A magnetic acceleration plasma facility for plasma shock peening

Set-up, function and application potential of pulsed magnetoplasmadynamic self-field accelerators are described. The focus is on the facility MAX (Magnetoplasmadynamic Accelerator-eXperiment). Here, a high power coaxial accelerator is investigated regarding space propulsion and processes aiming for metal treatment as potential applications.A certain amount of gas is accelerated via magnetic fields while the overall kinetic energy of the plasma has to be maximized. During plasma generation numerous parasitic effects are associated with the discharge of the device. Hence, the characterization of the facility in terms of power balance, functional behaviour and kinetic energy of the plasma is mandatory. The kinetic energy is of importance for both space propulsion and the mentioned plasma material treatment processes. Electrodynamic properties enabling the simulation with a snowplow model have been determined experimentally. The model provides a relation between the plasma movement and the electrodynamic properties. Results of the model are current and voltage histories but also statements on the kinetic energy of the plasma. Based on this calorimeters were designed, manufactured and integrated using adequate measurement technology, e.g. fast thermocouples and an infrared camera allowing for the determination of the temporal and spatial temperature histories on the calorimeters. A thermal analysis model was developed and applied to the calorimeter and compared with the measurements. Hence, the thermal energy could be determined which consequently led to an efficiency of 12% for a load voltage of 12 kV and an ambient pressure of 10⁻⁵ mbar.     

Experimental and numerical study of an electrothermal pulsed plasma thruster for small satellites

A pulsed plasma thruster (PPT) with a propellant feeding mechanism was designed using two poly tetrafluoroethylene (PTFE) bars as propellants. The PPT was mounted on a thrust stand with a 1-m-long perpendicular pendulum which was developed for precise measurements of impulse bits. Initial thrust performance showed thrust-to-power ratio of 43–48 μN/W, specific impulse of 470–500 s and thrust efficiency of 10–12% with energy of 4.5–14.6 J. Ten thousand shots achieved a total impulse of approximately 3.6 Ns, and the PTFE bars were consumed approximately 2 mm in length. However, uneven receding of the PTFE surface was observed. In order to investigate physical phenomena in a whole system, an unsteady numerical simulation of initial discharge, generation of plasma, heat transfer to the PTFE, heat conduction inside the PTFE, ablation from the PTFE surface and acceleration of plasma was performed. The calculated results were used to explain the physical phenomena in the cavity, especially ablated mass of the PTFE.     

Phase-locked ions and foil transparency in the radiation pressure acceleration regime

The transverse expansion of a thin foil accelerated in the RPDA regime can be exploited in order to increase the ion energy and the acceleration efficiency at the expense of decreasing the number of accelerated particles. In the relativistic regime, the ions become phase-locked with respect to the electromagnetic wave. The use of an optimal laser pulse shape makes it possible to keep the expanding foil opaque to the laser radiation. This provides a new approach in order to enhance the energy of laser accelerated ions significantly.     

High density plasma etching of amorphous CoZrNb films for thin film magnetic devices

Inductively coupled plasma reactive ion etching of CoZrNb magnetic thin films was studied using a TiN hard mask in a Cl₂/O₂/Ar gas mix. The etch rates of CoZrNb films and TiN hard mask gradually decreased with increasing Cl₂ or O₂ gas concentrations. When O₂ gas was added in the Cl₂/Ar gas mix, the etch rate of TiN hard mask was suppressed effectively so that the etch selectivity of CoZrNb film to TiN hard mask was enhanced. The addition of O₂ into the gas mix also led to the anisotropic etching of the CoZrNb films and it was confirmed by Auger electron spectroscopy that there were no redeposited materials on the sidewall of the etched films. Highly anisotropic etching of CoZrNb films was achieved at room temperature under the optimized etching conditions.     

Effects of RF bias power on electron energy distribution function and plasma uniformity in inductively coupled argon plasma

Changes in the plasma non-uniformity and the electron energy distribution function (EEDF) by increasing RF bias power were observed in inductively coupled plasma using spatially resolved radial EEDF measurements. As the bias power was increased at a fixed ICP power at a low gas pressure, The EEDF was evolved from a bi-Maxwellian to a Maxwellian distribution. The plasma density was decreased in all radial positions and thus plasma non-uniformity was slightly changed. However, strongly improved plasma spatial non-uniformity was observed at a high gas pressure with a decrease in the center-plasma density and an increase in the radial edge-plasma density. This result could be understood by combined effects of the ion acceleration loss and the non-uniform power deposition due to the RF bias power.     

