Helium Plasma Research Articles

Dynamic features of the electron drift and electron properties in a HiPIMS discharge

Information on the evolution of electron properties during high-power impulse magnetron sputtering (HiPIMS) operation of planar magnetrons enables the study of fundamental physical processes. In this work, incoherent Thomson scattering is implemented for the non-invasive, spatiotemporally-resolved characterization of electron properties and drifts in the HiPIMS regime of a planar magnetron. In the ionization region of argon and helium plasmas, the azimuthal electron drifts are directly measured perpendicular to the magnetic field and are found to evolve according to a changing balance of E × B and diamagnetic electron drifts, while radial electron drifts, measured parallel to the magnetic field, can be attributed to plasma expansion/contraction and centrifugal forces. The evolutions of electron density and temperature in the afterglow plasma phase show the existence of two time scales for the variation of plasma properties. These characterizations provide detailed information on electron properties and dynamics in regions of the magnetic trap ordinarily inaccessible to invasive diagnostics.

Evaluation of self-absorption effect of He I resonance line by measurement of forbidden emission in helium arc jet plasma

We have proposed a new method to estimate the optical escape factor (OEF) in high-density helium (He) plasma. Plasma with an electron temperature of ca. 3 eV and density of 5 × 1013 cm−3 was generated by a cascaded arc discharge and rapidly cooled by the introduction of additional He gas, which resulted in a transition from ionizing to recombining plasma. With an increase in the gas pressure, the plasma became optically thick, and the He I forbidden line (spin-exchange intercombination line, 1 1S-2 3P: 59.1 nm) with resonance lines were simultaneously observed using a vacuum ultraviolet spectrometer. Comparison of the intensity ratio of the He I 58.4 nm resonance line to the forbidden emission with those by determined from the collisional-radiative model considering the self-absorption enabled the successful estimation of the OEF. The OEF was decreased with the ambient He gas pressure and was 6.6 × 10−4 and 3.5 × 10−5 for He gas pressures of 1.59 and 20.22 Pa, respectively.

Effects of plasma-induced grafting modification on the adhesive strength and mechanical properties of fiber posts.

This study aimed to assess the effects of plasma grafting modification on the micro-push-out adhesive strength and mechanical properties of fiber posts and to assess the stability of these treatment effects. Glass-fiber posts were divided into four groups based on the treatment methods used, as follows: (1) Group NT: no treatment; (2) Group PT: Helium (He) plasma treatment; (3) Group PIG: He-plasma-induced post-irradiation grafting; and (4) Group SIG: He-plasma-induced syn-irradiation grafting. The treated fiber posts were bonded using self-adhesive resin cement exposure to air for 0, 1, 6 or 12 hours separately after surface treatment. Micro-push-out adhesive strength, flexural modulus, and flexural strength were measured. Plasma treatment, post-irradiation grafting, and syn-irradiation grafting improved adhesive strength at the 0-hours level. However, the improved adhesive strength disappeared in group PT after exposure for one or more hours. In group PIG, the adhesive strength after 1-hour exposure was 20.5% lower than that of 0-hour exposure (adhesive immediately after treatment), and no statistically significant differences in adhesive strength were observed between the 1, 6, and 12-hour exposure. In group SIG, no statistically significant differences in adhesive strength were observed among the 0, 1, and 6-hour exposure. Although the adhesive strength was 23% lower at the 12-hour exposure than that of 0-hour exposure in group SIG, the adhesive strength of fiber posts received syn-irradiation grafting still presented the best adhesive strength compared with the other treatment methods. The three-point flexural modulus and strength remained unaffected by the treatment methods used. Plasma-induced syn-irradiation grafting provided the ideal improvement and stability in adhesive strength in fiber posts. In addition, plasma-induced grafting modification successfully overcame the surface aging effect caused by plasma treatment alone without affecting the bulk mechanical properties of fiber posts.

