Influence of the position relationship between gas–liquid interface and laser focus on plasma evolution characteristics in jet LIBS technology

In order to understand the characteristics of breakdown process,plasma evolution and spectral emission in liquid jets laser-induced breakdown spectroscopy methods under the influence of the position variation between laser focus and gas–liquid interface,this work takes the plasma generated by laser-induced liquid jets as the object of study and discusses the changes in the spatial and temporal evolution characteristics and spectral radiation of the plasma when the position parameters between the laser focal point and the gas–liquid interface are different.The initial breakdown position is always between the front interface and the focus when the laser focus moves along the vertical direction of the interface,forming a phenomenon like’interface effect’.The relationship between laser pulse energy and breakdown probability exhibits a law similar to a‘hysteresis curve’in the study of breakdown threshold.In this work,plasma with smaller size,rounder shape,stronger radiation,higher temperature,and higher density can be produced when the focus position is in the liquid column 0.2 mm away from the front interface.Simultaneously,the spectral signal intensity and signal-to-back ratio of the characteristic peaks of target elements in water reach maximum values,and the spectral signal becomes more stable(relative standard deviation value reaches 2%).The Ca element’s ion radiation at 393.366 nm and atomic radiation at 422.673 nm are studied using narrow-band filtering imaging and time-space resolution spectroscopy.The findings demonstrate that the laws of ion and atomic radiation are not perfectly consistent in space and time.

Influences of the cold atmospheric plasma jet treatment on the properties of the demineralized dentin surfaces

Improvement of the bonding strength and durability between the dentin surface and the composite resin is a challenging job in dentistry. In this paper, a radio-frequency atmosphericpressure glow discharge(RF-APGD) plasma jet is employed for the treatment of the acid-etched dentin surfaces used for the composite restoration. The properties of the plasma treated dentin surfaces and the resin–dentin interfaces are analyzed using the x-ray photoemission spectroscopy, contact angle goniometer, scanning electron microscope and microtensile tester.The experimental results show that, due to the abundant chemically reactive species existing in the RF-APGD plasma jet under a stable and low energy input operating mode, the contact angle of the plasma-treated dentin surfaces decreases to a stable level with the increase of the atomic percentage of oxygen in the specimens; the formation of the long resin tags in the scattered clusters and the hybrid layers at the resin–dentin interfaces significantly improve the bonding strength and durability. These results indicate that the RF-APGD plasma jet is an effective tool for modifying the chemical properties of the dentin surfaces, and for improving the immediate bonding strength and the durability of the resin-dentin bonding in dentistry.

Unraveling the surface chemistry processes in lithiated and boronized plasma material interfaces under extreme conditions

The review of recent theoretical and experimental research on the complex surface chemistry processes that evolve from low-Z materialconditioning on plasma-facing materials under extreme fusion plasma conditions is presented. A combination of multi-scale computationalphysics and chemistry modeling with real-time diagnosis of the plasma-material interface in tokamak fusion plasma edge is complemented byex-vessel in-situ single-effect experimental facilities to unravel the evolving characteristics of low-Z components under irradiation. Effects of thelithium and boron coatings at carbon surfaces to the retention of deuterium and chemical sputtering of the plasma-facing surfaces are discussedin detail. The critical role of oxygen in the surface chemistry during hydrogen-fuel irradiation is found to drive the kinetics and dynamics ofthese surfaces as they interact with fusion edge plasma that ultimately could have profound effects on fusion plasma confinement behavior.Computational studies also extend in spatio-temporal scales not accessible by empirical means and therefore open the opportunity for a strategicapproach at irradiation surface science studies that combined these powerful computational tools with in-vessel and ex-vessel in-situ diagnostics.

Comparison of Damages on Tungsten Surface Exposed to Noble Gas Plasmas

Tungsten was exposed to pure Ar or Ne plasmas over 1550 K at several incident ion energies. Even under the irradiation condition that the tungsten nanostructure is formed by He plasma irradiation, holes/bubbles and fiberform nanostructures were not formed on the surface by exposure to Ar or Ne plasmas. In addition, the results from energy dispersive X-ray spectroscopy supported the facts that Ar and Ne did not remain in the sample. We will discuss the reason for the differences in the damage to the tungsten surface exposed to noble gas plasmas.

A Molecular Dynamics Study on the Dust-Plasma/Wall Interactions in the EAST Tokamak

The interactions between the W nano-dust and deuterium plasma at different lo- cations of the EAST tokamak are simulated using a molecular dynamics code. It is shown that nano-dust particles, with the radius, Rd, ~5 nm, can exist for at least several nano-seconds under the interactions from the ions without being ablated in some specific places of the tokamak edge plasma, while those with Rd ≥~25 nm may be ablated if the plasma temperature T~ 50 eV and density n^10^19 m-3. In addition, the collisions of tungsten nano-dust grains with a tungsten wall at 100 m/s or I000 m/s impinging speeds are simulated. It is demonstrated that the dust will stick to the wall, and the collision will not cause substantial damage to the wall, but it may be able to cause partial destruction of the dust grains themselves depending on their incident speeds.