Effects of erosion surface on near wall conductivity (NWC) in the Hall-type stationary plasma thrusters

The effect of erosion structure of the wall surface on near wall conductivity was studied in this paper. The electron dynamics process in the plasma was described by the test particle method where electrons were randomly injected into the wall. Monte Carlo method was used to calculate the model. The simulation results showed that the axial current density increased in the case of erosion. Furthermore, the current layers were observed and the layer thickness was approximately equal to the radical electron motion in a half of the electron Larmor period. Also, the current density increases as the erosion structure gradually growed.     

Effect of the electrode material on the atmospheric plasma conversion of NO in air mixtures

Non-thermal atmospheric pressure plasma is widely used for conversion of hazardous gases. Results from different laboratories confirm importance of energy non-equilibrium in the plasma where dominant energy carriers are electrons and a dominant chemistry is based on formation and interactions of radicals. Because of rather high electric fields required for generation and sustaining of air discharges at atmospheric pressure many plasma systems were found rather to create a lot of NO instead of removing it. A widely supported way to clean NO and NO₂ from air mixtures is a plasma assisted catalytic reduction where the cold plasma is combined with the solid-state catalyst. In an ideal case the plasma acts as an oxidation catalyst where an atomic oxygen from air oxidizes NO to NO₂ and the solid-state catalysts are then capable to convert all NO₂ to N₂ and O₂. In most cases it is also necessary to involve auxiliary gases, e.g., propylene, to make the process efficient enough. This work introduces an original cold plasma system based on atmospheric hollow cathodes generated by a nanopulse DC power with controllable voltage and pulse frequency. The system was optimized in both the geometry and the applied power. However, the material of electrodes was found to be the most important factor affecting the plasma performance and consequently the chemical kinetics. A 100% conversion of NO to NO₂ was achieved with a graphite electrode, without using any auxiliary gas and without catalyst. Plasma performance and conversion efficiency are compared for several electrode materials.     

Synthesis of nitrogen-rich carbon nitride thin films via magnetic field-assisted inductively coupled plasma sputtering

Carbon nitride (CN_x) thin films were synthesized by magnetic field-assisted inductively coupled plasma (ICP) sputtering. The electron density, electron temperature and optical emission intensity of the plasma state were significantly changed by varying the external magnetic field applied. The CN_x thin film with the highest nitrogen content (N/C=1.16) was obtained when the electron density was at its highest and the electron temperature at its lowest. Additionally, the optical emission from atomic nitrogen was the strongest under the same condition.     

High temperature mechanical properties and surface fatigue behavior improving of steel alloy via laser shock peening

Laser shock peening was carried out to reveal the effects on ASTM: 410L 00C_r12 microstructures and fatigue resistance in the temperature range 25–600 °C. The new conception of pinning effect was proposed to explain the improvements at the high temperature. Residual stress was measured by X-ray diffraction with sin²ψ method, a high temperature extensometer was utilized to measure the strain and control the strain signal. The grain and precipitated phase evolutionary process were observed by scanning electron microscopy. These results show that a deep layer of compressive residual stress is developed by laser shock peening, and ultimately the isothermal stress-controlled fatigue behavior is enhanced significantly. The formation of high density dislocation structure and the pinning effect at the high temperature, which induces a stronger surface, lower residual stress relaxation and more stable dislocation arrangement. The results have profound guiding significance for fatigue strengthening mechanism of components at the elevated temperature.     

Time and energy resolved ion mass spectroscopy studies of the ion flux during high power pulsed magnetron sputtering of Cr in Ar and Ar/N2 atmospheres

Mass spectroscopy was used to analyze the energy and composition of the ion flux during high power pulsed magnetron sputtering (HIPIMS/HPPMS) of a Cr target in an industrial deposition system. The ion energy distribution functions were recorded in the time-averaged and time-resolved mode for Ar⁺, Ar²⁺, Cr⁺, Cr²⁺, N₂⁺ and N⁺ ions. In the metallic mode the dependence on pulse energy (equivalent of peak target current) was studied. In the case of reactive sputtering in an Ar/N₂ atmosphere, variations in ion flux composition were investigated for varying N₂-to-Ar flow ratio at constant pressure and HIPIMS power settings. The number of doubly charged Cr ions is found to increase linearly with increasing pulse energy. An intense flux of energetic N⁺ ions was observed during deposition in the reactive mode. The time evolution of ion flux composition is analyzed in detail and related to the film growth process. The ionization of working gas mixture is hampered during the most energetic phase of discharge by a high flux of sputter-ejected species entering the plasma, causing both gas rarefaction and quenching of the electron energy distribution function. It is suggested that the properties (composition and energy) of the ion flux incident on the substrate can be intentionally adjusted not only by varying the pulse energy (discharge peak current), but also by taking advantage of the observed time variations in the composition of ion flux.     

