Dense Argon Research Articles

Computational modeling study of the radial line slot antenna microwave plasma source with comparisons to experiments

The radial line slot antenna plasma source is a high-density microwave plasma source comprising a high electron temperature source region within the plasma skin depth from a coupling window and low electron temperature diffusion region far from the window. The plasma is typically comprised of inert gases like argon and mixtures of halogen or fluorocarbon gases for etching. Following the experimental study of Tian et al. [J. Vac. Sci. Technol. A 24, 1421 (2006)], a two-dimensional computational model is used to describe the essential features of the source. A high density argon plasma is described using the quasi-neutral approximation and coupled to a frequency-domain electromagnetic wave solver to describe the plasma-microwave interactions in the source. The plasma is described using a multispecies plasma chemistry mechanism developed specifically for microwave excitation conditions. The plasma is nonlocal by nature with locations of peak power deposition and peak plasma density being very different. The spatial distribution of microwave power coupling depends on whether the plasma is under- or over-dense and is described well by the model. The model predicts the experimentally observed low-order diffusion mode radial plasma profiles. The trends of spatial profiles of electron density and electron temperature over a wide range of power and pressure conditions compare well with experimental results.

Collection efficiency of photoelectrons injected into near- and supercritical argon gas

Injection of photoelectrons into gaseous or liquid dielectrics is a widely used technique to produce cold plasmas in weakly ionized systems for investigating the transport properties of electrons. We report measurements of the collection efficiency of photoelectrons injected into dense argon gas for T = 152.7 K, close to the critical temperature T(c) ≈ 150.9 K, and for T = 200.0 K. The high-field data agree with the Young-Bradbury model and with previous measurements below T(c) and at an intermediate temperature above T(c). The effective, density-dependent electron-atom momentum transfer scattering cross section can be deduced. However, the weak-field data near T(c) show large deviations from the theoretical model. We show that the electron behavior at weak field is influenced by electrostriction effects that are only important near the critical point.

Observation of threshold energy and hysteresis in high current ion beam guiding and transmission through a micro-glass-capillary

The energy dependent guiding of high current density (0.1–3 A/m2) argon ion beams through a micro-glass-capillary is studied. It is observed that ion transmission through the capillary takes place only if its energy is greater than the retarding potential barrier, which depends upon the amount of charge deposited on the capillary inner wall. Foremost evidence of the observation that the transmitted current exhibits hysteresis with ion energy is presented. Particle in cell simulations carried out by solving Poisson's and Newton's force equation self-consistently, agree reasonably well with the experimental results.

Optical emission diagnostics with electric probe measurements of inductively coupled Ar/O2/Ar-O2 plasmas

Physical properties of low-pressure inductively coupled argon, oxygen, and Ar-O2 mixture plasmas are investigated using optical emission spectroscopy (OES) combined with an rf-compensated Langmuir probe measurement. In each gas discharge, the electron density and the electron temperature were obtained by using the probe. The electron temperature was also obtained by OES models and compared with that measured by the probe. The electron temperature was observed to decrease with increasing power and pressure and also observed to decrease with increasing Ar content. Argon metastable densities were calculated based on an optical transition model. In Ar-O2 discharges, the dissociation fraction of O2 molecules was estimated using optical emission actinometry. The dissociation fraction was observed to increase with increasing power and Ar content.

Density of metastable atoms in the plume of a low-pressure argon microplasma

Spatially resolved measurements of the density of metastable excited atoms in the plume of an argon microplasma are presented. The microplasma device is operated at a relatively low pressure, on the order of 1 Torr, and is exhausted into a vacuum. Line-integrated densities of excited argon neutrals in the exhaust plume are measured using tunable diode laser absorption spectroscopy. The densities of argon metastables in both 1s5 and 1s3 states are measured. These line-integrated density measurements are converted to three-dimensional density maps using the Abel inversion. The density of 1s5 argon peaks at a value of approximately 1018 m−3 near the outlet orifice, while the 1s3 density is roughly five times lower everywhere. It is found that, far from the face of the microplasma outlet orifice, metastable density follows axial and angular distributions consistent with that expected of vacuum gas expansion as predicted by classic rarified flow theory. Integrated metastable density is found to be conserved as the plume expands through 4 mm, suggesting a net production of excited species in the first millimetre and a constant population further downstream.

