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     

Radiation effects of electrons and protons on the ceramic oxide superconductor YBaCuO

The irradiation of the high T_c superconducting material YBaCuO has been carried out by using 200 keV proton, and 400 keV and 8 MeV electron beams. The temperature of zero resistance increases from 86.7 to 89.8 K with proton implantation while 8 MeV electron irradiation reduces the zero resistance temperature by 3 K with an irradiation dose of 2.25×10¹⁴e^-/cm². However, wich an irradiation dose of 1.35×10¹⁵e^-/cm² the 8 MeV electron beam can make the superconductor become insulating. The in situ examination of a high resolution transmission electron microscope has proved that the amorphous region in the system has ordered arrangement whereas the crystalline region turns disordered under 400 keV electron irradiation with very high doses up to 10²⁶ e^-/cm². The experiments demostrate that proton and electron irradiations exhibit quite different effects both in its structure and property.     

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.     

氮气开关流柱形成过程的理论研究

使用蒙特卡罗-粒子模拟方法对氮气开关中的流柱形成过程进行模拟, 并结合计算结果对其进行理论分析. 发现在流柱击穿发生前(即空间电荷场远小于本底电场), 等离子体的电离频率、电子平均能量及其迁移速度等都近似为常数, 因此可以解析求解电子数密度方程对等离子体的演化行为进行分析. 在击穿发生后, 随机碰撞过程会破坏初始等离子体区域分布的对称性, 并出现分叉的等离子体区域结构. 在放电过程中, 随着等离子体密度增加, 其内部基本保持电中性且电场不断减小, 靠近阴阳极两端电荷分离产生的净电荷密度不断增加, 场强也不断增加, 且靠近阳极端的电荷密度(绝对值)和场强都大于阴极端. 通过改变极板间电压发现, 平均电子能量随极板间场强增加而增加, 电子迁移速度随着场强近似线性增加, 电离频率随场强的变化快慢介于E⁴与E⁵之间.     

Defects in H implanted GaAs studied by ion-beam and low-energy positron techniques

Ion-beam and low-energy positron-beam techniques have been used to study damage and implanted ion distributions and their annealing behavior in semi-insulating GaAs after the room temperature implantation of 3 × 10¹⁵−1 × 10¹⁷ 60 keV H⁺ cm^-2. The redistribution of the implanted H during annealing was observed to be connected to the migration of implantation-induced defect-complexes. A huge increase in the displaced atom concentration in the region of the H concentration was observed after annealings. A monovacancy overlayer, dissociation of H-vacancy complexes, and formation of stable vacancy-H agglomerates were observed in the different parts of the slowing-down region of the implanted H.     

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     

Conductivity of two-dimensional crystals over liquid helium for different holding fields

The results are presented of the experimental study of electron crystals over liquid helium with surface electron density of 3.2×10⁸ and 6.4×10⁸ cm⁻² (melting temperatures 0.4 and 0.58 K) at temperature 83 mK and for holding electric fields 300–1200 V/cm. The measurements are performed in the frequency range 1–14 MHz where the coupled phonon–ripplon resonances are observed in the experimental cell. The real and imaginary parts of the complex conductivity of the crystal have been obtained as a result of analysis of an electron layer response on exciting ac voltage with frequency corresponding to the mode (0,1) of the coupled phonon–ripplon oscillations. An analysis of the results allows us to suppose that structural defects of the electron crystal play an essential role in dissipation processes.     

SiC电子输运特性的Monte Carlo数值模拟

利用系综MonteCarlo法研究了2H ,4H和6HSiC的电子输运特性.在模拟中考虑了对其输运过程有着重要影响的声学声子形变势散射、极化光学声子散射、谷间声子散射、电离杂质散射以及中性杂质散射.通过计算,获得了低场下这几种不同SiC多型电子迁移率同温度的关系,并以4H SiC为例,重点分析了中性杂质散射的影响.最后对高场下电子漂移速度的稳态和瞬态变化规律进行了研究.将模拟结果同已有的实验数据进行了比较,发现当阶跃电场强度为10×106V·cm-1时,4H Sic电子横向瞬态速度峰值接近33×107cm·s^-1,6H Sic接近30×107cm·s^-1.     

Hydrogen action in the surface space charge region of highly doped silicon

The action of atomic hydrogen on clean cleaved (1 1 1) surfaces of highly doped silicon samples, both phosphorus ([n] = 2 × 10¹⁹ cm^-3) and boron ([p] = 4 × 10¹⁹ cm^-3) doped has been compared to the case of lightly doped samples ([n] = 1 × 10¹⁴ cm^-3). Once cleaved under ultra high vacuum, the samples were exposed to increasing doses of atomic hydrogen up to saturation. Before and after each hydrogen exposure, the Si(1 1 1) 2 × 1 surface was studied by low energy electron diffraction (LEED) and photoemission yield spectroscopy (PYS). The compared PYS measurements show that H atoms adsorbed on the Si(1 1 1) surface at room temperature do totally compensate the shallow-acceptor impurities (boron) and only partially the shallow-donor impurities (phosphorus) in the space charge region. They also remove the surface dangling bond states. These effects are reversible upon heating under vacuum. Both surface stresses and space charge electric field play a role in this compensation effect.     

