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     

Self-sustained self-sputtering: a possible mechanism for thesuperdense glow phase of a pseudospark

Self-sustained self-sputtering occurring during high current pseudospark operation (≈10⁴ A/cm², I>10³ A) is shown to be a possible mechanism for the superdense glow. The mean-free-path for ionization of cathode material sputtered in the low-current hollow-cathode phase can be shorter than the cathode-anode gap distance, and ionized atoms can return to the cathode surface, self-sputtering with a yield greater than one. The self-sputtered cathode atoms become ionized in the beam of electrons accelerated in the cathode sheath. A large fraction of the discharge current at the cathode surface can be carried uniformly over the surface by ions and a very high electron emission density is not required to maintain the high current     

Experimental studies of long-lifetime cold cathodes for high-powermicrowave oscillators

Operation of explosive-emission cold cathodes made from various materials was studied at a large number of pulses at current densities of ~1.0⁴ A/cm². The cathode voltage and the beam current were ~500 kV and 5 kA, respectively, with a pulsewidth of ~20 ns. At a small number of pulses (⩽10³), cathodes of like geometry (even made from different materials) demonstrated similar emission properties. For most of the materials tested, with a large number of pulses (⩾10³), the current risetime increased to the fullwidth of the voltage pulse and the maximum current of the vacuum diode decreased. When using a graphite cathode, the maximum current remained invariant until 10⁸ pulses. Mass losses were measured for a series of cathode materials. The results obtained offered the possibility to realize long-lived operation of an X-band relativistic backward-wave oscillator with an almost invariant output power of 350-400 MW during 10⁸ pulses at a pulse repetition rate of 100-150 p.p.s     

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     

空间电荷效应对热场致发射中诺廷汉效应的影响

热场致发射阴极所产生的强流电子束具有很强的空间电荷效应,为研究该效应对热场致发射过程中诺廷汉（Nottingham）效应的影响机理,在理论分析的基础上,用数值方法研究了不同逸出功和多个外加电场条件下考虑空间电荷效应对诺廷汉效应结果的影响,并与不考虑空间电荷效应时的情形进行了对比. 结果表明：空间电荷效应的强弱会显著影响到阴极表面的稳态电场,进而对诺廷汉效应产生不可忽略的影响;当逸出功在3.0–4.52 eV、外加电场在3×10⁹–9×10⁹ V/m范围内时,考虑空间电荷效应的影响后,热场致发射电子所带走的平均能量较不考虑空间电荷效应时增加0–2.5 eV,且温度越高或外加电场越大时,该增加值越大;考虑空间电荷效应对诺廷汉效应的影响后,热场致发射电子从阴极带走的平均能量随外加电场的增加呈非线性下降规律;当阴极表面温度较高时,诺廷汉效应中的冷却效应随二极管间隙距离的变大而增强. 关键词： 热场致发射 诺廷汉效应 空间电荷效应 阴极表面电场     

Flashboard plasma characterization using spectroscopy

The plasma produced by a 10 cm×6 cm planar flashboard has been investigated by emission spectroscopy. The plasma composition, density, and temperature have been determined with time and space resolution using measurement of the relative intensity of spontaneous emission in different atomic and ionic transitions together with calculations based on a collisional-radiative equilibrium model. The (1-2)×10¹³ cm^-3 and (3-4) eV plasma flows away from the flashboard surface at a speed of about 10 cm/μs. A 1.7 cm/μs transverse velocity has been obtained from the Doppler width of an emission line     

沉积工艺参数对碳纳米管薄膜场发射性能的影响

利用微波等离子体化学气相沉积(MWPCVD)方法,在不锈钢衬底上直接沉积碳纳米管膜。通过SEM、拉曼光谱和XRD表征,讨论了制备温度和甲烷浓度对碳纳米管膜场发射的影响。结果表明:不同条件下制备的碳纳米管膜的场发射性能有很大差异,保持氢气的流量(100sccm)、生长时间(10min)、反应室压力不变,当甲烷流量为8sccm、温度为700~800℃时,场发射性能最好,开启场强仅为0.8V/μm,发射点分布密集、均匀。     

Study of field-induced explosive electron emission mechanisms ofpseudospark superemission cathodes

