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     

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     

真空二极管辐射微波的机理分析

在研究真空开关的过程中,发现真空二极管能辐射出宽带微波.这种器件只由带触发装置的阴极和平板阳极组成,不存在金属波纹慢波结构,所以真空二极管的辐射机理与等离子体填充微波器件不同,不能直接套用等离子体填充微波器件的相关理论.本文描述了真空二极管产生辐射的物理过程,建立了真空二极管辐射的数学模型,通过求解波动方程得到产生辐射的色散关系,并绘制出了色散曲线.将理论分析得到的色散曲线与已经测得的微波辐射进行比较,两者能很好地符合.理论分析和实验结果表明,电子束和磁化等离子体的相互作用是真空二极管产生微波辐射的原因.     

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     

Control of gigawatt REB pulse duration using gas injection

A technique for controlling the voltage and current pulse duration of a gigawatt relativistic electron beam (REB) was investigated. The pulse duration of a beam of 250 keV and 10 kA was controlled from 60 ns to 10 ns by injecting gases (air,, argon, and hydrogen) into the diode gap at a pressure ranging from 10^-5 torr to 10^-1. The observed dependence of the pulse duration on the nature and pressure of the gas is explained in terms of volume ionization of the gas by beam electrons. It is concluded that the pulse duration is governed by the time at which the plasma density created by the beam reaches a critical value of the order of beam electron number density     

Development of an intense pulsed metallic ion beam source

Intense pulse metallic ion beams (Al⁺, Cu⁺, and Pb⁺) were produced by a magnetically insulated ion diode having a metal anode. Metal ion plasmas on the anode could be generated through enhanced electron bombardment by using a radial cathode. The energy, current density, and duration time of the lead ion beam were 30~140 keV, ~7.5 A/cm² (total ion current ⩾0.5 kA), and 800 ns, respectively. The ion current density exceeded the space-charge-limited current by a factor of 50. The lead ions in the first-to-sixth states of ionization were detected by a Thomson-parabola ion-spectrometer together with light loss, such as C⁺ and O ⁺. The ratio of the ion current of heavy metals to the total ion current was measured using a magnetic mass analyzer with a charge collector. The ratio was about 90% for a lead ion beam and 20~50% for Al and Cu ion beams     

1000 V p-GaN混合阳极AlGaN/GaN二极管

GaN材料具有优异的电学特性,如大的禁带宽度(3.4 eV)、高击穿场强(3.3 MV/cm)和高电子迁移率(600 cm~2/(V·s)). AlGaN/GaN异质结由于压电极化和自发极化效应,产生高密度(1×10~(13)cm~(-2))和高迁移率(2000 cm~2/(V·s))的二维电子气(2DEG),在未来的功率系统中, AlGaN/GaN二极管具有极大的应用前景.二极管的开启电压和击穿电压是影响其损耗和功率处理能力的关键参数,本文提出了一种新型的具有高阻盖帽层(high-resistance-cap-layer, HRCL)的p-GaN混合阳极AlGaN/GaN二极管来优化其开启电压和击穿特性.在p-GaN/AlGaN/GaN材料结构基础上,通过自对准的氢等离子体处理技术,在沟道区域形成高阻盖帽层改善电场分布,提高击穿电压,同时在阳极区域保留p-GaN结构,用于耗尽下方的二维电子气,调控开启电压.制备的p-GaN混合阳极(p-GaN HRCL)二极管在阴阳极间距Lac为10μm时,击穿电压大于1 kV,开启电压+1.2 V.实验结果表明, p-GaN混合阳极和高阻GaN盖帽层的引入,有效改善AlGaN/GaN肖特基势垒二极管电学性能.     

Pulse intense metal-ion diode with a vacuum arc plasma anode

An intense pulsed ion beam of metal was extracted from a magnetically insulated ion diode operated in a mode of plasma prefill generated from a vacuum arc discharge, anode plasma source. With this ion diode, an intense metal-ion beam of a high melting-point metal (Ta) was obtained. A variety of operational modes appeared, depending on the amount of plasma in the diode gap at the initiation of the high-voltage pulse. The energy, current, and duration time of the ion beam were 20~100 keV, ~1 kA, and 1 μs, respectively. Measurements of ions were performed with an ion energy analyzer or a biased ion collector located at the end of a long drift tube and a Thomson parabola ion spectrometer. The Ta ions in the first to fifth states of ionization were detected accompanied by C⁺, O⁺, F⁺, and H⁺ . A Ta ion beam current of about half the total ion flux was obtained in this experiment     

无箔二极管结构设计研究

用数值计算方法研究了无箔二极管中几何结构参量对束流的影响，得出了无箔二极管结构参数选择的部分规律，同时又结合实际给出了一个无箱二级管模型的最佳组合参数．     

Angular velocity spread of relativistic electron beam generated by high current diode

Theoretical analysis and numerical simulations of the Relativistic Electron Beam (REB) generation in a high current diode immersed in an external magnetic field has been done. The calculations have confirmed that the generated beam is homogeneous and monoenergetic in a broad central region. In the case of cylindrical diode the mixing of electron trajectories has been observed only in a narrow periphery beam region. The angle between particle trajectories and external longitudinal magnetic field varies chaotically from 0° to –25°. This phenomenon suppresses the excitation of two stream instability excited by the REB in the plasma column.     

