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     

Neutral copper vapor density and electric recovery after forcedextinction of vacuum arcs

Dielectric recovery data were obtained for vacuum arcs between chromium copper butt contacts 30 mm in diameter and 2 mm apart. The 50-Hz arc current was forced to zero at its maximum of 200 A in about 1 μs. Following current zero, high-voltage pulses of a sufficient amplitude to always cause breakdown were applied to the gap. Gap recovery is characterized by the measured breakdown voltage as a function of time. Dielectric strength of the gap rises sharply within the first few microseconds after current zero, reaching its final value in about 10 μs. Neutral copper concentration in the center of the gap was measured by laser-induced fluorescence under conditions very similar to those of the recovery measurements. In contrast to the fast gap recovery, the copper vapor concentration does not change substantially during the first 100 μs from its value of 1.4×10¹⁸ m ^-3 near current zero. It is concluded that the neutral copper vapor concentration does not play a decisive role in gap recovery under these experimental conditions. This is corroborated by the fact that the mean free path for electron-impact ionization of copper atoms exceeds the gap length by four orders of magnitude     

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     

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     

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     

Charge state and residence time of metal ions generated from amicrosecond vacuum arc

Metal ions generated from a microsecond vacuum arc were measured using a time-of-flight (TOF) method. A point-plane vacuum gap was fired by an impulse voltage to generate metal ions. The risetime and time constant for the decay of the arc current were 0.1 and 4.5 μs, respectively. TOF ion currents were measured for variable ion extraction times after the arc ignition. At a lead cathode, Pb⁺ and Pb ⁺⁺ ions were detected for ion extraction times less than 45 μs. The average charge-state fractions of the Pb⁺ and Pb ⁺⁺ ions were 91 and 9%, respectively. At a copper cathode, Cu ⁺, Cu⁺⁺, and Cu⁺⁺⁺ ions were detected for ion-extraction times less than 12.5 μs, and the average charge-state fractions were 42, 41, and 17%, respectively. The residence times of the generated lead and copper ions were also discussed     

Residence time of metal ions generated from microsecond vacuum arcs

Vacuum gaps of 1 mm with lead or copper cathode are fired by a 13 μs duration sinusoidal arc or a 10 μs duration exponentially-decaying arc, and time-of-flight (TOF) ion measurements are made at variable times after the arc ignition. At the lead cathode, Pb⁺ and Pb⁺⁺ ions are generated and the upper limit on the times for Pb⁺ ion detection are 48 μs and 46 μs from the arc ignition for the sinusoidal and exponential arcs, respectively. At the copper cathode, Cu⁺, Cu⁺⁺, and Cu⁺⁺⁺ ions are generated and detected within 15 μs and 13 μs from the arc ignition for the sinusoidal and exponential arcs, respectively. The residence time of the Pb⁺ ions in the ion acceleration region is approximately 35 μs, regardless of the waveform of the arc current. The residence time of the copper ions, described by the time constant of the time-of-flight ion current delay characteristics, is 3 μs     

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     

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     

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     

Ion and electron energies in gated field emitter failures

In previous work (1992), the authors studied the characteristics of gated field emitter failures and developed a theory to explain failure initiation. During a failure, the voltage between the emitter tip and gate (spaced 1 μm apart) was found to drop from -140 V to ≈-10 V. The current density was found to be ~10¹² A/m² during the failure, and plumes of ions and electrons were injected into vacuum. The ratio of ion current to electron current was found to be 10%. Those results indicated that the failures were similar to cathodic vacuum arcs. In the present study the energies of the ions and electrons are measured using a retarding potential energy analyzer. The results show that there are ions with energies as high as 80 eV and electrons with energies of 6 eV. The high-energy ions confirm that emitter failures are cathodic vacuum arcs     

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     

Investigation of arc roots of constricted high current vacuum arcs

The anodic and cathodic arc roots of constricted high current vacuum arcs were investigated with a fast framing charge-coupled device camera of 1 μs exposure time. The experiments were performed with cup-shaped contacts, with sinusoidal currents of amplitudes between 20 and 100 kA, and a sine halfwave duration of 10-12 ms. The arcs were drawn by contact separation and accelerated by the Lorentz force between the arc current and the transverse magnetic field generated by the contrate contact. The anode and cathode arc roots behave reproducibility and arc scaleable within the range of currents investigated. Both types of arc roots are elliptical, with a major to minor axis ratio of 1.4. The major axis points are in the direction of arc propagation. Anodic and cathodic arc root cross-sectional areas as a function of current can both be described by a potential law with a common exponent of 0.76. For currents of 20-100 kA, mean current densities of 81-121 and 41-60 kA/cm ² were found in anode and cathode arc roots, respectively. Estimations of their temperature and vapor densities were performed. For the investigated current range T_A≈3300-3600 K, n_A ≈1.6*10¹⁹-2.2*10¹⁹cm^-3 and T _C≈3200-3400 K, n_C≈0.8*10¹⁹-1.2*10 ¹⁹ cm^-3 were found for anode and cathode, respectively     