Plasma wake-field acceleration of high energies: Physics and perspectives

Requirements of high-energy physics impose some restrictions on parameters of future plasma-based accelerators; these restrictions are analyzed for the accelerator driven by particle beams. With two-dimensional simulations of driver dynamics it is shown that properly prepared 10 GeV electron driver can accelerate up to 5 × 10⁹ particles with energy gain of about 10 GeV in 10 m long plasma section and with energy spread less than 5%. On the basis of these simulations, the draft concept of high-gradient linear collider of TeV-range energy is briefly considered.

To prove the possibility of precise driver handling and to test simulation results, the experiment is under preparation at Budker Institute, Novosibirsk. The modulated electron beam (0.8 GeV) is expected to excite the electric field up to 0.5 GeV/m at the distance of about 1 m.     

Effect of solute mixing in the liquid propellant of a pulsed plasma thruster

Sodium chloride or ammonia was dissolved in the water propellant of pulsed plasma thrusters to improve the performance. Pulsed plasma thrusters using liquid propellant utilize water as attractive alternative instead of Teflon. Water propellant enables in controlling propellant mass flow and leads to high specific impulse. However, liquid propellant pulsed plasma thrusters have larger plasma resistance and lower thrust power ratio than the common Teflon propellant thruster. Here, sodium chloride and ammonia solution of water were examined to decrease that plasma resistance. As a result, emission lines attributed from the solute were observed using sodium chloride aqueous solution propellant, and a 5% reduction of the plasma resistance was shown, and the thrust to power was increased. However, ammonia aqueous solution decreased the thruster performance.     

Formation and expansion of the plasma column under electron beam-metal interaction

The process of expansion of low-energy electrons and ions produced from electron beam evaporation of a metal target in the technological vacuum chamber of an electron beam welding machine is studied. Analyzing the spatial distribution of the particle density that is measured, the radial collisionless plasma motion from the beam region is shown to be prevailing. A model of the free plasma expansion out of the cylindrical source is developed. The relative distribution of the electron density and the electrostatic potential obtained are compared with the experimental results. The knowledge of the plasma parameter distributions in the technological vacuum chamber can be used for remote diagnostics and control of the technology process.     

Influence of Ar plasma treatment on the wetting behavior of pharmaceutical powders

In this study, corn starch and ibuprofen are treated in Argon (Ar) plasma to enhance the wettability of the powders without using any additive or guest particle. The powder and pellets were exposed to the Ar plasma for different time intervals (5–20?min) at optimized pressure and voltage. While the morphological changes due to plasma exposure are captured by SEM, the AFM measurement shows the variations in surface roughness for both corn starch and ibuprofen powders. The XPS and surface energy results confirm that the surface groups change is dominating under initial exposure, but longer exposure creates more damaging effect than building the new active sites at the surface. The contact angle (CA) measurement shows that with increase in treatment time, the CA decrease from 48° to 33° for corn starch and 69° to 51° for ibuprofen. A comparison of experimentally obtained CA and that of theoretically calculated CA shows that there exists some difference. This may be attributed to the absence of surface group contribution in the Wenzel model. The present study shows that short exposure is sufficient enough to improve the wetting where the surface group plays a dominant role over surface roughness.     

Advanced research and development for plasma processing of polymers with combinatorial plasma-process analyzer

A plasma-process analyzer has been developed on the basis of combinatorial method, in which process examinations with continuous variations of plasma-process conditions can be carried out on a substrate holder with an inclined distribution of process parameters. Combinatorial plasma-process analyses have been demonstrated for examinations of plasma-polymer interactions in terms of etching characteristics and surface morphologies in order to show feasibility and effectiveness of the methodology as advanced research and development for next-generation plasma nano processes. The etching properties and surface morphologies have been investigated for polyethylene terephthalate (PET) films exposed to argon-oxygen mixture plasmas. The etching depth data obtained from three independent batches of the experiments showed universal and almost linear dependence with increasing product of (ion saturation current) × (exposure time); i.e. ion dose. Surface roughness of the polymer slightly increased with increasing ion dose, while the mean spacing after plasma exposure was found to decrease monotonically with increasing ion dose but was saturated at the level of approximately 250 nm.     

