High Collision Frequency Research Articles

Fabrication of vertically aligned single-walled carbon nanotubes in atmospheric pressure non-thermal plasma CVD

A fabrication technique of high-purity vertically aligned single-walled carbon nanotubes (VA-SWCNTs) using atmospheric pressure plasma enhanced chemical vapor deposition is presented. Although densely mono-dispersed Fe–Co catalysts of a few nanometers is primarily responsible for VA-SWCNT growth, carbon precipitation was virtually absent in the thermal CVD regime at 700 °C. On the other hand, high-purity VA-SWCNTs without measurable defects were grown at 4 μm min −1 by applying atmospheric pressure radio-frequency discharge (APRFD) which has been previously developed for this purpose. The results proved that cathodic ion sheath adjacent to the substrates, where a large potential drop exists, also plays an essential role for the controlled growth of SWCNTs, while ion damage to the VA-SWCNTs is inherently avoided due to high collision frequency among molecules in atmospheric pressure. Operation regime of APRFD and tentative reaction mechanisms for VA-SWCNT growth are discussed along with optical emission spectroscopy of near substrate region.

Piecewise continuous distribution function method: Fluid equations and wave disturbances at stratified gas

Wave disturbances of a stratified gas are studied. The description is built on a basis of the Bhatnagar–Gross–Krook (BGK) kinetic equation which has reduced down the level of fluid mechanics. The double momenta set is introduced inside a scheme of iterations of the equations operators, dividing the velocity space along and opposite gravity field directions. At both half-spaces the local equilibrium is supposed. As a result the momenta system is derived. It reproduces Navier–Stokes and Barnett equations at the first and second order at high collision frequencies. The homogeneous background limit gives the known results obtained by direct kinetics applications of Loyalka and Cheng as well as the recent higher momentum fluid mechanics results of Chen, Rao and Spiegel. The ground state declines from exponential at the Knudsen regime. The WBK solutions for ultrasound in exponentially stratified medium are constructed in explicit form, evaluated and plotted.

Stress and large-scale spatial structures in dense, driven granular flows

We study the appearance of large-scale dynamical heterogeneities in a simplified model of a driven, dissipative granular system. Simulations of steady-state gravity-driven flows of inelastically colliding hard disks show the formation of large-scale linear structures of particles with a high collision frequency. These chains can be shown to carry much of the collisional stress in the system due to a dynamical correlation that develops between the momentum transfer and time between collisions in these frequently colliding particles. The lifetime of these dynamical stress heterogeneities is seen to grow as the flow velocity decreases toward jamming, leading to slowly decaying stress correlations reminiscent of the slow dynamics observed in supercooled liquids.

Collisionality dependence of the polarization current caused by a rotating magnetic island

The effect of collisions on the polarization current caused by a rotating island in a Tokamak plasma is studied with guiding center particle simulations. The simulations are performed with a δf code which includes a Monte Carlo model for Coulomb collisions. The transition between the two limiting cases of low and high collision frequency ν (compared to the island rotation frequency ω) is found to occur at ν≈ω in agreement with a recent analytic theory.

Drift waves in a high-density cylindrical helicon discharge

A low-frequency instability is investigated in a helicon plasma, which is characterized by comparably high plasma-β and high collision frequencies. Single movable Langmuir probes and a poloidal probe array are used for studies of spatiotemporal dynamics and for characterization of the background plasma parameters. All experimentally observed features of the instability are found to be consistent with drift waves. A linear nonlocal numerical model for drift modes, based on the two-fluid description of a plasma, is used for comparison between the experimental observations and theory. Comparing numerical and experimental frequencies, it is found that the experimentally observed frequencies are consistent with drift waves. The numerical results show that the high electron collision frequencies provide the strongest destabilization mechanism in the helicon plasma.