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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。     

Observation of tungsten particles in vacuum arcs by laser inducedfluorescence

We have clarified the relation between the decay of tungsten ion density in the vicinity of current zero and vacuum arc mode in high current period by using a laser induced fluorescence method in tungsten vacuum arcs of 60 Hz sinusoidal current with the peak value of 3.3, 6.7, and 9.8 kA. In the case of 6.7 kA, the arc mode was the anode spot mode. Because of the generation of the anode spot, the tungsten ion density near the anode was higher than near the cathode and the density near the anode was about ten times as high as the case of 3.3 kA which was the diffuse mode. In the case of 9.8 kA, which was the intense arc mode, the density near the anode was not significantly different from the case of 6.7 kA. The density near the cathode was higher than near the anode and tungsten ions were observed till about 30 μs after current zero while they disappeared at current zero in the other cases     

不同电场下碳纳米管场致发射电流密度研究

本文运用密度泛函理论和金属电子论, 深入研究了碳纳米管场致发射电流的变化规律. 结果显示其发射电流密度取决于体系的态密度、赝能隙、管长和局域电场, 在不同范围电场下的变化规律不同. 在较低电场下, 发射电流密度随电场增强而近似线性增大(对应的宏观电场须小于18 V· μm^-1); 但在较高电场下, 发射电流密度随外电场增加呈现非周期性振荡增长趋势, 碳纳米管表现为电离发射. 本文进一步研究了金属性碳纳米管电导率在不同电场下的变化规律.     

Performance of dual-band short-or mid-wavelength infrared photodetectors based on InGaAsSb bulk materials and InAs/GaSb superlattices

In this paper,we demonstrate bias-selectable dual-band short-or mid-wavelength infrared photodetectors based on In_(0.24)Ga_(0.76)As_(0.21)Sb_(0.79)bulk materials and InAs/GaSb type-II superlattices with cutoff wavelengths of 2.2μm and 3.6μm,respectively.At 200 K,the short-wave channel exhibits a peak quantum efficiency of 42%and a dark current density of5.93×10~(-5)A/cm~2at 500 mV,thereby providing a detectivity of 1.55×10~(11)cm·Hz~(1/2)/W.The mid-wave channel exhibits a peak quantum efficiency of 31%and a dark current density of 1.22×10~(-3)A/cm~2at-300 mV,thereby resulting in a detectivity of 2.71×10~(10)cm·Hz~(1/2)/W.Moreover,we discuss the band alignment and spectral cross-talk of the dual-band n-i-p-p-i-n structure.     

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。     

Effects of a Family Non-universal Z' Boson in \bar{B}_d→μ+μ- and B- →π-μ+μ-,ρ-μ+μ- Decays

Motivated by the recently updated experimental measurement on \bar{B}_d→μ⁺μ^- and B^-→π^-μ⁺μ^- decays by CDF and Belle collaborations, we revisit these
decays, as well as B^-→ρ^-μ⁺μ^- decay, within the Standard Model, and evaluate the effects of a family non-universal Z' boson. Under the constraint from {\cal B}(B^-→π^-μ⁺μ^-), we find the ranges of the Z' couplings S^LR_μμ< -5.2×10^-2 or D^LR_μμ < -8.1× 10^-2 are excluded. Within the allowed Z' parameters spaces, comparing with the SM predictions, we find that {\cal B}(\bar{B}_d→ μ⁺μ^-, {\cal B}(B^-→ π^-μ⁺μ^-), and {\cal B}(B^-→ρ^-μ⁺μ^-) could be enhanced by a factor about
226%, 245%, and 254%, respectively, byZ' contributions. However, they are hardly to be reduced. Furthermore, the zero crossing in the normalized forward-backward asymmetry spectrum of B^- →ρ^-μ⁺μ^- decay at low dimuon mass always exists.     

Spectroscopic investigation of a low current arc in pure fluorine

A pure fluorine 10-A DC arc has been operated in an Al₂O₃ tube of 18 mm diameter at atmospheric pressure. This arc was used to perform radially resolved spectroscopic end-on measurements in the visible and UV-spectral regions. An excitation temperature of 7800 K on the arc axis was determined from the intensity of atomic fluorine lines, and an ion density of 1.7×10 ²¹ m^-3 was determined from the half-width of the contamination line H_β. A Boltzmann plot of the affinity continuum in the UV-spectral region yields two groups of electrons. A group of hot electrons is characterized by a temperature that agrees with the excitation temperature, and a group of cold electrons has a temperature in accordance to the gas temperature of 4000 K. The absolute intensity of the affinity continuum gives and electron density of 1.3×10²⁰ m^-3 on the arc axis, which is lower than the density of both positive and negative ions in the discharge. From the difference between the electron and gas temperature, an elastic collision cross section between the electrons and F-atoms of 2×10^-20 m² is determined