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     

Comparison of nanosecond laser ablation at 1064 and 308 nm wavelength

To study the solid Cu ablation in vacuum, two different laser sources operating at 1064 and 308 nm wavelength are employed at similar values of laser fluences. The infrared laser is a Q-switched Nd:Yag having 9 ns pulse width (INFN-LNS, Catania), while the ultraviolet one is a XeCl excimer having 20 ns pulse width (INFN-LEA, Lecce). Both experiments produced a narrow angular distribution of the ejected material along the normal to the target surface. The ablation showed a threshold laser power density, of about 7 and 3 J/cm² at 1064 and 308 nm, respectively, below which the ablation effect was negligible. The laser interaction produces a plasma at the target surface, which expands very fast in the vacuum chamber. Time-of-flight (TOF) measurements of the ion emission indicated an average ion velocity of the order of 4.7×10⁴ and 2.3×10⁴ m/s for the infrared and ultraviolet radiation, respectively. We also estimated approximately the corresponding temperature of the plasma from which ions originated, i.e. about 10⁶ and 10⁵ K for IR and UV wavelength, respectively. A discussion of the analysis of the ablation mechanism is presented. At the used laser power densities the produced Cu ions showed ionisation states between 1+ and 5+ in both cases.     

Electron concentration in YBa2Cu3O6.9 single crystals determined from field electron emission spectroscopic measurements

Results of field electron emission spectroscopic investigations of YBa₂Cu₃O_6.9 single crystal specimens oriented perpendicular to the c-axis are presented. It is shown that the shift of the total energy distribution of field-emitted electrons from YBa₂Cu₃O_6.9 along the energy scale with increasing electric-field strength at the surface of the specimen is due to the penetration of the electric field into the subsurface region of the YBa₂Cu₃O_6.9 specimen. The electron concentration in YBa₂Cu₃O_6.9 found from spectroscopic measurements is n=(2−4) × 10²¹ cm⁻³. Field electron emission spectroscopy and microscopy of YBa₂Cu₃O_6.9 allows one to draw the conclusion that the electron concentration in the bulk of the material does not change with cooling of specimens from 300 K to 115 K.     

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.     

Observation of Raman forward scattering and electron accelerationin the relativistic regime

Raman forward scattering (RFS) is observed in the interaction of a high intensity (>10¹⁸ W/cm²) short pulse (<1 ps) laser with an underdense plasma (n_e~10¹⁹ cm ^-3). Electrons are trapped and accelerated up to 44 MeV by the high-amplitude plasma wave produced by RFS. The laser spectrum is strongly modulated by the interaction, showing sidebands at the plasma frequency. Furthermore, as the quiver velocity of the electrons in the high electric field of the laser beam becomes relativistic, various effects are observed which can be attributed to the variation of electron mass with laser intensity     

电离辐射对部分耗尽绝缘体上硅器件低频噪声特性的影响

本文针对辐射前后部分耗尽结构绝缘体上硅(SOI)器件的电学特性与低频噪声特性开展试验研究. 受辐射诱生埋氧化层固定电荷与界面态的影响, 当辐射总剂量达到1 M rad(Si) (1 rad = 10^-2 Gy)条件下, SOI器件背栅阈值电压从44.72 V 减小至12.88 V、表面电子有效迁移率从473.7 cm²/V·s降低至419.8 cm²/V· s、亚阈斜率从2.47 V/dec增加至3.93 V/dec; 基于辐射前后亚阈斜率及阈值电压的变化, 可提取得到辐射诱生界面态与氧化层固定电荷密度分别为5.33×10¹¹ cm^{- 2}与2.36×10¹² cm^-2. 受辐射在埋氧化层-硅界面处诱生边界陷阱、氧化层固定电荷与界面态的影响, 辐射后埋氧化层-硅界面处电子被陷阱俘获/释放的行为加剧, 造成SOI 器件背栅平带电压噪声功率谱密度由7×10^{- 10} V²·Hz^-1增加至1.8×10^-9 V² ·Hz^-1; 基于载流子数随机涨落模型可提取得到辐射前后SOI器件埋氧化层界面附近缺陷态密度之和约为1.42×10¹⁷ cm^-3·eV^-1和3.66×10¹⁷ cm^-3·eV^-1. 考虑隧穿削弱因子、隧穿距离与时间常数之间关系, 本文计算得到辐射前后埋氧化层内陷阱电荷密度随空间分布的变化.     