Investigation of diamond-like-carbon films prepared by unbalanced magnetron sputtering

Using unbalanced magnetron sputtering method, smooth, dense and uniform diamond like carbon films (DLC) with high diamond phase content are deposited.Based on testing and analysis of deposition mechanism, we conclude that with smaller s puttering clusters and higher collision frequency between the sputtering cluster and environment molecules, and more suitable momentum of the sputtering cluste r impacting on the substrate, higher content of diamond phase in the film will result. Scanning electron micrograph shows that there are no obvious grains in this film. The micro-hardness is about 11 GPa, bulk elastic modulus is about 120 GPa as tested by nano-indentation. The optical transmittance is about 89%, ref raction coefficient is 1.952, and the deposition rate is 0.724μm/h as calculate d from the FTIR spectra.

Collisional multipactor inside ambient gas

The influence of collisions during rf-induced multipactors in the presence of ambient gas is studied in some detail. The effect of the randomization of the electron rf-interaction phases due to collisional kicks is given particular emphasis, since phase locking among successive generations of rf-driven electrons is essential for avalanching. The systematic collisional friction shifts the parameter region for multipactor eruption to higher rf-fields compared to vacuum. Random velocity fluctuations prevent convergence to exact phase locking, causing the impact phase to wander around its nominal resonant value. The rms width of the phase spread is determined by the balance between the rf-induced dynamical phase focusing and random phase diffusion. For low collision frequencies phase wandering in the unfavorable for secondary emission regime reduces the effective secondary yield and may eventually cause multipactor quenching. For high collision frequencies multipactor is completely suppressed due to loss of coherence with the rf.

Impact of the Collisional Plasma on the Propagation of Millimeter Waves

The plasma generated in the low-altitude atmosphere is of high collision frequencies. In this paper, the transmission coefficients of millimeter (MM) waves normally incident upon the plasma with high collision frequencies are calculated and analyzed. The experimental results of reflection and attenuation are presented for the eight-millimeter waves propagating through the plasma. Both the calculated experimental results indicate that the MM-waves concerned are attenuated significantly and reflected weakly, when propagating through the plasma of high collision frequencies.

Evolution of vacancy densities in powder particles during mechanical milling

This paper investigates the change in the density of vacancies during ball milling. A model for the evolution of vacancies has been proposed in this paper. Results from the simulation show that the density of vacancies increases asymptotically with collision times and the rate of increase becomes slower with further ball milling. The accumulation of vacancies in the powder particles is dramatically accelerated at lower milling temperature. Vacancies are shown to accumulate more easily in powder materials with higher vacancy migration energies. The present simulation indicates that higher collision frequency results in a faster accumulation of vacancies and therefore higher density of vacancies can be obtained. In addition, higher impact energy introduces higher density of vacancies per collision. Low milling temperature, high collision frequency and high collision energy may be advantage for the formation of new alloys, but low migration rate of vacancies at low temperature may reduce the rate of the formation of new alloys if the milling temperature is too low.

Ion plating discharges: evidence of cluster formation during metal evaporation

Previous work has demonstrated the possible presence of metal clusters in ion plating discharges that utilise evaporative sources. In the present study, cathode sheath thickness measurements under ion plating conditions and mass spectroscopy studies (up to 2500 a.m.u.) of gas evaporation have provided further evidence of metal clusters in the vapour phase. The view that these clusters nucleate and grow by vapour cooling, through metal atoms losing energy by collisions with gas atoms, is supported by the following observations: (i) for evaporation at a fixed gas pressure, the abundance of clusters seems to be enhanced if argon is used instead of neon, indicating the greater cooling effect from a gas with larger atomic mass; (ii) cluster detection rate appears to be increased with increasing gas pressure, and this can be attributed to higher collision frequencies leading to enhanced cooling.