Predictive Modeling of EAST Divertor Target Power Loading with Varying Chemical Sputtering Conditions

The Experimental Advanced Superconducting Tokamak （EAST） is being built in China to achieve high power and long pulse operation for studies of reactor-relevant issues under steady-state conditions. A major concern for EAST is the power handling capability of the divertor target plates, which is a critical issue for future high-powered steady-state tokamaks, such as ITER. Detailed modeling using B2/EIRENE code package and the most recent chemical sputtering data shows that the presence of strong chemical sputtering at the main chamber wall leads to strong carbon radiation in the periphery of the confined plasma, significantly reducing the heat fluxes to the target plates and facilitating plasma detachment at a lower density desired for lower hybrid current drive in EAST, with only a slight increase in Zeff at the edge. The target heat load can be further reduced by operating with a double-null divertor configuration, which also leads to a significant reduction in the edge Zeff. However, the code predicts that the double-null operation would result in a strong divertor asymmetry in target power loading, favoring the outside targets.

Fusion Material Studies Relating to Safety in Russia in 2002

The paper is a summary of Russian material studies performed in frames of activi-ties aiming at substantiation of safety of the International Thermonuclear Experimental Reactor(ITER) after 2001. Subthreshold sputtering of tungsten by 5 eV deuterons was revealed at temper-atures above 1150℃. Mechanism of globular films formation was further studied. Computationsof tritium permeation into vacuum vessel coolant confirmed the acceptability of vacuum vesselcooling system for removal of the decay heat. The most dangerous accident with high-currentare in toroidal superconducting magnets able to burn out a bore up to 0.6 m in diameter in thecryostat vessel was determined. Radiochemical reprocessing of V-Cr-Ti alloy and its purificationfrom activation products down to a contact dose rate of ～ 10 μSv/h was developed.

MD simulation on the interactions between CH＿2 groups and the(001) surface of tungsten

This work studies the angle dependence of the interactions between impinging CH2 particles of 150 eV with the tungsten surface. The simulations show that the carbon atoms are much more easily bonded to the tungsten atoms than hydrogen atoms, though a few of the latter can also penetrate into the tungsten material. When the incidence angle is greater than 75%, the incident CH2 particles are reflected without break-ups. Below this angle, a W-C layer of about 0.5 nm is formed with another C, H-rich layer depositing on top of it. The molecular dynamics （MD） approach has proved to be a powerful tool to solve the structural problems at atomic length scale of various materials. Some of its possible applications to the railway track materials have also been discussed.

Tribo-Mechanical and Electrochemical Properties of Carbonitrided 316 Austenitic Stainless Steel by rf Plasma for Biomedical Applications

AISI 316 austenitic stainless steel was carbonitrided using rf plasma with purpose of using low-cost orthopedic implant materials in biomedical applications besides the manufacturing requests. The plasma treatment process was accomplished at low working gas pressure of 0.075 mbar in nitrogen-acetylene gaseous mixture. The plasma-processing time was fixed at 10 min while the plasma-processing power was varied from 450 to 650 watt. The effect of plasma treatment power on the structure, tribological, mechanical, electrochemical and biocompatibility of AISI 316 has been investigated. The structural results demonstrated the formation of nitrogen and carbon solid solutions, chromium nitride, iron carbide and iron nitride phases in the treated samples. The microhardness of the treated layer increases with increasing the processing power to reach a maximum value of approximately 1300 HV0.1 at 600 W which represents more than 6-folds increase in microhardness in comparison with the untreated matrix. The wear and corrosion resistance of the treated AISI 316 were enhanced compared to the untreated one. The friction coefficient was reduced from nearly 0.5 for the untreated substrate to nearly 0.3 for the carbonitrided sample. The surface energy and wettability of the carbonitrided samples were augmented as the plasma-processing power increased. Furthermore, the numbers of grown mesenchymal stem cells are higher for carbonitrided samples compared to the untreated one. The formation of nitrogen and carbon solid solution, chromium nitride, iron nitride and iron carbide hard phases after carbonitriding process is responsible for achieving good mechanical, tribological, biocompatibility and electrochemical properties for AISI 316 alloys.

A review of late-stage tungsten fuzz growth

Tungsten will be used as the plasma-facing divertor material in the International Thermonuclear Experimental Reactor(ITER)fusion reactor.Under high temperatures and high ion fluxes,a‘fuzz’nanostructure forms on the tungsten surface with dramatically different properties and could contaminate the plasma.Although simulations and experimental observations have provided understanding of the initial fuzz formation process,there is debate over whether tungsten or helium migration is rate-limiting during late-stage growth,and the mechanisms by which tungsten and helium migrations occur.Here,the proposed mechanisms are considered in turn.It is concluded that tungsten migration occurs by adatom diffusion along the fuzz surface.Continual helium migration through the porous fuzz to the tungsten bulk is also required for fuzz growth,for continued bubble growth and rupture.Helium likely migrates due to ballistic penetration,although diffusion may contribute.It is difficult to determine the limiting process,which may switch from helium penetration to tungsten adatom diffusion above a threshold flux.Areas for further research to clarify the mechanisms are then considered.A greater understanding of the fuzz formation mechanism is key to the successful design of plasma-facing tungsten components,and may have applications in forming porous tungsten catalysts.