Optical emission spectroscopy diagnosis of low-pressure microwave discharge helium plasma based on collisional-radiative model

An inverse model based on a low-pressure helium collisional-radiative (CR) model was developed. Parts of the rate coefficients were recalculated from the cross-sections. The dominant processes in the revised model were extracted to simplify the calculation and to develop an inverse model. The model can calculate the electron density and temperature of low-pressure helium plasma by inputting the population densities of levels and which can be measured by optical emission spectroscopy (OES) measurement in the visible wavelength range. The results demonstrate that the electron temperature obtained by the model is extremely close to the original value in the CR model. The output values of the electron density were of the same order and magnitude as the input values. The electron density and temperature measured by OES measurement in the experiment using the developed inverse model are consistent with the results measured by the probe method.

Effect of Fe11+ ion combined with helium and deuterium plasmas irradiation on the transient thermal shock behaviors of pure and potassium-doped tungsten

The large-scale warm-rolled pure tungsten (PW) and hot-rolled potassium-doped tungsten (W-K) plates were prepared by powder metallurgy technology to meet the demands of International Thermonuclear Experimental Reactor (ITER) in engineering application. In order to evaluate the performance of plasma-facing materials (PFM) in a more realistic service environment as much as possible, the influence of combined irradiation effects of high-energy iron (Fe) ion, deuterium (D) and helium (He) plasmas on the transient thermal shock behavior in the PW and W-K was investigated. The results indicate that the PW can generate more vacancy- and dislocation-type defects than that of W-K after irradiation. The cracking thresholds for un-irradiated PW and W-K are >0.66 and 0.44–0.55 GW/m2, respectively. However, the cracking threshold for both samples decreased to 0.22–0.33 GW/m2 after combined irradiation, indicating that the combined irradiation can significantly reduce the thermal shock resistance of PW and W-K. In addition, according to the statistical results of the crack density and width on the surface of irradiated samples, irradiated W-K owns better thermal shock resistance than irradiated PW under the same thermal shock condition. These results were also verified by the determination of residual stresses. Besides, the relationship between the evolution of irradiation-induced defects and K bubbles is also discussed in detail. These studies can provide an important reference for the identification of future PFM and numerical simulation under combined irradiation plus high thermal load.

Helium-induced morphology evolution in tungsten under thermal treatment

Surface and near-surface morphology evolution of helium-irradiated tungsten due to thermal cycling up to the ITER-relevant temperature of 1350 K was studied using electron microscopy and positron annihilation spectroscopy techniques. Holes at the surface and bubbles in the near-surface of recrystallized polycrystalline tungsten samples were created by 75 eV helium plasma irradiation with the fluence of 3 × 1023 He m−2 at the surface temperature of 1073 K. Subsequent annealing experiments were combined with a detailed electron microscopy analysis to investigate the shape and density changes of holes and helium bubbles with respect to grain orientation. We show that the initially circular holes and round bubbles became faceted upon heating with 1 K s−1 ramp up to 870 K. Annealing cycles up to 1350 K induced resulted in bubbles removal in the first 5 nm below the surface and surface smoothing. Electron energy loss spectroscopy measurements allowed estimation of helium gas density and pressure inside bubbles. Positron annihilation spectroscopy allowed to investigate the nature of defects and their evolution in the helium-irradiated tungsten with thermal cycling.

(Invited) Role of Trace Impurities in Microwave-Excited Atmospheric Pressure Plasmas: Application to 3D Nano-Printing

