Low Density Mode Research Articles

Bimodal distribution of size-resolved particle effective density: results from a short campaign in a rural environment over the North China Plain

Abstract. Effective density is one of the most important physical properties of atmospheric particles. It is closely linked to particle chemical composition and morphology and could provide special information on particle emissions and aging processes. In this study, size-resolved particle effective density was measured with a combined differential mobility analyzer–centrifugal particle mass analyzer–condensation particle counter (DMA–CPMA–CPC) system in autumn 2019 as part of the Multiphase chemistry experiment in Fogs and Aerosols in the North China Plain (McFAN). With a newly developed flexible Gaussian fit algorithm, frequent (77 %–87 %) bimodal distribution of particle effective density is identified, with a low-density mode (named sub-density mode) accounting for 22 %–27 % of the total number of observed particles. The prevalence of the sub-density mode is closely related to fresh black carbon (BC) emissions. The geometric mean for the main density mode (ρ‾eff,main) increases from 1.18 ± 0.10 g cm−3 (50 nm) to 1.37 ± 0.12 g cm−3 (300 nm) due to a larger fraction of high-density components and a more significant restructuring effect at large particle sizes but decreases from 0.89 ± 0.08 g cm−3 (50 nm) to 0.62 ± 0.12 g cm−3 (300 nm) for the sub-density mode (ρ‾eff,sub), which could be mainly ascribed to the agglomerate effect of BC. ρ‾eff,main and ρ‾eff,sub show similar diurnal cycles with peaks in the early afternoon, mainly attributed to the increasing mass fraction of high material density components associated with secondary aerosol production, especially of secondary inorganic aerosol (SIA). To investigate the impact of chemical composition, bulk particle effective density was calculated based on measured chemical composition (ρeff,ACSM) and compared to the average effective density at 300 nm (ρ‾eff,tot,300nm). The best agreement between the two densities is achieved when assuming a BC effective density of 0.60 g cm−3. The particle effective density is highly dependent on SIA and BC mass fractions. The influence of BC on the effective density is even stronger than SIA, implying the importance and necessity of including BC in the estimate of effective density for ambient particles.

Performance Analysis of NOMA Enabled Fog Radio Access Networks

In the fifth generation (5G) era, mobile networks will provide diversified services such as enhanced Mobile BroadBand (eMBB) and ultra-reliable low-latency communication (URLLC). Furthermore, the 5G architecture will be fog-like, enabling a functional split of network functionalities between cloud and edge nodes. In this article, we propose a non-orthogonal multiple access (NOMA)-enabled fog-cloud structure in a novel density-aware fog-based radio access networks (F-RAN) to tackle different aspects of high and low-density regions, in order to meet the heterogeneous requirements of eMBB and URLLC traffic. A framework of two different problem formulations is considered to cater the high throughput and low-latency requirements in a high and low-density mode respectively. In the first problem, we study the joint caching placement and association strategy aiming at minimizing the average delay. To deal with the first problem, we apply McCormick envelopes and Lagrange partial relaxation method to transform it into three convex sub-problems, which is then solved by proposing a distributed algorithm. The second problem is to jointly optimize transmission mode selection, subchannel assignment and power allocation to maximize the sum data rate of all fog user equipments (F-UEs) while satisfying fronthaul capacity and fog-computing access point (F-AP) power constraints. Moreover, for given transmission mode selection and subchannel assignment, the optimal power allocation is derived in a closed-form expression. Simulation results are provided to validate the effectiveness of the proposed NOMA-enabled F-RAN framework and reveal that the ultra-low latency and high throughput can be achieved by properly utilizing the available resources.