Various mechanisms of electron emission, including the field, field-enhanced thermionic, and explosive electron emissions from pseudospark cathodes, are discussed and compared. The mechanism of the field-induced explosive electron emission due to microstructure on the cathode surface is considered to be more likely the pseudospark superemissive mechanism. A high-mean electric field up to 3-5 MV/cm on the cathode surface in the end of hollow cathode phase is enough to initiate the mechanism. The cathode spot initiation delay time (<10 ns) and explosive emission threshold current (~10⁸ A/cm² ) prior to the high current conducting phase are given by solving the initial value problem of the one-dimensional heat conduction equation, thus explaining the existing experimental data of the pseudospark cathode superemission. In the case of multigap discharge, the above mechanism occurs on nearly all cathode and interelectrode surfaces. Experimental evidence in single- and multigap pseudospark discharges supports the suggested explanation     

Two-dimensional observation of copper vapor in vacuum arcs bylaser-induced fluorescence

The density distribution and the velocity of copper neutral atoms emitted from a single cathode spot in 40 A vacuum arc were measured by the two-dimensional (2-D) laser-induced fluorescence method. The density was calibrated from the two-dimensional fluorescence image observed by a CCD camera. The gap space was almost filled with the copper atoms, and the density reached 5×10¹⁹/m³. We varied the wavelength of the laser light and measured the velocity of the copper atoms emitted from the cathode spot using the effect of Doppler shift. The velocity of the copper atoms was about 10 km/s     

Development of a barium-free high-temperature cesium Tacitron

A barium-free, high-temperature, cesium Tacitron has been developed using a platinum, hollow cathode, emitter. The hollow cathode emitter used in our investigation is designed to enhance the current emission of a Tacitron cathode without the use of barium in the switch. In a barium-cesium Tacitron, the barium is known to cover the surface of a molybdenum emitter lowering its work function. The barium however limits the lifetime of the Tacitron, unless sophisticated seal technology is used in the manufacture of the Tacitron. The hollow cathode emitter was operated at current densities from 2.5 A/cm² up to 7.0 A/cm². Continuous operation of the Tacitron was demonstrated at 100-150 V and repetition rates of up to 5.8 kHz with measured voltage drops of 3.5-9.0 V. The results of the experimental characterization are compared to the computer model, and the applicability of this unique Tacitron design is discussed     

Twenty-five years of progress in vacuum arc research andutilization

Progress in understanding and applying vacuum arcs is reviewed. Laser diagnostics have demonstrated the existence of micron-sized regions in the cathode spot plasma having electron densities exceeding 10²⁶ m^-3. The expanding plasma produces a highly ionized jet whose ions typically have charge states of 1-3 and energies of 50-150 eV. Gas dynamic and explosive emission models have been formulated to explain cathode spot operation. In cases where the arc is constricted at the anode, forming an anode spot, or the anode is thermally isolated, forming a hot anode vacuum arc, material emitted from the anode may dominate the interelectrode plasma. Evaporation from liquid droplets may also provide a substantial component of the plasma, and the presence of these droplets can have deleterious consequences in applications. The vacuum arc has been extensively utilized as a plasma source, particularly for the deposition of protective coatings and thin films, and as a switching medium in electrical distribution circuit breakers     

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     

The Two-Nucleon Mechanism for the Neutrinoless Double Beta Decay and the BCS Nuclear Wave Function

A method of using the Bardeen-Cooper-Schrieffer (BCS) nuclear wave function to treat the two-nucleon mechanism for neutiinoless double beta decay process 0⁺ → 0⁺ is proposed.The neutrinoless decay mode and the neutrinoless decay accompanied by a Majoron emission mode of ⁸²Se are studikd. Our cdculated results show that to reproduce the experimental value of γ^(ov) > 1.8 × 10²² yr for neutrinoless double beta decai of ⁸²Se the Majorana neutrino mass m_v < 6.2 eV and the mixing parameter of right-handed current η < 7.0 × 10^-6 In the emission with a Majoron mode the effective Majoron coupling to neutrino is deduced from the experimental value of γ^(ov,H) > 4.4 × 10²⁰ yr for ⁸²Se with the result H0> < 6.2 × 10^-4.     