A Molecular Dynamics and Lattice Boltzmann Multiscale Simulation for Dense Fluid Flows

A molecular dynamics (MD)-lattice Boltzmann (LB) hybrid scheme has been adopted to simulate dense fluid flows. Based on the domain decomposition method and the Schwarz alternating scheme, the “Maxwell Demon” approach is used to impose boundary conditions from the continuum to the atomistic region, while the “reconstruction operator” is implemented to construct the single-particle distribution function of the LB method from the results of the MD simulation. Couette flows and the flow of a dense fluid argon around a carbon nanotube (CNT) are solved to validate the hybrid method. When the mesh of the LB domain is refined and the size of corresponding sampling cells of the MD domain is reduced, the fluctuations of the results between two successive iterations of the hybrid method become more severe, although the results get closer to the MD reference solutions. To decrease the fluctuation due to the mesh refinement, a new weighting function is proposed for the sampling of MD simulation results. Numerical practice demonstrates its feasibility.

Soft X-ray characterization of an elongated gas-puff target dedicated for laser–matter interaction experiments and high harmonic generation

A slit-shaped elongated nozzle, capable of forming an extended gas density profile, is presented and studied in detail. Argon and xenon target density backlighting measurements revealed the elongated density profiles with gas density that can be tuned by changing valve backing pressure and valve timing. Two dimensional density maps for Ar and Xe targets, obtained at soft X-ray wavelengths around 7Å, are shown and spatially characterized. Moreover the influence of valve backing pressure and valve timing on gas puff target density is studied. This geometry of the nozzle has potential applications in high harmonic generation for an efficient conversion of femto-second duration, intense infrared pulses to extreme ultraviolet (EUV) region.

Electron density measurement of inductively coupled plasmas by terahertz time-domain spectroscopy (THz-TDS)

The electron densities of argon inductively coupled plasmas were measured by terahertz time-domain spectroscopy (THz-TDS). At a low pressure, the electron densities were also measured with a Langmuir-type double probe and the validity of THz-TDS electron-density measurement in a plasma has been corroborated. As the input radio-frequency (RF) power increases, the plasma density and gas temperature increase, which makes the probe measurement less reliable or even impossible, due to the large heat load to the probe surface. On the contrary, the THz-TDS measurement is unaffected by the gas temperature and becomes more reliable due to the higher electron density at higher input power for plasma generation.

High resolution 3D gas-jet characterization

We present a tomographic characterization of gas jets employed for high-intensity laser-plasma interaction experiments where the shape can be non-symmetrically. With a Mach-Zehnder interferometer we measured the phase shift for different directions through the neutral density distribution of the gas jet. From the recorded interferograms it is possible to retrieve 3-dimensional neutral density distributions by tomographic reconstruction based on the filtered back projections. We report on criteria for the smallest number of recorded interferograms as well as a comparison with the widely used phase retrieval based on an Abel inversion. As an example for the performance of our approach, we present the characterization of nozzles with rectangular openings or gas jets with shock waves. With our setup we obtained a spatial resolution of less than 60 μm for an Argon density as low as 2 × 10(17) cm(-3).

