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     

The anode surface temperature of CuCr contacts at the limit ofcurrent interruption

This paper estimates anode surface temperature at the current interruption limit by measuring the melting time of a contact material after current interruption and simulating the anode surface temperature. As a result, the minimum anode surface temperature of CuCr(50/50) contact material was about 1750 K. We also calculated the metal vapor density between electrodes with a simplified model. The calculation results showed that the critical vapor density was about 3×10²⁰ atoms/m³. This vapor density is equivalent to the averaged pressure of 8 Pa, which is close to the value of the Paschen minimum     

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     

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     

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     

温稠密钛电导率计算

温稠密物质是惯性约束核聚变、重离子聚变、Z箍缩动作过程中物质发展和存在的重要阶段. 其热力学性质和辐射输运参数在聚变实验和内爆驱动力学模拟过程中有至关重要的作用. 本文通过建立非理想Saha方程, 结合线性混合规则的理论方法模拟了温稠密钛从10^-5-10 g·cm^-3, 10⁴ K到3×10⁴ K区间的粒子组分分布和电导率随温度密度的变化, 其中粒子组分分布由非理想Saha方程求解得到. 线性混合规则模型计算温稠密钛的电导率时考虑了包括电子、原子和离子之间的多种相互作用. 钛的电导率的计算结果与已有的爆炸丝实验数据相符. 通过电导率随温度密度变化趋势判断, 钛在整个温度区间, 密度0.56 g·cm^-3时发生非金属相到金属相相变. 对于简并系数和耦合系数的计算分析, 钛等离子体在整个温度和密度区间逐渐从弱耦合、非简并状态过渡到强耦合部分简并态.     

Plasma deposition of thin films utilizing the anodic vacuum arc

Anodic vacuum arcs operating with cold cathodes in the spot mode and hot evaporating anodes are investigated to explore their technical potential as a plasma deposition technique. This discharge provides a unique source of a highly ionized, metal vapor plasma by autogeneration of the working gas to evaporation of the anode. This gas-free and droplet-free metal vapor plasma expands into the ambient vacuum (10^-4 mbar) and produces thin metallic films at the surface of substrates. An analysis of Al and Cu plasmas at the position of a possible substrate for arc currents between 20 and 200 A leads to the following results: electron densities, 10¹⁵-10¹⁸/ m³; degree of ionization, 0.5-25%; directed ion energy, 5 eV; and electron temperatures, 0.2-1 eV. Metallic coatings generated with deposition rates between 0.1 and 100 nm/s show the following properties: purity, 99.9%; polycrystalline structure with grain sizes between a few and a few hundred nm, same mass density as the respective bulk material, electrical conductivity rather close to that of the bulk material, and excellent optical properties. The coatings show good adhesion, which can be enhanced by a plasma-supported pretreatment of the substrate surface and by an acceleration of the ions towards the substrate     

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     

Experimental study of a plasma-filled backward wave oscillator

Experimental studies of a plasma-filled X-band backward-wave oscillator (BWO) are presented. Depending on the background gas pressure, microwave frequency upshifts of up to 1 GHz appeared along with an enhancement by a factor of 7 in the total microwave power emission. The bandwidth of the microwave emission increased from ⩽0.5 GHz to 2 GHz when the BWO was working at the RF power enhancement pressure region. The RF power enhancement appeared over a much wider pressure range in a high beam current case (10-100 mT for 3 kA) than in a lower beam case (80-115 mT for 1.6 kA). The plasma-filled BWO has higher power output than the vacuum BWO over a broader region of magnetic guide field strength. Trivelpiece-Gould modes (T-G modes) are observed with frequencies up to the background plasma frequency in a plasma-filled BWO. Mode competition between the T-G modes and the X-band Tm₀₁ mode prevailed when the background plasma density was below 6×10¹¹ cm^-3 . At a critical background plasma density of ≃8×10¹¹ cm^-3 power enhancement appeared in both X-band and the T-G modes. Power enhancement of the S-band in this mode collaboration region reached up to 8 dB. Electric fields measured by the Stark-effect method were as high as 34 kV/cm while the BWO power level was 80 MW. These electric fields lasted throughout the high-power microwave pulse     

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     

EAST装置低杂波电流驱动效率分析

在EAST上通过分析剩余环电压与低杂波功率之间的关系,计算得到了低杂波电流驱动效率。采用归一化功率,即功率对等离子体电流、电子密度、等离子体大半径以及有效电荷数归一化,将所有数据绘制在同一曲线中,这样可以得到不同等离子体参数下的低杂波电流驱动效率。实验得到低杂波电流驱动效率η₀=(0.5~1.3)×10¹⁹ A.m^-2.W^-1,在等离子体电流I_p=277kA、低杂波功率P_LH=681kW条件下,实验得到长达3s的低杂波全波驱动。     

S—Ka频段电磁波在等离子体中传输特性的实验研究

在感应耦合等离子体风洞上开展了等离子体中电磁波传输特性实验研究,获得了不同频率电磁波在等离子体中的传输衰减.通过微波诊断技术,获得了等离子体射流的电子数密度和碰撞频率.通过矢量网络分析仪和标准增益天线组成的电磁波传输特性测试系统,获得了电磁波经过等离子体之后的衰减,研究了电子数密度范围7.0×10~(10)-1.0×10~(13)cm~(-3)、等离子体碰撞频率在109 Hz量级的等离子体对2.6—40 GHz不同频率电磁波传输特性的影响,分析了经典传输理论和薄层理论预测结果与实验结果的差异.该实验工作为等离子体中电磁波传输特性的理论研究和数值仿真提供了基础数据.     