Electron localization of linear symmetric molecular ion H_3~(2+)

Electron localization in the dissociation of the symmetric linear molecular ion H_3~(2+) is investigated. The numerical simulation shows that the electron localization distribution is dependent on the central frequency and peak electric field amplitude of the external ultrashort ultraviolet laser pulse. When the electrons of the ground state are excited onto the 2pσ~2Σ_u~+ by a one-photon process, most electrons of the dissociation states are localized at the protons on both sides symmetrically. Almost no electron is stabilized at the middle proton due to the odd symmetry of the wave function. With the increase of the frequency of the external ultraviolet laser pulse, the electron localization ratio of the middle proton increases, for more electrons of the ground state are excited onto the higher 3pσ~2Σ_u~+ ustate. 50.9% electrons of all the dissociation events can be captured by the middle Coulomb potential well through optimizing the central frequency and peak electric field amplitude of the ultraviolet laser pulse. Besides, a direct current(DC) electric field can be utilized to control the electron motions of the dissociation states after the excitation of an ultraviolet laser pulse, and 68.8% electrons of the dissociation states can be controlled into the middle proton.     

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.     

Chemical effects of Ne bombardment on the MoS2(0001) surface studied by high-resolution photoelectron spectroscopy

The effect of 1 keV Ne⁺ bombardment on the clean MoS₂(0001)-1 × 1 surface with fluences between 4 × 10¹⁴ and 4 × 10¹⁶ Ne⁺/cm² was studied using high-resolution photoelectron spectroscopy excited with synchrotron radiation. Spectra of the Mo 3d and S 2p core levels were measured with photon energies that ensured that the kinetic energy of the photoelectrons was the same, resulting in the same depth being probed for both core levels. For lower fluences (i.e., 2 × 10¹⁵ Ne⁺/cm²), S vacancy defect formation occurs in the MoS₂ lattice, with the concurrent formation of a small amount (< 10%) of dispersed elemental molybdenum [Mo(0)]. For fluences greater than l × 10¹⁶ Ne⁺/cm², the Mo(0) is the predominant species in the surface region, while the remaining species consist of amorphous MoS_2−x and polysulfide species. Valence band spectra taken with photon energies of 152 and 225 eV were consistent with the core level results. The movement of the valence band maximum toward the Fermi level indicated the formation of a metallic surface region. Annealing the sample to temperatures up to 1000 K resulted in the formation of metallic Mo coexisting, in approximately equal amounts, with reformed MoS₂ in a surface with no long-range order as determined by LEED. Finally, a qualitative depth distribution of the chemical species present after Ne⁺ bombardment was determined by varying the photon energies used for the core level spectra. The results indicate that the preferential sputtering of sulfur over molybdenum occurs predominantly through a mechanism involving chemical bonding effects, specifically, through the preferential emission of polysulfide ions over other species in the bombarded region.     

Specific activity and derived intervention levels for Cesium-137 in Costa Rican export goods: Tuna fish, coffee and powdered milk

Cesium-137 is a fission product of ²³⁵U and ²³⁹Pu. After a major nuclear accident, it is released into the atmosphere and in the far field region it will produce radioactively contaminated food and drinking water. This paper will study the specific activity of ¹³⁷Cs in three Costa Rican export products: tuna fish, coffee and powdered milk. The average specific activities found are as follows: 0.89±0.41, 1.16±0.76 and 4.53±2.00 Bq kg⁻¹. They represent low values compared to their derived intervention levels: 1.25 × 10⁷, 1.25 × 10⁷ and 2.44 × 10⁵ Bq kg⁻¹, respectively.     

Surface properties of Si from photoelectric emission at room temperature and 80 °K

The position of the Fermi level with respect to the energy bands at a semiconductor surface as well as changes in the work function can be determined from the energy distributions of photoelectrically emitted electrons. Prior studies involved the photoelectric yield spectrum and required assumptions concerning the photoelectric threshold; the present method is free of such assumptions. Measurements at room temperature indicate that the Fermi level lies 0.23 eV and 0.41 eV above the top of the valence band for degenerate p and n-type materials, respectively. These results confirm those of Allen and Gobeli¹). Cooling to 80 °K increases the work function of p-type material by 0.025 eV while that of n-type Si remains unchanged; the results show that the electron and hole gases in the surface states are degenerate. The density of surface states lies between 7 × 10¹³ and 10¹⁵ eV⁻¹ cm⁻². On the cesiated surface, the Fermi level lies 0.16 eV below the conduction band at room temperature and coincides with its bottom at 80 °K.