Formation of Nano-Crystalline Phase in Hydrogenated Amorphous Silicon Thin Film by Plasma Focus Ion Beam Irradiation

A 3.3 kJ Mather type dense plasma focus device is used to generate a pulsed argon ion beam of 100 KeV in this work. Hydrogenated amorphous silicon (a-Si:H) film prepared by plasma enhanced chemical vapor deposition (PECVD) on c-Si substrate was irradiated with the argon ion beam produced by this dense plasma focus device. The effects of exposure to a single, 5 and 10 shots of dense plasma focus argon ion beam irradiation on the surface morphology, crystallinity and chemical bonding properties of the a-Si:H films were studied using Field Emission Scanning Electron Microscope (FESEM), X-ray Diffraction (XRD), Raman scattering and Fourier Transform Infrared (FTIR) spectroscopy, respectively. Formation of nano-crystalline silicon phase along with increase in structural order and hydrogen content in the film structure has been observed when the a-Si:H film was irradiated with a single shot of dense plasma focus argon ion beam. Exposure to 5 and 10 shots of the dense plasma focus argon ion beam irradiation reduced the hydrogen content resulting in a decrease in crystallinity and structural order in the film structure.

The effect of processing conditions on the microstructure and impact behavior of melt infiltrated Al/SiCp composites

In the current research, pressureless melt infiltration (PMI) was applied to study the effect of different processing conditions on the final properties of Al/SiC composites, fabricated through the infiltration of aluminum melt into SiC particulates porous preforms. Charpy impact test was used to explore the impact behavior of the Al/SiC composites, obtained from variable processing conditions. Conducting the process at a higher infiltration temperature (1350 °C) increased the final relative density of composites up to the value of 0.97 of theoretical density (TD). The application of a post sintering procedure in nitrogen atmosphere after the completion of infiltration resulted in a slight increase (∼1) in the final density of composites compared to the only infiltrated ones. Instead, the final density of argon sintered composites has undergone a 0.41% reduction. This can be originated from the occurrence of chemical reactions in the N 2 atmosphere resulting in the formation of consequent phases, contrary to the argon neutral gas. Results concerning with the impact resistance demonstrated a remarkable superiority for the impact energy of the composites subjected to the combined infiltration and sintering (MIS) procedure compared to the infiltrated ones. While such an observation was found to be identical through sintering in both atmospheres, the appearance of brittle phases formed through sintering procedure in nitrogen gave rise to higher impact energy for the argon sintered composites.

Micro-fluidic target chamber machined by proton beam writing for the in situ analysis of gas absorption in synthetic crystals

Many synthetic crystals used for chemical and industrial applications have special internal structures, e.g. nano-pores, which allow separating different gases and fluids. Ion beam analytical methods can be used to study the gas diffusion and absorption in these materials in situ and to visualize their inner surfaces which affect these processes. For this purpose, a small target chamber was constructed in PMMA (polymethyl methacrylate) using proton beam writing (PBW). This micro-fluidic structure enables the establishment of a defined atmosphere around a crystal and allows the simultaneous ion beam analysis. In order to confine the gas from the high vacuum in the measurement chamber Si3N4-windows of 200nm thickness were thermally bonded to the structured PMMA block yielding a closed target chamber with the possibility to accomplish PIXE and RBS measurements. In addition, two capillaries were connected to the chamber for gas inlet and evacuation. First tests showed that the construction is leak-proof and allows to establish a defined atmosphere. After that, the Argon gas diffusion into Zn(tbip)-crystals was studied. These measurements have shown unexpectedly high nickel concentrations in the host crystal which reduces the Argon density in these areas after absorption, because the Ni atoms decrease the pore size by replacing Zn-atoms in the Zn(tbip)-lattice. It could be demonstrated that gas diffusion and absorption in organic crystals can be studied in situ with high lateral resolution using ion beam analysis in a dedicated target chamber machined by PBW.

Injection of photoelectrons into dense argon gas

The injection of photoelectrons into a gaseous or liquid sample is a widespread technique that is used to produce a cold plasma in a weakly ionized system to study the transport properties of electrons in a dense gas or liquid. We report here the experimental results of the collection efficiency of photoelectrons injected into dense argon gas at T = 142.6 K as a function of the externally applied electric field and gas density. We show that the experimental data can be interpreted in terms of the so-called Young–Bradbury model, only if multiple scattering effects due to the dense environment are taken into account when computing the scattering properties and energetics of the electrons.