Spectroscopic investigation on the near-substrate plasma characteristics of chromium HiPIMS in low density discharge mode

High power impulse magnetron sputtering (HiPIMS) discharge promises high ionization fraction and energetic ions in comparison with dc magnetron sputtering discharge. But acknowledge on the characteristics of HiPIMS plasma in the near-substrate region (substrate vicinity), which is of great importance for film deposition, is still limited. Here, optical emission spectroscopy (OES) combined with the collisional-radiative modelling are developed and used to determine the electron temperature and the number density of neutral sputtered atom for the chromium HiPIMS plasma in substrate vicinity. The OES analysis demonstrated the HiPIMS discharge of Cr sputtering process in low density mode was dominated by the electron impact ionization of argon atoms and excitation of chromium atoms. As the HiPIMS plasma in the substrate vicinity is far from the local thermal equilibrium state, the relative intensities of transition lines to ArI 4p states was used to calculate the electron temperature. Subsequently, the neutral chromium atoms density about 1017 m−3 was reported in the near-substrate region. Our findings have important implications for species generation in low density HiPIMS discharge, with applications in synthesis of dense chromium coatings.

Application of field-portable-XRF for the determination of trace elements in deciduous leaves from a mine-impacted region

Deciduous leaves (n = 87) from beech (Fagus sylvatica), birch (Betula spp.) and oak (Quercus spp.) trees have been collected from three metal mine-impacted sites in southwest England and tested for concentrations of trace elements (As, Cu, Pb and Zn) using a field-portable-x-ray fluorescence (FP-XRF) spectrometer configured in a low density mode and housed in a stand. When intact leaves were analysed directly, mean detection limits ranged from about 10 (As) to 70 μg g−1 (Cu) on a fresh weight basis; after freeze-drying, respective limits increased to about 20 and 120 μg g−1 on a dry weight basis. Within these constraints, As and Zn were detected in samples from all genera, with concentration differences between fresh and dry states attributed to the mass of water present and its propensity to attenuate x-rays. A comparison with As and Zn concentrations in local soils and determined by XRF in a higher density mode revealed different accumulation and exclusion characteristics among the three genera of tree. In contrast, and despite soil concentrations that were similar to those of Zn, Cu was detected in only two dried leaves and Pb evaded detection throughout. Pooled results from the study showed good agreement with independent results derived from ICP following acid digestion, with a slope defining the overall relationship that was close to unit value. Accordingly, the XRF approach is able to provide a rapid assessment of the levels of certain trace elements in leaves from contaminated sites, with the configuration deployed on site having potential to deliver immediate results.

Multi-Objective Resource Allocation in Density-Aware Design of C-RAN in 5G

In this paper, a multi-objective resource allocation algorithm in a novel density-aware design of virtualized software-defined cloud radio access network (C-RAN) is proposed. We consider two design modes based on the average density of users: 1) high-density mode when a large number of low-cost remote radio heads (RRHs) without baseband processing capability are controlled by one single base station and 2) low-density mode when a small number of RRHs with baseband processing capability are deployed. In high-density mode, the challenge of front-haul capacity limitation is tackled via separating control plane and data plane in a heterogeneous structure. Besides, the fully centralized processing and management, and energy-efficient use of infrastructure in low traffic time by turning off RRHs are achieved. In the low-density mode, the transmission delay due to the large distance between the sparse RRHs and cloud unit, is more critical. This practical issue is handled by sharing the baseband processing and resource management among these units in a hierarchical structure. This resulting heterogeneous /hierarchical virtualized software-defined cloud-RAN (HVSD-CRAN) offers various tradeoffs in resource management objectives such as throughput and delay versus power and cost. Consequently, we resort to multi-objective optimization theory to propose a resource allocation framework in HVSD-CRAN.

Bromine in plastic consumer products – Evidence for the widespread recycling of electronic waste