Configuration of sodium fluoride crystal characterized by surface crystals on mother crystal

In the reaction crystallization of sodium fluoride in a semi-batch reactor, the effect of the crystallization conditions, such as the reactant concentration and agitation speed on the crystal configuration, are characterized by the appearance of irregular surface crystals on the mother crystals. As the reactant concentration and agitator speed increased, the number of surface crystals on the mother crystals increased; on the other hand the mean crystal size, which included the mother and surface crystals, decreased. These experimental results can be explained by secondary nucleation on the mother crystals and agglomeration of free nuclei and small crystals. A higher interfacial supersaturation contributes to secondary nucleation and a higher collision frequency contributes to agglomeration. These conditions arise with an increase in the reactant concentration and agitator speed, and result in more surface crystals and smaller mean crystal sizes.

Secondary nucleation of gibbsite crystals from synthetic Bayer liquors: effect of alkali metal ions

The effect of alkali metal ions (Na + versus K +) on secondary nucleation of gibbsite ( γ-Al(OH) 3) from synthetic Bayer liquors has been investigated under seeded, isothermal, batch crystallisation conditions. The nucleation kinetics showed a fourth-order dependence upon Al(III) relative supersaturation and a strong temperature effect. An activation energy of 132 kJ mol −1, which was independent of alkali metal ion, was calculated. Secondary nucleation and subsequent crystal growth rates however, were greater in sodium than in potassium aluminate solution. The Arrhenius, pre-exponential factor was at least three times larger in sodium than in potassium aluminate solutions at equivalent crystal surface area, similar supersaturation and temperature. The results indicated that secondary nucleation of Al(OH) 3 is a chemical reaction-controlled process which is alkali metal ion-mediated. Na + ions provide a more favourable pathway than potassium ions for the formation of Al(III)-containing clusters, higher collision frequency between the species and crystal surface, and faster growth of potential secondary nuclei in the solutions.

Measurements of mid-latitude E-region, sporadic-E, and TID-related drifts using HF Doppler-sorted interferometry

Abstract Modern HF digital ionosondes have been used for Doppler-sorted interferometry (DSI) to automatically measure F -region plasma drifts by detecting the Doppler shift and angle-of-arrival of echoes. We report on the use of a Digisonde 256 receiving on a seven-element antenna array located at the Australian mid-latitude station Beveridge (37.5° S, 144.9 ° E, −48.0 ° CGM latitude) to extend the routine application of drift measurements to the E -region, and especially sporadic- E (Es) . Obtaining good quality E -region drift measurements required many more soundings independent in the frequency and time domains than are usually made for F -region measurements. This is because the E region isoionic surfaces were usually more horizontally stratified than those in the more disturbed F -region, and so did not return as many of the oblique echoes upon which the accuracy of the technique depends. Smoothness of the ionosphere was less of a problem when performing Es measurements because of the patchy, cloud-like property of the layers. When making F -region drift measurements the motions are usually thought of as being uniform throughout the volume of ionosphere sampled for echoes. Application of the technique to E -region measurements is interesting because of the enhanced irregular neutral winds associated with gravity waves which grow in amplitude with height. These irregular winds control the motion of the plasma via high collision frequencies, and lead to the formation of Es layers at mid-latitudes. Therefore, the possibility of sharp vertical gradients in plasma drifts must be considered when making and analysing measurements. During numerous campaigns conducted throughout 1994/95, horizontal drifts in the range of ≈30−250 m/s were measured for Es patches lasting up to 6h. The drift measurements sometimes indicated the presence of wave-like motions in the ionosphere with periods in the range 5–90 min. A TID was observed to propagate towards the station from the south, and clearly showed an association between long-period motions in the F -region (≈90 min), and the arrival of a patch of slant- Es and slant- F irregularities. A predominant drift direction was towards the NW or W, and was sometimes seen to veer through S towards the SE during intervals of ≈ 1–5 h. There is evidence of these veering events occurring diurnally and semidiurnally. Our results can be understood in terms of the formation of & under the influence of gravity waves and tides.

