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     

Experimental observations of steady anodic vacuum arcs withthermionic cathodes

Stationary plasma discharges have been investigated in a high vacuum ambient (background gas pressure <10^-2 Pa), with an externally heated cathode and a consumable hot evaporating anode. With various anode materials like chromium or copper, and electrode separations between 0.5 and 3 mm, the nonself-sustained discharge operates with DC arc currents in the range of 220 A. The waveform of the arc voltage is strongly influenced by the magnetic field of the cathode heating current, and arc voltages between a minimum of 3 V and a maximum exceeding 100 V have been observed. The voltage-current characteristics (V_CC) and the influence of the electrode separation have been measured separately for the minimum and the maximum of the arc voltages and show a different behavior. The metal plasma expands into the ambient vacuum toward the walls of the vacuum vessel and offers a macroparticle free deposition source of thin films. The arc voltage can be varied by external manipulations of the arc discharge, and the mean ion energy of the expanding metal plasma shows a linear dependence of the mean arc voltage     

Experimental observation of high-voltage, low-current vacuum arcs

A poorly explored type of discharge has been investigated in high vacuum (10^-7 to 10^-6 torr), with a DC high voltage across 0.2- to 0.8-mm gaps. The discharge has been found to be quite different from other widely known types of vacuum and gas discharges by the combination of its voltage-current characteristics (hyperbola-type), source and carriers of current (mostly electrons), and spatial potential distribution (a considerable electric field across the gap and a steep potential fall near the cathode)     

Measurement of neutrons and X-rays from a vacuum spark

The Jülich DPF I machine was modified to operate as a vacuum spark. It was established that with an operating pressure varying from 10^-4 to 10^-5 torr a voltage within the range of 14–17 kV was necessary to ensure a dense plasma phase as indicated by soft X-ray pinhole photography and current measurements. Operating with a LiD-filled channel in the anode, occasional bursts of neutrons were detected with a time-of-flight scintillator system. The yield, however, was erratic and below 10⁷ per discharge.     

Over recovery in low pressure spark gaps-confirmation of pressurereduction in over recovery

The over recovery in sparkgaps (Nagesh, 1997, Nagesh et al., 1999) operating along the left-hand side of Paschens' characteristics is due to pressure reduction in the gap after the first pulse discharge. This pressure reduction leading to over recovery in low pressure spark gaps has been verified using a low pressure spark gap with two spark gaps placed one above the other in the same chamber. The breakdown voltage strength characteristics of the second gap has been determined for gap spacings of 3.5 mm to 10 mm, diffusion of plasma in the direction of vacuum pumping and opposite, at a pressure of 2.1 Pa for hydrogen gas. The vacuum pumping direction has a great influence on the breakdown strength characteristics of second gap after the first gap discharge. The breakdown voltage of the second gap can exceed its self breakdown voltage only 1) when the diffusion of plasma and vacuum pumping direction are same and 2) when the second gap spacing is greater than or equal to first gap spacing. Shorter gaps can always have breakdown voltage lower than or equal to their self breakdown voltage. The experimental setups, behavior of self breakdown voltages of second gap due to breakdown in the first gap, over recovery characteristics of spark gaps, the results, and discussions are presented here     

EAST真空室内巡视系统的真空性能研究分析

介绍了EAST真空室内巡视系统(IVVS)系统对EAST真空室第一壁进行巡检操作的工作原理,并基于系统部分材料出气率测试对其真空性能进行了分析。获得了在有效抽速为1064L•s^–1的条件下,IVVS系统试验样机所采用的真空容器的极限真空度为3.37×10^–3Pa。为优化IVVS真空性能,结合材料放气率试验,将部分材料更换为放气率低的材料,对系统进行烘烤处理,并对其中的相关结构提出了优化方案。优化后系统的真空性能有明显提高,在同等抽速条件下的极限真空度在1×10^–4Pa范围内,能够满足EAST接受的真空环境要求。此外,抽气机组增添低温泵可以更进一步提高系统的真空性能。     

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介绍了EAST真空室内巡视系统(IVVS)系统对EAST真空室第一壁进行巡检操作的工作原理,并基于系统部分材料出气率测试对其真空性能进行了分析.获得了在有效抽速为1064L·s–1的条件下,IVVS系统试验样机所采用的真空容器的极限真空度为3.37×10–3Pa.为优化IVVS真空性能,结合材料放气率试验,将部分材料更换为放气率低的材料,对系统进行烘烤处理,并对其中的相关结构提出了优化方案.优化后系统的真空性能有明显提高,在同等抽速条件下的极限真空度在1×10–4Pa范围内,能够满足EAST接受的真空环境要求.此外,抽气机组增添低温泵可以更进一步提高系统的真空性能.     