A range of plastic consumer products and components thereof have been analysed by x-ray fluorescence (XRF) spectrometry in a low density mode for Br as a surrogate for brominated flame retardant (BFR) content. Bromine was detected in about 42% of 267 analyses performed on electronic (and electrical) samples and 18% of 789 analyses performed on non-electronic samples, with respective concentrations ranging from 1.8 to 171,000μgg−1 and 2.6 to 28,500μgg−1. Amongst the electronic items, the highest concentrations of Br were encountered in relatively small appliances, many of which predated 2005 (e.g. a fan heater, boiler thermostat and smoke detector, and various rechargers, light bulb collars and printed circuit boards), and usually in association with Sb, a component of antimony oxide flame retardant synergists, and Pb, a heavy metal additive and contaminant. Amongst the non-electronic samples, Br concentrations were highest in items of jewellery, a coffee stirrer, a child's puzzle, a picture frame, and various clothes hangers, Christmas decorations and thermos cup lids, and were often associated with the presence of Sb and Pb. These observations, coupled with the presence of Br at concentrations below those required for flame-retardancy in a wider range of electronic and non-electronic items, are consistent with the widespread recycling of electronic plastic waste. That most Br-contaminated items were black suggests the current and recent demand for black plastics in particular is met, at least partially, through this route. Given many Br-contaminated items would evade the attention of the end-user and recycler, their disposal by conventional municipal means affords a course of BFR entry into the environment and, for food-contact items, a means of exposure to humans.

Cadmium, lead and bromine in beached microplastics

Samples of microplastic (n = 924) from two beaches in south west England have been analysed by field-portable-x-ray fluorescence (FP-XRF) spectrometry, configured in a low-density mode and with a small-spot facility, for the heavy metals, Cd and Pb, and the halogen, Br. Primary plastics in the form of pre-production pellets were the principal type of microplastic (>70%) on both beaches, with secondary, irregularly-shaped fragments representing the remainder of samples. Cadmium and Pb were detected in 6.9% and 7.5% of all microplastics, respectively, with concentrations of either metal that exceeded 103 μg g−1 usually encountered in red and yellow pellets or fragments. Respective correlations of Cd and Pb with Se and Cr were attributed to the presence of the coloured, inorganic pigments, cadmium sulphoselenide and lead chromate. Bromine, detected in 10.4% of microplastics and up to concentrations of about 13,000 μg g−1, was mainly encountered in neutrally-coloured pellets. Its strong correlation with Sb, whose oxides are effective fire suppressant synergists, suggests the presence of a variety of brominated flame retardants arising from the recycling of plastics originally used in casings for heat-generating electrical equipment. The maximum bioaccessible concentrations of Cd and Pb, evaluated using a physiological extraction based on the chemical characteristics of the proventriculus-gizzard of the northern fulmar, were about 50 μg g−1 and 8 μg g−1, respectively. These concentrations exceed those estimated for the diet of local seabirds by factors of about 50 and 4, respectively.

In situ determination of trace elements in Fucus spp. by field-portable-XRF

Fresh and freeze-dried sample sections of the coastal macroalgae, Fucus serratus and F. vesiculosus, and the brackish water macroalga, F. ceranoides, have been analysed for trace elements by field-portable-X-ray fluorescence (FP-XRF) spectrometry using a Niton XL3t in a low density mode with thickness correction. When analysed fresh in a laboratory accessory stand for a period of 200 seconds, As, Br, Fe and Zn were registered in the apex, mid-frond and lower stipe of all species, with detection limits of a few μg g−1 (As) or a few tens of μg g−1 (Br, Fe, Zn); when analysed dry under the same conditions, concentrations returned were systematically higher and Cu and Pb were detected in a number of F. ceranoides sections. Concentrations arising from both approaches on a dry weight basis were highly correlated, with deviations from unit slope attributed to the absorption of fluorescent X-rays by internal and surficial water when analysed fresh. With algorithms correcting for the effects of water on mass and X-ray absorption, sections of F. vesiculosus and F. ceranoides were analysed in situ with the XRF connected to a mobile stand and laptop. Dry weight concentrations returned for As and Zn were significantly correlated with respective concentrations subsequently determined by ICP-MS following acid digestion and with a slope close to unity; lower concentrations of Fe returned by ICP were attributed to the incomplete acid digestion of silt particles that evaded an initial cleaning step, while Br concentrations could not be verified independently because of loss of volatile forms during digestion. The in situ determination of trace elements in fucoids by FP-XRF provides a rapid and non-destructive means of monitoring environmental quality and identifying hot-spots of contamination, and enables a research strategy to be developed iteratively that is informed by immediate results.