Ray trajectories in an absorbing ionosphere

Abstract The present paper deals with the simulation of electromagnetic ray propagation in a cold collisional ionosphere in the presence of the Earth's magnetic field. This subject has been extensively studied in the past. The novel aspect here is our attempt to assess the effect of absorption on the ray trajectories, not merely the field intensity. In addition to the theoretical interest in this problem, practical questions, such as target location by means of Over The Horizon Radar (OTHR) systems, in the presence of high losses, provide the motivation. The analytical investigation of such problems is limited by the complexity of the wave propagation field problem and the physics of the ionosphere, which combine to yield a complex dispersion relation, and the restricted capability of available computers and mathematical software packages for handling the ray tracing model. The present model is based on the familiar Appelton-Hartree, sometimes called the Appelton-Lassen, dispersion equation for the cold, collisional, magnetized ionosphere. The way that the ray tracing is performed (ray tracing being an approximation) and the model chosen by the researcher predetermines the resultant ray trajectories. Thus, in the presence of losses, certain decisions regarding the use of the Hamiltonian ray tracing model have to be made. Unlike some studies which first compute the lossless trajectories, and then add on a posteriori the attenuation along these trajectories, as a perturbation of the lossless solution, here the Hamiltonian ray tracing formalism is extended in order to include the absorption effects in the formalism a priori. For small absorption all models yield more or less the same results; therefore, in the present study high losses are considered in order to emphasize the effects. However, the present study contributes to our understanding of the basic problem of ray propagation in the presence of arbitrary losses. The extended Hamiltonian ray tracing formalism used here assumes complex space, and an additional constraint that guarantees real space and time subspace for the ray trajectories, as well as for the group velocity, whereas the propagation vector and the frequency may be complex. Other formulations for the ray equations formalism exist too. At this time it remains an open problem whether ray trajectories computed by those models will agree with the results obtained here or not. Furthermore, in the absence of sufficient direct ray trajectories empirical data, where high absorption cases are compared to lossless cases, the question as to which model better describes the physical reality must remain open. The variation of the ray paths with frequency, launching angle, collision frequency, electron density profile and other variables, are examined for Chapman type E and F layers. By using typical F layer parameters, it is found that, in certain cases, a high collision frequency affects the ray path by as much as 500 km. This result is important for sub-ionospheric propagation and for target location tracking.

Mapping of the residues involved in a proposed beta-strand located in the ferric enterobactin receptor FepA using site-directed spin-labeling.

Electron paramagnetic resonance (EPR) site-directed spin-labeling (SDSL) has been used to characterize a proposed transmembrane beta-strand of the Escherichia coli ferric enterobactin receptor, FepA. Each of nine consecutive residues was mutated to cysteine and subsequently labeled with the sulfhydryl-specific spin-label methanethiosulfonate (MTSL) and the purified protein reconstituted into liposomes. Continuous wave (CW) power saturation methods were used to determine exposure of the nitroxide side chains to a series of paramagnetic relaxation agents, including nickel acetylacetonate (NiAA), nickel ethylenediaminediacetate (NiEDDA), chromium oxalate (CROX), and molecular oxygen. The spin-label attached to Q245C, L247C, L249C, A251C, and Y253C had higher collision frequencies with molecular oxygen than with polar relaxation agents, indicating that these sites are exposed to the hydrophobic phase of the lipid bilayer. MTSL bound to residues S246C, E248C, E250C, and G252C had higher collision rates with the polar agents than with oxygen, suggesting that these sites are exposed to the aqueous channel. The alternating periodicity observed with the polar relaxation agents, NiAA and NiEDDA, and in opposite phase with oxygen, is consistent with beta-sheet structure. Depth measurements, based on the reciprocal concentration gradients of NiEDDA and O2 across the bilayer and calibrated for our system with phosphatidylcholine spin-labels, indicated that L249C was nearest the center of the bilayer and that Q245C and Y253C were located just below the bilayer surface in opposite leaflets of the membrane. Thus, we conclude that this approach, through mapping of individual residues, has the capability of defining beta-sheet secondary structure.