Because rare gases like helium, neon or argon have first-excited levels at high energy (respectively 19.8, 16,62 and 11.5 eV), impurities with much lower energy levels are more easily excited. Thus, if their concentrations are high enough (hundreds of ppm or more), these impurities drive the behavior of the discharge, leading for instance to emission spectra where no lines of rare gas are visible.Modelling these discharges is a challenge because of the complexity of the chemical behavior of these trace elements. In microwave discharges, several problems arise among which one finds the role of electron-electron collisions, super-elastic collisions and the possible non-local heating of the electron energy distribution function. Based on a collisional–radiative model proposed to describe the behavior of a helium plasma sustained in a resonant cavity at atmospheric pressure, we will discussed these different aspects. The role of impurities will be described next. The level of dry air used as impurity typically varies from 0 to 1500 ppm, which corresponds to the most commonly encountered range in atmospheric pressure discharge experiments. Results clearly show that the plasma chemistry and consequently the discharge evolution is highly affected by the concentration level of impurities (from tens of thousands of ppm) in the mixture.Next, the use of these plasmas for 3D-nanoprinting will be detailed. This technology, named DISCRIBE, might be used for drawing electrical contacts, building pillars as part of metasurfaces or even creating 3D parts of clocks in watchmaking industry. A plasma-assisted printer, operated at atmospheric pressure, was developed to write patterns with sub-micrometric resolution. A reactive gas is injected through a needle within an atmospheric argon microwave plasma. Forming sub-micrometric needles (down to 100 nm in diameter) by pulling glass capillaries heated locally by laser is simple as this technology is readily available in the medical field. By plasma-enhanced chemical vapor deposition, a coating is formed on a substrate (conveniently a silicon wafer, but any other substrate can be used) with a spot size that almost corresponds to the needle aperture. Then, when the substrate moves, a pattern is formed accordingly. The writing speed is dependent on the dynamic deposition rate, which can be as high as 2 nm·m·s-1.In the specific case of acetylene used as precursor, the shape of the hydrogenated-carbon spot is strongly dependent on the amount of oxygen-containing impurities present in the microwave argon plasma. The influence of the plasma composition and the local hydrodynamic flow pattern are essential to control the deposition conditions and these aspects will be discussed. In particular, the distance between the needle exit and the substrate turns out to be a key parameter in the deposition process. DISCRIBE offers the advantage of all PECVD processes: it allows deposition at atmospheric pressure and reduced temperature of multi-materials (semiconductors, metals, ceramics, plasma polymers) ony any kind of substrate.

Erosion of Fuzz Layers Formed in Steady-State Plasma Discharge

The erosion of nanostructured tungsten and titanium by high-heat plasma flux, laser, and arcing is investigated. To fabricate nanostructural fuzz layers and hierarchical granularity on the surfaces, samples were exposed to helium plasma in the steady-state plasma device PLM-M, which is a linear plasma trap of an eight-pole multicusp magnetic field with parameters similar to the scrape-off layer and divertor plasma in a tokamak. Arcing ignited with a Nd:YAG laser pulse on the target fuzzy surface in the helium plasma resulted in the melting of fibers and the creation of craters of several microns in depth and several tens of microns in diameter.

Carrier-envelope phase controlled dynamics of relativistic electron beams in a laser-wakefield accelerator

In laser-wakefield acceleration, an ultra-intense laser pulse is focused into an underdense plasma in order to accelerate electrons to relativistic velocities. In most cases, the pulses consist of multiple optical cycles and the interaction is well described in the framework of the ponderomotive force where only the envelope of the laser has to be considered. But when using single-cycle pulses, the ponderomotive approximation breaks down, and the actual waveform of the laser has to be taken into account. In this paper, we use near-single cycle laser pulses to drive a laser-wakefield accelerator. We observe variations of the electron beam pointing on the order of 10 mrad in the polarisation direction, as well as 30% variations of the beam charge, locked to the value of the controlled laser carrier-envelope phase, in both nitrogen and helium plasma. Those findings are explained through particle-in-cell simulations indicating that low-emittance, ultra-short electron bunches are periodically injected off-axis by the transversally oscillating bubble associated with the slipping carrier-envelope phase.