高功率脉冲磁控放电等离子体注入与沉积CrN薄膜研究

采用高功率脉冲磁控放电等离子体离子注入与沉积的方法在不锈钢基体上制备了高膜基结合力的CrN硬质薄膜,并研究了不同的Ar/N流量比对薄膜形貌、结构及性能的影响.采用扫描电子显微镜、X射线衍射对其表面形貌和结构进行分析,发现制备的薄膜表面光滑、致密,相结构单一,主要是CrN(200)相.对薄膜的结合力、硬度、弹性模量、耐磨...     

Characteristics of a low-cost cold atmospheric plasma and its application

Cold atmospheric plasma (CAP) can be generated and operated easily without using a vacuum device. However, its characteristics and applications are not yet completely clear. In this paper, a low-cost system, equipped with two aluminum housings with single- and dual-inlet cold atmospheric plasma sources (CAPS), was designed for investigating CAP characteristics. A commercial alternating current (AC) plasma power source was used in this study. Compressed air, oxygen, and nitrogen were used as the gas sources for measuring the temperature distribution empirically. The results showed that different gas-supplying methods affect the temperature distribution significantly. The temperature of the air plasma source is ~ 60 °C at the outlet and decreases slowly with the gap distance. Additionally, the emission optical spectrogram of CAPS is similar to that of high-temperature plasma. These CAPS show the potential applications of air plasma for metal surface modification and medical treatment for urticaria in the future.     

Mitigating the geometrical limitations of conventional sputtering by controlling the ion-to-neutral ratio during high power pulsed magnetron sputtering

High power pulsed magnetron sputtering has been used to grow thin chromium layers on substrates facing and orthogonal to the target. It is demonstrated that at low peak target current density, j_T < 0.6 A/cm² corresponding to a low ion-to-neutral flux ratio, films grown on substrates facing the target exhibit in-plane alignment. This is due to the rectangular shape of the target that yields an asymmetry in the off-normal flux of sputtered species. With increasing j_T the biaxial alignment degrades, as the major portion of the incoming flux (ions) can be effectively steered by the electric field of the substrate to remove asymmetry imposed by geometrical restrictions. Eventually, at j_T = 1.7 A/cm² a fiber texture is obtained. For films grown on substrates orthogonal to the target, the large column tilt characteristic for growth at low j_T, decreases with increasing ion content in the flux and almost disappears at the highest value of j_T. The latter indicates that material flux to the substrate is highly ionized so that deposition takes place along substrate normal despite the high nominal inclination angle. Thus, in the limit of high j_T the artifacts of conventional physical vapor deposition, resulting from the line-of-sight deposition, are effectively eliminated and the film growth proceeds more or less unaffected by the substrate orientation. Samples mounted orthogonally thus possess a similar texture, morphology, and topography as those facing the target.     

激光冲击强化技术的应用现状与发展

激光冲击强化是一种利用激光诱导等离子体冲击波来提高材料疲劳寿命的新型表面改性技术,具有强化效果显著、可控性强、适应性好等优点,对提高结构可靠性和部件疲劳强度、延长材料使用寿命具有重要作用。近年来,该技术受到了广泛重视,得到了快速发展。本文简要介绍了激光冲击强化技术的基本原理、特点与应用领域;总结了国内外激光冲击强化技术的发展状况与研究成果;并针对国内外激光冲击强化技术的现状,提出了一些现在需要解决的强化工艺问题;最后对激光冲击强化技术的应用前景进行了展望。

     

Modeling of the supersonic argon plasma jet at low gas pressure environment

Numerical simulation results are presented concerning the heat transfer and fluid flow within the supersonic argon plasma jet encountered in low pressure (or soft vacuum) plasma spraying (LPPS). The plasma parameters at the inlet section of the plasma jet are taken from our modeling results of the subsonic-to-supersonic d.c. arc plasma torch. The mach number, temperature and static pressure at the center of the plasma jet on the torch exit section are 2.8, 13 200 K and 6000 Pa, respectively, whereas the environment (i.e. vacuum chamber) pressure is 0.1 atm. Those parameters are typical for LPPS. The plasma jet is assumed to be axi-symmetrical and in local thermodynamic equilibrium state. The All-Speed SIMPLE algorithm is coupled with the FAST-2D program to simulate the whole plasma jet containing both the supersonic and subsonic flow regions. Modeling results clearly show that there exist several successive temperature, velocity and static wave crests and troughs. The fluctuation magnitudes of those parameters reduce rapidly in the flow direction, along with the flow transformation from the supersonic flow regime into the subsonic flow regime. The existence of a series of compression and expansion waves in the region near the torch nozzle exit shows clearly the over-expanded characteristics of the supersonic plasma flow.