Ambipolar ion acceleration in an expanding magnetic nozzle

The helicon plasma stage in the Variable Specific Impulse Magnetoplasma Rocket (VASIMR®) VX-200i device was used to characterize an axial plasma potential profile within an expanding magnetic nozzle region of the laboratory based device. The ion acceleration mechanism is identified as an ambipolar electric field produced by an electron pressure gradient, resulting in a local axial ion speed of Mach 4 downstream of the magnetic nozzle. A 20 eV argon ion kinetic energy was measured in the helicon source, which had a peak magnetic field strength of 0.17 T. The helicon plasma source was operated with 25 mg s−1 argon propellant and 30 kW of RF power. The maximum measured values of plasma density and electron temperature within the exhaust plume were 1 × 1020 m−3 and 9 eV, respectively. The measured plasma density is nearly an order of magnitude larger than previously reported steady-state helicon plasma sources. The exhaust plume also exhibits a 95% to 100% ionization fraction. The size scale and spatial location of the plasma potential structure in the expanding magnetic nozzle region appear to follow the size scale and spatial location of the expanding magnetic field. The thickness of the potential structure was found to be 104 to 105 λDe depending on the local electron temperature in the magnetic nozzle, many orders of magnitude larger than typical laboratory double layer structures. The background plasma density and neutral argon pressure were 1015 m−3 and 2 × 10−5 Torr, respectively, in a 150 m3 vacuum chamber during operation of the helicon plasma source. The agreement between the measured plasma potential and plasma potential that was calculated from an ambipolar ion acceleration analysis over the bulk of the axial distance where the potential drop was located is a strong confirmation of the ambipolar acceleration process.

Ultrafast observation of shocked states in a precompressed material

We apply ultrafast single shot interferometry to determine the pressure and density of argon shocked from up to 7.8 GPa static initial pressure in a diamond anvil cell. This method enables the observation of thermodynamic states distinct from those observed in either single shock or isothermal compression experiments. In particular, this method enables access to high density, relatively low temperature states of light materials, such as isentropically compressed states of giant planets. Further, since excitation by a shock wave is intrinsically ultrafast and this method has picoseconds time resolution, it has the potential to observe the collective dynamics of materials undergoing shock induced phase transitions and chemistry on ultrafast time scales. We also present a straightforward method for interpreting ultrafast shock wave data which determines the index of refraction at the shock front, and the particle and shock velocities for shock waves in transparent materials. Based on these methods, we observe shocked thermodynamic states between the room temperature isotherm of argon and the shock adiabat of cryogenic argon at final shock pressures up to 28 GPa.

Comment on “Brillouin scattering studies of liquid argon at high temperatures and high pressures” [J. Chem. Phys. 129, 154503 (2008)

In Ref. 1, the authors report on Brillouin scattering measurements on argon at high pressure P and temperature T conditions from which they obtain experimental data for the sound velocity and refractive index of the fluid phase along several isotherms up to 503 K and respective melting pressures. Starting from the refractive index data, they derive the density of fluid argon along the studied isotherms using the Lorentz–Lorenz relation, under the assumption that the electronic polarizability of argon does not vary with P-T conditions. Applying the same reasoning to the data obtained at the solid-fluid coexistence, they obtain values for the density of the fluid and solid phases at coexistence, from which they derive the volume jump Vm at melting or melting volume . This results in a highly unusual observation, that is, the increase in Vm with temperature along the melting line. In this comment I show that this finding is neither supported by available experimental data nor theoretical calculations, and that this is due to an erroneous determination of the fluid densities. Because it is an inert, weakly interacting system with a simple closed-shell electronic configuration, argon has longplayed the role of reference system in condensed-matter physics. In particular, its thermodynamic properties such as P-V-T equation of state EOS and melting curve have been often used as benchmark data for testing various theories. In parallel, a large number of experimental studies have been devoted to the accurate determination of thermodynamic data, which cover a large P-T range see, for example, Ref. 2 for a good number of references for the fluid phase and melting curve . Figure 1 shows the available literature data for the melting volume, obtained by direct volumetric measurements, and compare them to the values of Ref. 1. It is clear that all previous data sets show a regular decrease in Vm with Tm or Pm . In addition to experimental data, which are limited to 320 K, I have plotted the theoretical calculations of Zha et al. using the exponential-6 Buckingham pair potential fitted to shock and static high-pressure data. This potential is an “effective” pair potential as it includes the contribution from many-body interactions averaged over one pair. The calculations agree very well with experimental data, both for the melting curve and volume behaviors, and extend up to 3000 K–53 GPa for the sake of clarity, the T range of Fig. 1 was limited to 600 K . The solid line in Fig. 1 corresponds to the empirical law proposed by Cheng et al., who showed that the experimental data were very well reproduced by the form