Generation of a submillisecond electron beam with a high-density current in a plasma-emitter diode under the conditions of open plasma boundary emission

The generation of a 250-μs-wide electron beam in a plasma-emitter diode is studied experimentally. A plasma was produced by a pulsed arc discharge in hydrogen. The electron beam is extracted from a circular emission hole 3.8 mm in diameter under open plasma boundary conditions. The beam accelerated in the diode gap enters into a drift space in the absence of an external magnetic field through a hole 4.1 mm in diameter made in the anode. The influence of electron current deposition at the edge of the anode hole on the beam’s maximum attainable current, above which the diode gap breaks down, is studied for different accelerating voltages and diode gaps. The role of processes occurring on the surface of the electrodes is shown. For an accelerating voltage of 32 kV, a mean emission current density of 130 A/cm² is achieved. The respective mean strength of the electric field in the acceleration gap is 140 kV/cm. Using the POISSON-2 software package, the numerical simulation of the diode performance is carried out and the shape of steady plasma emission boundaries in the cathode and anode holes is calculated. The influence of the density of the ion current from the anode plasma surface on the maximum attainable current of the electron beam is demonstrated.     

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     

Production of high-current large-area H- beams by abucket-type ion source equipped with a magnetic filter

A negative-ion-based neutral beam injection (NBI) system is planned for plasma heating of the Large Helical Device (LHD). We have developed a negative ion source, which is 1/3 the scale of the source for the NBI. A magnetic filter held was generated by external permanent magnets to lower the electron temperature in a large-area bucket plasma source (35 cm×62 cm) for efficient H^- production. We investigated the magnetic field configuration and found a low electron temperature high density plasma (<1 eV, 10¹²/cm³) could be achieved with an optimized configuration, The filter strength (B_max=70 G, line-integral flux=780 G cm at the center axis of the source) was proved to be enough to lower the electron temperature below 1 eV at high arc discharge power (100 kW) and low pressure (0.4 Pa). We injected cesium vapor into the plasma source to enhance H^- production efficiency and obtained a 16.2-A H^- beam current (31 mA/cm², 47 kV) using a large-area, four-grid electrostatic extraction system (25 cm×50 cm). This satisfied the development target (>15 A: 1/3 current of LHD ion source). Based on the results, we are designing a negative ion source for the LHD     

General scaling of pulse shortening in explosive-emission-drivenmicrowave sources

Microwave generation in devices that depend on synchronization between an electron beam and a resonant cavity or slow wave structure can be disrupted by changes in either. Explosive-emission-driven microwave sources use plasma as the electron source in the diode. This plasma is conductive enough to act as the boundary for both the applied diode voltage and the microwave electric field. The motion of this plasma can effectively change the dimensions of either the electron beam diode or the cavity and will thereby cause resonance destruction. This shortens the microwave pulse length τ_μ. A general model of the process predicts that, for a Child-Langmuir diode, microwave power falls as P∝τ_μ^-5/3 and that pulse energy falls as E∝τ_μ^-2/3. Therefore, energy efficiency declines as the pulse length is extended. We compare with data from magnetrons, MILO's and BWO's, and find that over some regions of operation the pulse length and energy from these explosive-emission-driven microwave sources agree with the plasma motion model scaling. At these applied drive voltages and output powers the microwave pulse length can be increased by finding cathode materials that generate slower plasmas     

Atomic collision processes relevant for heavy ion beam probes

Atomic collision processes of fast Tl and Cs ions with particles in a high temperature fusion plasma are investigated. At low beam energies (<5 MeV), ion impact collisions and charge exchange processes can be neglected compared to electron ionization processes. At beam energies above 5 MeV and high plasma ion temperatures, collisions with ions start to contribute significantly to signal generation and attenuation. Also, collisions with the neutral background gas in the beamlines can attenuate the ion beam significantly and lower the signal level, if the vacuum pressure is above 10^-4 Torr. For the heavy ion beam probes operating today, only electron impact ionization processes are important and accurate predictions of the secondary signal level and electron density profile measurements are possible because of the good knowledge of electron impact ionization cross sections for Cs ⁺ and Tl⁺ ions     

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.     

Calculation of Excited States of He Atoms in a Strong Magnetic Field

A recently developed B-spline algorithm is extended and utilized to calculate excited states of He atoms in the presence of strong magnetic fields. Binding energies are presented for He in the five excited atomic states 2¹0⁺, 1¹0^-, 2¹0^-, 1¹(-1)⁺, and 2¹(-1)⁺ with magnetic field strength ranging from 0.0001 to 10 a.u. The obtained energies are compared with available theoretical data, and found to be in good agreement. We investigate influence of magnetic fields on atomic structures of multielectron atoms, and illustrate that how electron probability density distributions change with increasing magnetic field strength. The current approach is directly applicable to simulations of discrete spectra for He atoms in the atmospheres of magnetized white dwarf stars.     

Beam profile monitor for pulsed CO2-laser

Commercial beam profile monitors for pulsed CO₂ lasers (10.6 μm wavelength) are expensive and suffer from a low damage threshold. To overcome these problems, a robust detector with a high damage threshold was developed. The detector is made of a special type of graphite which is evaporated during the interaction with the laser beam. A plasma is formed which radiates visible light. Using a conventional fast shutter imaging system, this plasma light directly shows the spatial intensity distribution of the laser beam. The detector works in the power range of 10⁵-10⁷ W/cm²      

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