Inductive plasma sources for plasma implantation and deposition

External and reentrant radio frequency inductive plasma sources are developed for plasma ion implantation and deposition processes in a 1.8 m³ vacuum vessel. Plasma densities in the range 10¹⁶ -10¹⁷ m^-3 desirable for the above processes. External plasma sources could not yield the required plasma densities because of high particle losses in the transition region between the source and the main vessel. The particle losses are clarified through experiments and analysis, with and without multipole magnetic confinement. Reentrant plasma sources eliminate transmission losses and yield high plasma densities with good spatial uniformity     

Photo-ionized lithium source for plasma accelerator applications

A photo-ionized lithium source is developed for plasma acceleration applications. A homogeneous column of lithium neutral vapor with a density of 2×10^15-3 is confined by helium gas in a heat-pipe oven. A UV laser pulse ionizes the vapor. In this device, the length of the neutral vapor and plasma column is 25 cm. The plasma density was measured by laser interferometry in the visible on the lithium neutrals and by CO₂ laser interferometry on the plasma electrons. The maximum measured plasma density was 2.9×10 ¹⁴ cm^-3, limited by the available UV fluence (≈83 mJ/cm²), corresponding to a 15% ionization fraction. After ionization, the plasma density decreases by a factor of two in about 12 μs. These results show that such a plasma source is scaleable to lengths of the order of 1 m and should satisfy all the requirements for demonstrating the acceleration of electrons by 1 GeV in a 1-GeV/m amplitude plasma wake     

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     

The spatial and temporal development of pseudospark switch plasmas

Optical spectroscopy is used to investigate the spatial and temporal development of high-current pseudospark switch plasmas. At a peak discharge current of 12 kA in amplitude and a current reversal of 20%, the electron density is measured from Stark width broadening of the hydrogen Balmer beta line. The peak electron density of ~4×10¹⁵ cm^-3 is measured briefly after the current maximum. The discharge initially starts on the symmetry axis of the cathode hole. A cylindrical plasma column is observed, which is produced mainly by ionizing collisions of beam electrons formed in the hollow cathode during the early part of the discharge. This plasma column rapidly expands in the radial direction, until it contacts the edge of the cathode hole. The same behavior is found when the Balmer beta line intensity is evaluated rather than the line shape. Although statistically distributed, localized bursts of light are found occasionally, an axially symmetric, homogeneous light intensity distribution is always predominant, and the local arcing is merely superimposed on it. These results confirm that the discharge remains diffuse during most of the current pulse     

Measurement of the tungsten ion concentration after forcedextinction of a vacuum arc

The concentrations of singly ionized and neutral tungsten atoms were measured by laser-induced fluorescence after the forced extinction of vacuum arcs between tungsten-copper butt contacts, 28-mm in diam. and 10-mm apart. The 50-Hz current was forced to zero at its maximum of 200 A in 1.3 μs by application of a reverse voltage. Near current zero, the ion concentration of 4×10¹⁷ m^-3 is of the same order of magnitude as the atomic tungsten concentration, which is 6×10¹⁷ m^-3. While the concentration of the neutrals remains virtually constant during 20 μs after current zero, the ion concentration decays by three orders of magnitude in the same time. The decay-time constant varies from 1.9 μs close to the postarc cathode to 3.6 μs near the postarc anode. It is concluded that the dielectric recovery of vacuum gaps after diffuse arcs is mainly controlled by residual charge carriers     

Development of a high current pseudospark switch and measurement ofelectron density

The pseudospark, a low-pressure gas discharge in a special geometry, is suitable for high-current switching. A single-channel prototype is tested with a 3.3-μF capacitor bank, voltages up to 30 kV, and peak currents up to 120 kA. The electrical circuit, not comprising any load resistor causes weakly damped sinusoidal pulses of 5-μs duration at 90% current reversal. For lifetime tests, a switch with an alumina insulator and copper seals is used. Hydrogen is the working gas. Several electrode materials like molybdenum, tungsten, graphite and chromium-copper are tested. Optical investigations of the discharge and of plasma parameters are done with an O-ring sealed pseudospark switch. The light of the discharge is observed spectrally integrated with a streak camera. Spectral resolution is obtained by using a high-speed shutter in combination with a monochromator. The radial electron density is determined by measuring the Stark broadening of the Balmer H_β-line. At 60 kA a maximum electron density of about 2×10¹⁷cm^-3 is calculated     

Pressure limits for the vacuum arc deposition technique

Observations of the cathodic copper plasma expansion at low pressures of He, Ar, and SF₆ showed that, for background gas mass densities of ρ_g=1 to 4×10^-4 kg/m ³ and higher, the plasma and gas are separated into two volumes. A shock wave acts as a boundary between the two volumes. The boundary attains a stationary position once its expansion velocity decreases to the velocity of sound in the background gas. This position corresponds to a distance R_c to the cathode that agrees with a snowplow expansion model, giving R_c ^βf=E_r, where f is a function of the arc current and background gas characteristics, E _r is the erosion rate of the cathode, and β varies between 2.1 and 2.5. The interaction model is based on kinetic energy exchanges between two gas-like volumes without other energy losses. A maximum pressure limit for vacuum arc deposition is set for ρ_g /I=2 to 9×10^-6 kg/m³ A     

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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。