The effect of the driving frequency on the confinement of beam electrons and plasma density in low-pressure capacitive discharges

The effect of changing the driving frequency on the plasma density and the electron dynamics in a capacitive radio-frequency argon plasma operated at low pressures of a few Pa is investigated by particle-in-cell/Monte-Carlo collision simulations and analytical modeling. In contrast to previous assumptions, the plasma density does not follow a quadratic dependence on the driving frequency in this non-local collisionless regime. Instead, a step-like increase at a distinct driving frequency is observed. Based on an analytical power balance model, in combination with a detailed analysis of the electron kinetics, the density jump is found to be caused by an electron heating mode transition from the classical α-mode into a low-density resonant heating mode characterized by the generation of two energetic electron beams at each electrode per sheath expansion phase. These electron beams propagate through the bulk without collisions and interact with the opposing sheath. In the low-density mode, the second beam is found to hit the opposing sheath during its collapse. Consequently, a large number of energetic electrons is lost at the electrodes resulting in a poor confinement of beam electrons in contrast to the classical α-mode observed at higher driving frequencies. Based on the analytical model this modulated confinement quality and the related modulation of the energy lost per electron lost at the electrodes is demonstrated to cause the step-like change of the plasma density. The effects of a variation of the electrode gap, the neutral gas pressure, the electron sticking and secondary electron emission coefficients of the electrodes on this step-like increase of the plasma density are analyzed based on the simulation results.

Wear Relief for High-Density Phase Change Memory Through Cell Morphing Considering Process Variation

Due to the scalability and large leakage power, dynamic random-access memory (DRAM) has a lot of challenges in scaling. As an alternative, phase change memory (PCM) has demonstrated promising potential to serve as the main memory in deep submicrometer regime. The broad resistance range of PCM cells enables several cell modes with various densities, pertaining to multiple level cell (MLC), triple state cell (TSC), and single level cell (SLC). High-density mode outperforms low-density ones in terms of capacity and cost-per-bit, but suffers from a weaker cell endurance. Wear leveling strategies are proposed to enhance the memory endurance but encounter more challenges with the aggravating process variation. Due to endurance variations, physical domains are fabricated with irregular tenacity. As a result, balanced write traffic, which is the objective of traditional wear leveling, cannot fully exploit the PCM endurance since the weak parts will be worn out sooner than others. In this paper, considering process variation, we propose a cell morphing based wear leveling scheme. Cell morphing refers to the cell mode transformation between high density (e.g., MLC) and low densities (e.g., TSC and SLC). Instead of redistributing write operations, the proposed wear leveling scheme dynamically transforms weak and frequently written portions into low-density mode for endurance benefits. Multitier cell morphing schemes are proposed to support mode transformation among multiple density levels. The experimental results show 236% endurance improvement for single-tier cell morphing and 209% for two-tier cell morphing with 2% low-density page percentage, when compared with the most related work.

Time and spatially resolved LIF of OH in a plasma filament in atmospheric pressure He–H2O

The production of OH in a nanosecond pulsed filamentary discharge generated in pin–pin geometry in a He–H2O mixture is studied by time and spatially resolved laser-induced fluorescence. Apart from the OH density the gas temperature and the electron density are also measured. Depending on the applied voltage the discharge is in a different mode. The maximum electron densities in the low- (1.3 kV) and high-density (5 kV) modes are 2 × 1021 m−3 and 7 × 1022 m−3, respectively. The gas temperature in both modes does not exceed 600 K. In the low-density mode the maximum OH density is at the centre of the discharge filament, while in the high-density mode the largest OH density is observed on the edge of the discharge. A chemical model is used to obtain an estimate of the absolute OH density. The chemical model also shows that charge exchange and dissociative recombination can explain the production of OH in the case of the high-density mode.