Density, viscosity and ultrasonic velocity studies of aqueous solutions of sodium acetate at different temperatures

The densities and viscosities of aqueous solutions of sodium acetate have been measured at 298.15, 303.15, 308.15 and 313.15 K and at atmospheric pressure. The molality range has been studied between 6.09 × 10^− 2 to 7.314 × 10^− 1 mol kg^− 1. The experimental values of density have been used to calculate apparent molar volume, partial molar volume, solute–solute interaction parameter, and Hepler's constant. The viscosity data have been analyzed with Jone–Dole equation. Furthermore, ultrasonic velocity measurements of aqueous solutions of sodium acetate have been made at 298.15 and 308.15 K and at atmospheric pressure. From experimental values of ultrasonic velocity, apparent molar isentropic compressibility and limiting apparent molar isentropic compressibility have been calculated. All the parameters calculated from density, viscosity, and ultrasonic velocity indicate that the sodium acetate is water structure maker.     

Evidence for the radioactive decay of Ra by C emission

Using a magnetic spectrometer and an intense ²²⁶Ra source, first evidence for the radioactive decay of ²²⁶Ra by ¹⁴C emission was obtained leading to a measured branching ratio relative to -particles of (3.2±1.6)×10⁻¹¹. In addition, the decay of ²²²Ra by ¹⁴C emission, previously reported, was clearly observed and an upper limit was set for the emission of ³⁴Si from ²⁴¹Am.     

Optimization of a poly (p-phenylene benzobisoxazole)-based light-emitting device with a complex cathode structure

In this work,we report the preparation of a series of electroluminescent(EL)devices based on a high-performance polymer,poly(p-phenylene benzobisoxazole)(PBO),and their optoelectronic properties,which have been rarely explored.The device structure is optimised using a complex cathode structure of tris-(8-hydoxyquinoline)aluminium(Alq3)/LiF/Al.By tuning the thickness of the Alq3layer,we improve the device efficiency dramatically in an optimized condition.Further analysis reveals that the Alq3layer in the complex cathode structure acts as a hole blocker in addition to its electron-injection role.A green light emission with a maximum brightness of 8.7×103cd/m2and a moderate current efficiency of 4.8 cd/A is obtained.These values are the highest ever reported for PBO devices.The high operational stability demonstrated by the present device makes it a promising tool for display and lighting applications.A new material is added to the selection of polymers used in this field up to now.     

980nm脉冲激发下Er3+掺杂材料中4I13/2→4I15/2跃迁的瞬态行为(英文)

在过去的几十年人们对Er³⁺掺杂的玻璃材料进行了广泛的研究,因为Er³⁺的⁴I_13/2→⁴I_15/2跃迁能够给出适合红外光通讯窗口的1.5μm的发射。据我们所知,目前关于脉冲激光激发下Er³⁺掺杂材料1.5μm发射的动力学行为研究报道仍很少。我们引入了转移函数理论,研究了980nm脉冲激发下Er³⁺的⁴I_13/2能级荧光的动力学行为。发现在980nm脉冲激发后,其荧光衰减遵循双指数规律,⁴I_13/2能级布居分为指数上升和指数下降两个过程。     

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     

Coherent and incoherent XUV emission in helium and neon,laser-driven plasmas

The authors present results of measurements of high-order harmonic generation and XUV spontaneous emission in helium and neon plasmas excited by a short pulse laser at intensities between 10¹⁴ and 5×10¹⁷ W/cm². They compare the observed behavior of the harmonics with recent single atom calculations in helium. A wavelength dependence to the efficiency of harmonic generation that has not been previously reported is observed. Line emission from excited state transitions in Ne⁷⁺ in a short pulse laser-driven plasma is also observed. In particular, strong emission and a rapid recombination rate for the 9.8-nm (3d-2p) transition that is a possible candidate for a recombination-pumped X-ray laser is shown     

Extending plasma accelerators: guiding with capillary tubes

In order to extend plasma accelerators, the laser beam has to be guided inside gas or plasma over a distance of the order of the dephasing length, which is typically much larger than the diffraction length z_R of the laser. A capillary tube can be used as a waveguide for high-intensity laser pulses over distances well in excess of z_R. Experimental demonstration of monomode guiding over 100 z_R of 10¹⁶ W/cm² pulses has been obtained in evacuated capillary tubes (45-70-μm inner diameter). A drop of transmission has been observed when the intensity of the amplified spontaneous emission (ASE) is high enough to ionize the capillary tube entrance. Propagation in helium gas-filled (10-40 mbar) capillary tubes has been studied at intensities up to 10¹⁶ W/cm²; a plasma column with on-axis density of the order of 10¹⁷ cm^-3 has been created on a length of the order of 4 cm. The use of a capillary tube for an extended accelerator is discussed for the ease of linear, resonant excitation of plasma waves by laser wakefield