単層カーボンナノチューブと周囲材料の界面熱抵抗(熱工学,内燃機関,動力など)

The thermal boundary resistance between a single-walled carbon nanotube (SWNT) and surrounding argon has been investigated by using molecular dynamics simulations. With a non-stationary approach, the thermal boundary resistance was quantified for a wide range of temperatures and argon densities, which covers various argon phases i.e., gas, liquid, solid and supercritical phases. The results show that, when the surrounding argon is in fluid phase, thermal boundary resistance is determined by the local density of the argon layer adjacent to the SWNT independently of the phase. On the other hand, when the surrounding material is solid, the modal thermal energy transfer manifests, which contributes to the density effect on the thermal boundary resistance.

3D Isosurface Visualization of Electron Density and Temperature Distribution in a Magnetized Sheet Plasma Ion Source

A 3D visualization of argon and nitrogen-argon plasma electron density (ne) and effective temperature (Teff) distributions were constructed using the I-V curves from Langmuir probe traces at specific discrete positions in the extraction region of a magnetized sheet plasma ion source. Argon and mixed N2-Ar sheet plasmas are characterized using the calculation of the electron energy distribution function (EEDF). By taking the current vs. voltage reading of a single probe in 68 discrete locations of the plasma, a 3-dimensional map of the electron density and electron temperature were constructed. The map was constructed to understand the global condition of the extraction region of the source. The technique can be applied to the determination and understanding of edge plasma parameters in magnetic confinement devices.

Electron density measurements in an atmospheric pressure argon discharge by means of plasma radiation

A method is developed for measuring the electron density in argon during atmospheric pressure dielectric barrier discharge. The oscillating electromagnetic signal is detected by an antenna at a distance from the plasma source. The electron density is then calculated from the ion oscillation frequency. The results show that the electron density in the plasma after the turnoff of the discharge channel is 2.55×1019 m−3 with an applied voltage of 2.73 kV, and it decays almost exponentially down to 1.00×1019 m−3 over 49.2 ns. The electron density is found to increase linearly with increasing voltage.

Operating a radio-frequency plasma source on water vapor

A magnetically enhanced radio-frequency (rf) plasma source operating on water vapor has an extensive list of potential applications. In this work, the use of a rf plasma source to dissociate water vapor for hydrogen production is investigated. This paper describes a rf plasma source operated on water vapor and characterizes its plasma properties using a Langmuir probe, a residual gas analyzer, and a spectrometer. The plasma source operated first on argon and then on water vapor at operating pressures just over 300 mtorr. Argon and water vapor plasma number densities differ significantly. In the electropositive argon plasma, quasineutrality requires n(i) approximately = n(e), where n(i) is the positive ion density. But in the electronegative water plasma, quasineutrality requires n(i+) = n(i-) + n(e). The positive ion density and electron density of the water vapor plasma are approximately one and two orders of magnitude lower, respectively, than those of argon plasma. These results suggest that attachment and dissociative attachment are present in electronegative water vapor plasma. The electron temperature for this water vapor plasma source is between 1.5 and 4 eV. Without an applied axial magnetic field, hydrogen production increases linearly with rf power. With an axial magnetic field, hydrogen production jumps to a maximum value at 500 W and then saturates with rf power. The presence of the applied axial magnetic field is therefore shown to enhance hydrogen production.