Simulation study of a capacitively coupled plasma torch array

Using capacitively coupled electrical discharges, an array of three plasma torches powered by a single 60-Hz source are lit up simultaneously to produce a dense plasma in the open air. The discharge voltage and current of each torch is measured for three cases of one to three torches being lit up in the array. The results determine the ν-i characteristic of the discharge which indicates that the torch is operating in a diffuse are mode. The torch array is modeled by an equivalent circuit for simulating its operation. The simulation results of the discharge voltage and current of a torch are shown to agree well with those from the experimental measurements for the three cases. The lump circuit model is then used to carry out numerical simulations of the discharge for a broad parameter space of plasma species. By fitting the simulation results, a function giving the parametric dependence of the consumed average power density 〈P〉 on the normalized average electron density 〈n_e〉 maintained in the plasma is determined to be 〈P〉 48 〈n_e〉 ^1.9α_^0.4(W/cm³), where 〈n_e〉 is normalized to 10¹³cm^-3 and α_, the electron-ion recombination coefficient normalized to 10^-7 cm³·s^-1, is used as a variable parameter in the simulation     

Initial stage of discharge current growth in a triggered vacuum gap

The dynamics of light emission accompanying the initial stage of electric discharge in a substantially undervoltaged vacuum gap was studied with a knock-down model using high-speed photorecording. Voltage across the gap was maintained within the range of 0.5-5 kV, which corresponded to the minimum operating voltage of vacuum-triggered gaps. It was found that front layers of a plasma cloud near a cathode, formed by a firing pulse, scattered at a speed of (5-8)×10⁶ cm/s. During firing, a channel directed to the opposite electrode was formed from the plasma cloud near the cathode. It was found that the average switch-on delay time of the triggered vacuum gap is ~d(1+h/d) I_f^-α, where d is the interelectrode gap length, h is the trigger assembly penetration height, and I_f is the firing current     

Stability assessment and operating parameter optimization on experimental results in very small plasma focus,using sensitivity analysis

Regarding the importance of stability in small-scale plasma focus devices for producing the repeatable and strength pinching, a sensitivity analysis approach has been used for applicability in design parameters optimization of an actually very low energy device (84 nF, 48 nH, 8–9.5 kV, ～2.7–3.7 J). To optimize the devices functional specification, four different coaxial electrode configurations have been studied, scanning an argon gas pressure range from 0.6 to 1.5 mbar via the charging voltage variation study from 8.3 to 9.3 kV. The strength and efficient pinching was observed for the tapered anode configuration, over an expanded operating pressure range of 0.6 to 1.5 mbar. The analysis results showed that the most sensitive of the pinch voltage was associated with $0.88\pm 0.8\text{mbar}$ argon gas pressure and 8.3–8.5 kV charging voltage, respectively, as the optimum operating parameters. From the viewpoint of stability assessment of the device, it was observed that the least variation in stable operation of the device was for a charging voltage range of 8.3 to 8.7 kV in an operating pressure range from 0.6 to 1.1 mbar.     

Pressure dependence of plasma parameters in medium-vacuum nitrogenarc discharge with the titanium cathode

The plasma properties of a medium-vacuum nitrogen arc discharge from a titanium cathode were studied. The arc chamber use was 400 mm in diameter and 600 mm in length. The cathode diameter and thickness were 64 and 25 mm, respectively. The experimental conditions are given as follows: pressure range=1×10^-3~2×10^-1 torr; N₂ gas flow rate=6 ml/min; arc current=50 A. Electric probe characteristics are measured as a function of pressure and distance from the cathode surface. The analytical results obtained show that the electron energy distribution takes 1-Mx at pressures above 1×10^-2 torr but 2-Mx at pressures under 4×10^-2 torr and that the electron density has a maximum value at a certain pressure. The Ti⁺, Ti⁺⁺, and N ⁺₂ ion spectral intensities are measured as a function of pressure and distance from the cathode surface. On comparison of these results and the electron density, the Ti⁺ spectral intensity turns out to be proportional to that of the electron density. This suggests that the major ion in the plasma volume is of the Ti⁺ species     

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根据EAST-NBI大面积束引出系统的特点及装配要求,研究合理的装配方法.通过多种装配方法来消除装配中产生的误差,以到达最大误差小于0.05mm,真空接口漏率小于1 × 10-7Pa·L·s-1,冷却水管耐压1 MPa,通过100kV的静电耐压测试,并进行了离子束引出,初步验证了引出系统装配的成功,支持兆瓦级强流离子源的研制.     