Effects from the Target Plate Geometry on Fluctuations of Helium Plasma in the Linear Divertor Simulator Magnum-PSI

Effects from the Target Plate Geometry on Fluctuations of Helium Plasma in the Linear Divertor Simulator Magnum-PSI

Implementation of digital holographic interferometry for pulsed plasma studies

The study of low-temperature plasma generated in pulsed mode at atmospheric pressure was carried out. The purpose of the presented research is to develop a method of digital holographic interferometry for registration and evaluation of parameters of low-temperature plasma at atmospheric pressure in pulsed mode. This type of plasma is currently applied in medicine and biology. Thus, there is a need to control the exposure dose and plasma environment formation regimes. As plasma parameters, it can be considered its electron concentration which can be calculated through the estimation of the refractive index of plasma pulse in relation to unperturbed state. The plasma pulses were activated in Helium. The plasma pulse frequency was 5 kHz and its duration was 750 ns. During an investigation a laboratory set-up for recording holographic images of plasma pulses was developed. Holograms are acquired on a digital camera and a pulsed laser INNOLAS SpitLight Hybrid II at a wavelength of 532 nm with pulse duration of 10 ns is used as a source of coherent radiation. In order to record plasma pulses, the laser, plasma generator and digital camera were strictly synchronized to each other. During the experiment, a series of about 500 holograms were acquired, and the reconstruction of the phase of the object field was calculated. Analysis of the sequence of holograms allowed calculations of phase difference (interferograms) related to the refractive index of low-temperature pulsed plasma in Helium. It is known that low-temperature plasma leads to low phase delay which forms low phase contrast of the evaluated interferograms. For this purpose, we carried out preliminary experiments with plasma-arc that has similar temporary parameters, however, with a higher phase contrast of the interferograms. The paper presents experimental results obtained by studying the phase contrast of the refractive index of pure Helium, plasma-arc and plasma pulses in Helium. Thus, the effectiveness of both the experimental set-up and the method to evaluate the interferograms related to the refractive index of the plasma pulse was verified. The data obtained can then be used to estimate the electron concentration of the plasma. However, it needs to increase the sensitivity of the method in order to enhance phase contrast. Increase of sensitivity can be done by means of extension of the spectral range, for example, toward to infra-red.

Modeling of the ion in cold helium plasma: dynamics of collisions and cross-sections

A detailed dynamical study is presented for /He collisions running in the electronic ground state of the collision complex. Hybrid, quantum–classical dynamical calculations have been performed considering a broad range of collision energies (E coll = 0.01–100 eV) and various initial rotational–vibrational excitations of the ion. Both non-reactive and reactive ( collision-induced dissociation) cross-sections have been calculated with the momentum-transfer approximation applied to the former ones. A thorough comparison with pseudo-experimental data obtained from mobility measurements reported in the literature via an inverse-method approach has been performed and the effect of the rotational alignment of the ion on calculated cross-sections has been assessed and analyzed.

Splitting of ionization waves at the edge of a dielectric plate oriented at grazing angles between the helium plasma jet and the plate

In this paper, we discuss the results from the two-dimensional computational investigation of the effects of a helium plasma jet interacting with a dielectric plate at a floating potential. The plate is oriented at grazing angles formed between the plasma plume and the plate. We show that for angles α > 70° the jet plasma envelopes the plate and the splitting of an ionization wave (IW) at its edge is observed. As a result, one of the IWs propagates at some elevation over the upper surface of the plate. The other IW slides along the bottom surface as a surface discharge. This effect is partially attributed to the electric field direction and buoyancy forces which are especially important for the light helium channel surrounded by air. As such, the treatment of the top and bottom surfaces is not identical even for the horizontally oriented plate (α = 90°). We discuss the production and delivery to the plate surfaces the charged and neutral species important for applications. For the conditions considered, both sides of the plate are treated simultaneously. We also show that the buoyancy forces affect the reactive oxygen and nitrogen species distributions. For the helium jet which bends upward due to these forces, the best results for simultaneous treatment of thin plates can be achieved at grazing angles α = 80°–83°, but not at the angle of 90°. The choice of these particular incidence angles can provide better results. Locating the plate horizontally in parallel to the plasma jet axis can be dis-advantageous.

