Engineering parameters predicted by the model were applied to lap weld of high- and low-density polyethylene transparent samples with thickness between 10 and 100 μm, and experimentally validated. Experimental set-up allowed reaching welding constant linear velocities up to 10 m/s.Theoretical and experimental data show coincidence. Weld strength increases for larger beam spot diameters, and elliptical beam spots increase weld efficiency, allowing higher processing speeds or decreasing required laser power. An angular deviation of elliptical beam spot with regard to the sample's movement direction causes an increase of weld strength and a decrease of welding speed.     

Electron density and temperature contour plots from alaser-produced plasma using collective ultraviolet Thomson scattering

Temporally integrated ultraviolet collective Thomson scattering measurements were performed with frequency-quadrupled Nd:YAG laser radiation on an underdense long-scalelength aluminum plasma (n_c ~10²¹ cm^-3, Z≈7, T_e≈T_i ⩾50 eV, L⩾100 μm). The plasma was preformed by an Nd:YAG fundamental beam (1.06 μm) with focusable intensities of 10 ¹¹ W/cm². Color images of two-dimensional (2-D) spatially resolved (30 μm) electron density and electron temperature were obtained. These are the shortest wavelength Thomson scattering measurements on a plasma to date     

Er~(3+)掺杂Na_5Lu_9F_(32)单晶体的红外光学特性

采用坩埚下降法成功地生长了Er~(3+)离子掺杂的Na_5Lu_9F_(32)(NLF)单晶体。测定了单晶体在400~2 500nm波段的吸收光谱与2.5~25μm红外波段的透过光谱。Na_5Lu_9F_(32)单晶体在400~7 150 nm宽波段范围具有好的光学透过性,在该波段的透过率达到90%。在透过光谱中几乎观察不到2.7μm中红外波段的吸收,说明单晶体中OH~-离子的含量极低。根据测定的吸收光谱,通过Judd-Ofelt理论计算了Er~(3+)在单晶体中的光学强度参数Ω_t(Ω_2=2.08,Ω_4=2.07,Ω_6=0.75),以及相应的辐射跃迁速率、荧光分支比和荧光寿命。根据Futchbauer-Ladenburg公式估算了样品的发射截面大约分别为1.42×10~(-20)cm~2(~4I_(13/2)→~4I_(15/2))和1.66×10~(-20)cm~2(~4I_(11/2)→~4I_(13/2))。在980 nm半导体激光器(LD)激发下,研究了单晶体的近红外1.5μm与中红外2.7μm的发射光谱特性。     

一种大型位置灵敏平行板雪崩计数器

中国科学院近代物理研究所气体探测器组研制了一种大型位置灵敏的平行板雪崩计数器（PPAC）。它主要由中心阴极和X,Y位置阳极构成,灵敏面积为244 mm×284 mm,阴极为双面镀金1.5 μm厚的Mylar膜,阳极为直径20 μm,间距1 mm的镀金钨丝。位置读出选用延迟块读出法。这种探测器经测试在灵敏面积内位置分辨小于4 mm,时间分辨为0.42 ns,已在兰州放射性束流线（RIBLL）上清华大学的核反应实验中使用。该实验中使用了主束40Ar直接轰击Au靶,产生裂变碎片,使用了4个这种大型位置灵敏的PPAC,用来探测裂变碎片的飞行时间和速度,实验表明,PPAC对裂变碎片有90.3%的效率,给出了裂变碎片的位置分布,为理论分析工作提供了非常好的实验数据。A kind of large position sensitive parallel plate avalanche counter(PPAC) was developed at Gas Detector Research Group, Institute of Modern Physics of CAS. Each of the detector with large sensitive area of 244 mm×284 mm, consists of one central cathode, X anode and Y anode. The cathode plane in the middle of detector is a 1.5 μm thick Mylar foil coated with a thin Au-layer on both sides. The position resolution of the PPAC is better than 4 mm, and the time resolution is 0.42 ns. The detectors were used in the fission experiment with 30MeV/u 40Ar beam bombarding on a gold target to measure the velocity of the fission fragments at Radioactive Ion Beam Line In Lanzhou(RIBLL). Four PPACs were used and the experiment result showed that a rather high detection efficiency (90.3%) is achieved. The position distribution of fission fragments is given by PPACs, which provides very good experimental data for further theoretical analysis.