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     

放射性次级束流分离器真空系统的压力分布

放射性次级束流分离器是强流重离子加速器装置中,连接增强器和高精度环形谱仪的束流输运线,用于传输重离子束流以及放射性次级束流。为了满足束流传输的要求,并维持相连增强器和高精度环形谱仪的极高真空,放射性次级束流分离器真空系统的平均压强应低于5×10-7 Pa。因此,需要验证真空系统设计方案的可行性,以及设计方案能否满足要求的压强范围。通过现有的同步储存环CSRm中的真空计监测数据以及软件BOLIDE的模拟结果对比,对真空压力计算软件VAKTRAK的使用方法和计算结果进行验证;采用VAKTRAK模拟计算不同真空参数下（流导、出气率以及泵速）放射性次级束流分离器真空系统的压力分布。根据计算结果,放射性次级束流分离器真空系统的平均压强可以达到1.79×10-7 Pa （H₂）,满足物理实验和工程设计的要求。通过模拟计算结果,放射性次级束流分离器真空系统的设计方案的可行性得到验证,系统设计的真空度满足要求。HIAF Fragment Separator(HFRS) is connected with Booster Ring(BRing) and Spectrometer Ring (SRing) in the HIAF and used to transfer the ion beams and radioactive secondary beams. To satisfy the requirements of beam transmission and maintain the extremely high vacuum of BRing and SRing, the average pressure of HFRS vacuum system should be lower than 5×10-7 Pa. Therefore, the feasibility of the design scheme and whether the design scheme would fulfill the required vacuum range or not should be verified. Based on the measured data on the current sychrontron CSRm and the simulation results of BOLIDE, the calculation results of VAKTRAK are verified and then VAKTRAK is used to calculate the pressure profiles of different parameters(such as the conductance, out-gassing and pumping speed) for HFRS. According to the calculation results, the average pressure of HFRS vacuum system could be 1.79×10-7(H₂) which achieves the required pressure for physics experiments and engineering design. According the calculation results of this paper, the feasibility of the designed HFRS vacuum system has been verified and the design of system satisfies the vacuum requirements.     

Influence of residual gases on cathode spot behavior

A residual gas in a vacuum arc chamber influences the behavior of the arc by two effects: it changes the state of the cathode surface, in particular the surface cleanness, and it influences the interelectrode plasma. Experiments are summarized dealing with the influence of the residual gas on the arc parameters in a pressure range of 10^-6-10^-5 Pa. With increasing pressure, general tendencies are a decrease in the fluctuations of the burning voltage, in the chopping current, and in the current density, and an increase in the arc lifetime, spot velocity, and spot diameter. The conditions at the cathode surface are decisive for the spot behavior and not the pressure. Surface contaminations render the arc more stable. The transition between the so-called cathode spot type 1 (on contaminated surfaces) and type 2 (on clean surfaces) was found to be smooth rather than abrupt     

Quantitative cleaning characterization of a lithium-fluoride iondiode

An ion source cleaning testbed was created to test plasma-cleaning techniques, and to provide quantitative data on plasma-cleaning protocols prior to implementation on the SABRE accelerator. The testbed was designed to resolve issues regarding the quantity of contaminants absorbed by the anode source (LiF), and the best cleaning methodology. A test chamber was devised containing a duplicate of the SABRE diode. Radio-frequency (RF) power was fed to the anode, which was isolated from ground and thus served as the plasma discharge electrode. RF plasma discharges in 1-3 mtorr of Ar with 10% O₂ were found to provide the best cleaning of the LiF surface. X-ray photoelectron spectroscopy (XPS) showed that the LiF could accrue dozens of monolayers of carbon just by sitting in a 2×10^-5 vacuum for 24 h. Tests of various discharge cleaning protocols indicated that 15 min of an Ar/O₂ discharge was sufficient to reduce this initial 13-45 monolayers of carbon impurities to 2-4 monolayers. Rapid recontamination of the LiF was also observed. Up to ten monolayers of carbon returned in 2 min after termination of the plasma discharge and subsequent pumping back to the 10^-5 torr range. Heating of the LiF also was found to provide anode cleaning. Application of heating combined with plasma cleaning provided the highest cleaning rates     

Filippov type plasma focus as intense source of hard X-rays(Ex≃50 keV)

This paper evaluates the use of a plasma focus machine (Filippov type) as a pulsed intense source of hard X-rays. It is shown experimentally that discharging a capacitor bank of W=50 kJ in the new arrangement of the discharge parameters, one can deliver into the test cavity (volume of 6*10³ cm³) an average of Φ _x=8*10^-4 J/cm². The delivered energy fluence varies in the range of 2*10^-4 J/cm²⩽Φ_x⩽2.9*10^-3 J/cm², depending on the probe position. The X-ray energy spectra were established by the differential absorption method using thermo-luminescent detectors (time-integrated spectra) or plastic scintillators (time-resolved spectra). The average energy of X-ray photons is 40 keVx^av<50 keV (20 keV⩽E _x⩽170 keV) and remains quasi-constant during the main part of the pulse (~40 ns). The electron energy that can produce such bremsstrahlung radiation is in the range of 95 keV⩽E_e⩽170 keV. However, time-resolved experiments reveal that at each instant of time only one energy of electron beam occurs. The aforementioned radiation characteristics reveal new potentials for the plasma-focus machine for a variety of applications. Some insight into the physics behind production mechanism of quasi-monoenergetic relativistic-electron beams will be concurrently reviewed     