Investigation of particle exhaust from EAST divertor

Particle control is one of the key issues for steady-state tokamak operation. The density decay time is used to evaluate the capability of the divertor to exhaust particle with all fueling turn off. Experiments of the EAST divertor's ability to exhaust helium particles were also performed during the EAST helium plasma operation. The experimental results show that the EAST divertor is less capable of exhaust helium particles than deuterium particles. And similar to deuterium, the helium particle exhaust performance is sensitivity on the different strike point location. During 2021 experiment campaign, a new lower tungsten divertor has been developed and installed on EAST, the particle exhaust capacity of the new divertor has also been investigated during this campaign.

Influence of air and water vapor on EEDF, plasma parameters, and the main RONS in atmospheric pressure low temperature helium plasmas: Global model approach

A zero-dimensional global model is developed with an aim to study the influence of air and water vapor on an electron energy distribution function, electron density, and temperature as well as on the chemical composition of atmospheric pressure helium plasmas, with special focus on the main reactive oxygen and nitrogen species important for applications of low temperature plasmas. The main channels for the production and consumption of electrons and electron energy gain and loss are examined by a global model based on a parametric study with variations of the mole fractions of air and water vapor. The calculations are done for 100, 1000, and 10 000 ppm of air in plasma, and for each of these values, the content of water vapor is taken to be 100, 1000, 2000, …, to 10 000 ppm. The variations of the most important production and consumption processes for reactive oxygen and nitrogen species are analyzed in detail. According to the results presented in this paper, water vapor highly contributes to electronegativity of the plasma through pronounced attachment of electrons, which then leads to an increase in electron temperature. For high water vapor content, vibrational excitations of water molecules are one of the main electron energy loss processes, leading to a further effect on the plasma composition. Water decreases concentrations of atomic oxygen, nitrogen, and ozone, while there is an increase in nitrogen oxide, OH, H2O2, and HO2 concentrations. Cross-sectional data for electron scattering on ground and excited state neutrals are mainly taken from the Quantemol-DB database.

Solid-state modification of tapioca starch using atmospheric nonthermal dielectric barrier discharge argon and helium plasma

Starch modification utilizes a substantial amount of water. This research aimed to investigate the effect of nonthermal dielectric barrier discharge (DBD) argon and helium plasma on tapioca starch's physicochemical properties and techno-functionality. Dry starch samples were treated with plasma generated from argon and helium gases at different voltage levels (10 and 15 kV) for 5, 10, and 15 min. The treatment did not destroy the starch’s crystalline structure, as evidenced by the birefringence under polarized light. Scanning electron micrographs showed dented starch granules. A marginal decrease in pH value of all treated samples was noted. A reduction in amylose and amylopectin molecular weight and increased reducing sugars were observed in the treated samples. Fourier transform infrared spectroscopy analysis showed a slight change in the 1047/1022 cm−1 absorbance ratio, indicating a change in short-range ordered degree of starch, while X-ray diffractometry showed a decrease in crystallinity. A significant reduction in peak viscosity, breakdown, and setback was observed. The gelatinized plasma-treated starch dispersion showed less shear-thinning and less thixotropic with increasing voltage and treatment times. This research proved that modification of starch in the solid-state using nonthermal plasma is feasible.

Multichannel probe measurements in a helium plasma in a hollow cathode discharge

A multichannel scheme and a technique for measuring the parameters of electrons in a discharge plasma in a hollow cathode with several Langmuir probes in different regions of the plasma simultaneously are developed. In the experimentally implemented system, the current-voltage characteristics (IVCs) of the probes are formed by combinations of noise and periodic voltage waveforms, which ensures a high dynamic range and accuracy of the measurement results. In different regions of the plasma space, the electron energy distribution functions (EEDFs), average energies <E>, plasma potentials Up, and electron concentrations Ne are determined simultaneously. The capabilities of the system are demonstrated for various probe locations. It is shown that in order to obtain consistent data, special processing (cleaning) of the probes is required, the states of which change with time. Methods of electronic and ionic cleaning and their combinations have been studied. If the probes are located in a plane equidistant from the cathode and anode at distances between them comparable to the transverse dimensions of the plasma, the EEDF shape and the Ne value are approximately preserved, but the potential value Up changes.