Low-frequency drift-wave instability in dc discharges

The result for the permittivity of weakly ionized, weakly magnetized plasmas obtained by Kiss'ovski and Shivarova (1995) is analysed aiming at a description of the behaviour of low-frequency instabilities in dc discharges maintained at comparatively high gas pressures. The spectrum and the condition of instability of low-frequency drift waves arising in the discharges under conditions of comparatively high collision frequency and finite ion gyroradius are derived. (Here and are the ion gyrofrequency and gyroradius, respectively, is the ion - neutral collision frequency and R is the plasma column radius.) Results from numerical analysis are compared with experimental data previously obtained (Kiss'ovski et al 1994).

Analysis of macromolecules using nanoelectrospray ionization mass spectrometry and low-energy collision activation.

The mass spectrometric analysis of several proteins using nanoelectrospray (nanoES) with elastic collisions showed an improvement in the sensitivity over nanoES without collisional activation. We believe this effect is due to a better declusterization/ionization process. Optimization of the collision parameters can be easily performed during the long experiment time allowed using the nanoES source. Moreover, an apparent shift in the charge-state distribution is observed, with lower charged ions becoming relatively more abundant with increasing either target gas pressure or kinetic energy of the precursor ions. Higher charge-state ions might be expected to have higher collision frequencies and correspondingly lose more kinetic energy than lower charge-state ions.

Nonlinear whistlers: Implications for 100 Hz electric fields observed in the Venus ionosphere

Conditions on the electric field of an electromagnetic wave in a plasma are found which make the wave magnetic field comparable in strength to the ambient field. The conditions arise from higher‐order forces than are used in linear theory. It is found that the strong 100‐Hz signals observed at night by the Pioneer Venus orbiter in the Venus ionosphere at 129 km and a large fraction of the signals observed at 160 km, if interpreted as whistlers, would meet these conditions. The observations would represent highly nonlinear states for whistlers. Previous analyses of the 100‐Hz signal interpreted as due to “linear” whistlers could therefore be flawed. This supports our contention [Cole and Hoegy, 1996a, b], arrived at by an independent argument, that the whistler hypothesis for the 100‐Hz signal is invalid. In addition, in a new contribution to linear theory of the dispersion relationship, it is found that in the Venus ionosphere, in the high collision frequency region near 130 km altitude, linear whistlers would propagate freely in any direction, contrary to assumptions otherwise in the literature. They are critically damped there. At 160 km, normal linear‐limit theory applies and manifests normal guidance by the ambient magnetic field, subject to ω<p cosθ, with little damping.

Radio frequency hollow cathodes for the plasma processing technology

The present paper summarizes the main features of the hollow cathode discharges generated by a radio frequency (r.f.) instead of a d.c. field. The pressure of gas inside the hollow cathode is almost independent on the reactor pressure, which allows to generate discharge at high collision frequency and transport it into the low pressure reactor. The discharge forced out from the hollow cathode forms a decaying plasma channel with extraordinary properties. Gas metastables excited inside the cathode can act in selected gas mixtures as a source of additional heat, thereby enhancing thermionic electron emission and ionization of the gas. An arc regime can be started from the glow discharge simply by increasing the r.f. power. Hollow cathode arc in the cathode metal vapor can be sustained even without working gas. Examples of utilization of hollow cathodes in the film deposition and dry etching technology are presented. Small size cylindrical r.f. cathodes allow special applications inside narrow (<10 mm in diameter) tubes. Linear arc discharge (LAD) produced by r.f. hollow cathode with magnetic focusing promises new applications either with or even instead of magnetrons.

A kinetic model of transient effects in tokamak edge plasmas

Fluid descriptions are not adequate for edge plasma studies when the high collision frequency assumption fails in the presence of rapid transient processes. The results of analytic and numerical modeling of some of the non-Maxwellian features of edge plasma transport associated with transient heating and cooling phenomena in the scrape-off layer are presented. It is found that kinetic effects alter the heat conduction coefficient and lead to interesting critical behavior of the distribution function. The numerics are performed with the recently developed time-dependent Fokker–Planck code ALLA.