Factors affecting and methods of improving the pulse repetitionfrequency of pulse-charged and DC-charged high-pressure gasswitches

The results of this paper describe some of the factors which affect the repetitive operation of high-pressure gas switches (spark gaps) for both pulse-charged and DC-charged operation. Also discussed are methods which may be employed to improve the pulse repetition frequency (PRF) of spark gaps operating under such conditions. Under pulse-charged conditions, the voltage recovery process of the spark gap has been shown to be restricted following partial density recovery by the residual ion population. This restriction may be minimized by applying a suitable bias voltage across the gap to remove the ion influence. It is also possible to manipulate the voltage-pressure (V-p) breakdown characteristic of a spark gap in order to improve the rate of rise of recovery voltage by reducing the recovery voltage dependence upon gas pressure. The combination of these effects has been shown to reduce the voltage recovery time of pulse-charged spark gaps from several hundred milliseconds to several milliseconds. Under DC-charged conditions, where no “dead time” is available for voltage recovery, it is possible to employ corona discharge effects, which occur in highly nonuniform fields, to stabilize and control the breakdown process. The use of corona stabilization has enabled the operation of a self-closing spark gap at a PRF of more than 5 kHz, without employing gas flow techniques. A triggered version of a corona-stabilized spark gap has also been developed which has demonstrated single run capabilities of 10⁷ (4 h continuous operation at 700 pps) and a lifetime of ~10 shots (maintenance free, sealed switch). The triggered corona switch has also demonstrated controlled switching up to a PRF of 1.2 kHz     

Electrical and optical characteristics of the radio frequency surface dielectric barrier discharge plasma actuation

Electrical characteristics and optical emission spectrum of the radio frequency(RF) surface dielectric barrier discharge(SDBD) plasma actuation are investigated experimentally in this paper. Influences of operating pressure, duty cycle and load power on the discharge are analyzed. When the operating pressure reaches 30 kPa, the discharge energy calculated from the Charge–Voltage(Q–V) Lissajous figure increases significantly, while the effective capacitance decreases remarkably. As the duty cycle of the applied voltage increases, the voltage–current waveforms, the area of Q–V loop and the capacity show no distinct changes. Below 40 W, effective capacitance increases with the increase of load power, but it almost remains unchanged when load power is between 40 W and 95 W. The relative intensity I_(391.4)~peak/I_(380.5)~peak changes little as the operating pressure varies from 4 kPa to 100 kPa, while it rises evidently with the pressure below 4 kPa, which indicates that the RF discharge mode shifts from filamentary discharge to glow discharge at around 4 kPa. With the increase of load power, the relative intensity I_(391.4)~peak/I_(380.5)~peak rises evidently. Additionally, the relative intensity I_(371.1)~peak/I_(380.5)~peak is insensitive to the pressure,the duty cycle, and the load power.     

锐钛矿金红石的高温原位X射线衍射研究

对TiO₂粉末进行了空气和真空条件下从室温到1200℃的加热原位X射线衍射实验, 得到了空气和真空条件下微米级锐钛矿颗粒转变为金红石的起始温度分别为850℃ 和855℃; 分别修正了空气条件下锐钛矿在(27–850℃)范围和金红石在(900–1200℃) 范围内的晶胞参数和真空条件下锐钛矿在(27–850℃)范围和金红石在(950–1200℃) 范围的晶胞参数, 从而得到了晶胞参数随温度变化的关系, 得到了锐钛矿和金红石在空气中和真空中的热膨胀系数, 并总结了热膨胀系数随温度变化的规律. 室温下锐钛矿在空气条件下的热膨胀系数为 α_a=4.55063×10^-6/℃, α_c=7.7543×10^-6/℃, β=16.85836×10^-6/℃; 真空下为 α_a=4.69429×10^-6/℃, α_c=9.02850×10^-6/℃, β=18.69688×10^-6/℃. 室温下, 金红石在空气条件下的热膨胀系数为 α_a=6.81243×10^-6/℃, α_c=8.71644×10^-6/℃, β=22.22178×10^-6/℃; 真空条件下为 α_a=6.05834×10^-6/℃, α_c= 8.39280×10^-6/℃, β=20.52362×10^-6/℃. 关键词： 2')" href="#">TiO₂ 原位X射线衍射 相转变 热膨胀