Cold Atmospheric Plasma Jet Treatment Improves Human Keratinocyte Migration and Wound Closure Capacity without Causing Cellular Oxidative Stress.

Cold Atmospheric Plasma (CAP) is an emerging technology with great potential for biomedical applications such as sterilizing equipment and antitumor strategies. CAP has also been shown to improve skin wound healing in vivo, but the biological mechanisms involved are not well known. Our study assessed a possible effect of a direct helium jet CAP treatment on keratinocytes, in both the immortalized N/TERT-1 human cell line and primary keratinocytes obtained from human skin samples. The cells were covered with 200 µL of phosphate buffered saline and exposed to the helium plasma jet for 10–120 s. In our experimental conditions, micromolar concentrations of hydrogen peroxide, nitrite and nitrate were produced. We showed that long-time CAP treatments (≥60 s) were cytotoxic, reduced keratinocyte migration, upregulated the expression of heat shock protein 27 (HSP27) and induced oxidative cell stress. In contrast, short-term CAP treatments (<60 s) were not cytotoxic, did not affect keratinocyte proliferation and differentiation, and did not induce any changes in mitochondria, but they did accelerate wound closure in vitro by improving keratinocyte migration. In conclusion, these results suggest that helium-based CAP treatments improve wound healing by stimulating keratinocyte migration. The study confirms that CAP could be a novel therapeutic method to treat recalcitrant wounds.

Efficient vibrational excitation of molecular nitrogen in low-pressure plasma with ultralow electron temperature

We investigated the vibrational temperature of molecular nitrogen in the downstream of helicon-wave excited helium and argon-based plasmas. It was confirmed by optical emission spectroscopy that the major part of the helium plasma was at a recombining state and it had an ultralow electron temperature of approximately 0.1 eV. In spite of the ultralow electron temperature, the vibrational temperature of molecular nitrogen, which was added into the helium plasma, was higher than that in the argon-based plasma at an ionizing state with an electron temperature of 1.7 eV. According to the relationship between the rate coefficient of electron impact vibrational excitation and the electron temperature, the higher vibrational temperature in the helium plasma is not attributable to the more efficient vibrational excitation. Therefore, the higher vibrational temperature is owing to the less efficient destruction of vibrational excited states in the helium plasma with the ultralow electron temperature.

Molecular dynamics simulations of cascade overlap with Void/Helium bubble

When tungsten (W) is used as a plasma-facing material in fusion reactors, it will suffer irradiation from the high-energy neutron and high-flux low-energy helium (He) plasma, affording voids and He bubbles with varying helium-to-vacancy ratios (He/V). With the accumulation of irradiation dose, an energetic neutron is very likely to trigger collision cascades close to the pre-existing voids/He bubbles. Therefore, in this study, we ran systematic molecular dynamics simulations of the cascades that overlapped with voids/He bubbles in W. We investigated the relationship between the evolution of the stress field and defect formation. Cascades overlapping with the bubbles with He/V values between 1 and 2 produced the least defects; alternatively, other bubbles could facilitate defect formation. Furthermore, two mechanisms for generating dislocation loops, the cascade-destruction and the over-pressured bubble-assist types, were identified and can be attributed to subcascade formation. When there are voids or under-pressured bubbles, the cascade-destruction mechanism can yield vacancy-type dislocation loops; however, the existence of He atoms can inhibit this behavior. In contrast, when the bubbles are over-pressured, the bubble-assist mechanism can contribute to the formation of self-interstitial atom-type dislocation loops. This study helps to improve our understanding of radiation damage in a complicated environment in experiments with multiple irradiations or fusion reactors.