HISTORY AND MODES OF STAR FORMATION IN THE MOST ACTIVE REGION OF THE SMALL MAGELLANIC CLOUD, NGC 346

We discuss the star formation history of the SMC region NGC 346 based on Hubble Space Telescope images. The region contains both field stars and cluster members. Using a classical synthetic CMD procedure applied to the field around NGC 346 we find that there the star formation pace has been rising from a quite low rate 13 Gyr ago to \approx 1.4 \times 10^{-8} Mo yr^{-1}pc^{-2} in the last 100 Myr. This value is significantly higher than in other star forming regions of the SMC. For NGC 346 itself, we compare theoretical and observed Color-Magnitude Diagrams (CMDs) of several stellar sub-clusters identified in the region, and we derive their basic evolution parameters. We find that NGC 346 experienced different star formation regimes, including a dominant and focused "high density mode", with the sub-clusters hosting both pre-main sequence (PMS) and upper main sequence (UMS) stars, and a diffuse "low density mode", as indicated by the presence of low-mass PMS sub-clusters. Quantitatively, the star formation in the oldest sub-clusters started about 6 Myr ago with remarkable synchronization, it continued at high rate (up to 2 \times 10^{-5} Mo yr^{-1} pc^{-2}) for about 3 Myr and is now progressing at a lower rate. Interestingly, sub-clusters mainly composed by low mass PMS stars seem to experience now the first episode of star formation, following multi-seeded spatial patterns instead of resulting from a coherent trigger. Two speculative scenarios are put forth to explain the deficiency of UMS stars: the first invokes under-threshold conditions of the parent gas; the second speculates that the initial mass function (IMF) is a function of time, with the youngest sub-clusters not having had sufficient time to form more massive stars.

Optical Coherence Tomography Fast versus Regular Macular Thickness Mapping in Diabetic Retinopathy

Objective: The purpose of the study was to investigate if absolute values and reproducibility of thickness maps obtained from 2 optical coherence tomography (OCT) scanning protocols, regular high-resolution and fast low-density mode, differ in patients with diabetic macular edema. Methods: A total of 26 consecutive patients undergoing fluorescein angiography and Stratus OCT scanning for the evaluation of diabetic macular edema at the Departments of Ophthalmology in Munich and Vienna were included. Results: Retinal thickness of the central field of the thickness map measured by fast retinal thickness protocol was 287 ± 97 and 290 ± 113 µm by the regular protocol. This difference as well as that for all other fields was not statistically significant. Three times repeated measurements applying both OCT scanning modes in 10 patients yielded very good intrasession correlation coefficients between 0.70 and 0.99, with corresponding intrasession standard deviations ranging between 6 and 16 µm. The fast mode yielded slightly less reproducible values than the regular mode. Visual acuity did not influence the results. Conclusion: In practice both scanning modes can be interchanged and absolute values can be compared directly. Best reproducibility is obtained with higher sampling density even in patients with reduced visual acuity due to diabetic macular edema.

Heating-mode transition in the capacitive mode of inductively coupled plasmas

The evolution of the electron energy distribution function (EEDF) against pressure is investigated in the capacitive mode of inductively coupled plasma (ICP). A significant change in the EEDFs is observed: a bi-Maxwellian EEDF at low pressure (⩽10 mTorr) evolves into a Druyvestein-like EEDF at high pressure (⩾50 mTorr) in the capacitive mode (low-density mode) while the EEDFs in the inductive mode (high-density mode) does not evolve like in the capacitive mode due to high electron–electron collisions. This EEDF transition in the capacitive mode of ICP is similar to that in the capacitive coupled plasma (CCP) reported in literature [V. A. Godyak and R. B. Piejak, Phys. Rev. Lett. 65, 996(1990)] as pressure increases. This observation directly shows that the electron heating mechanism of the capacitive mode in the ICP is the same as that in the CCP, as expected.

Different operational regimes in a helicon plasma source

The characterization of a helicon plasma source has been done in argon plasmas using a rf compensated Langmuir probe. It is now well known that helicon discharges exhibit three modes of operation: a capacitive mode (known as the E mode), which is a low-density mode (around 109 cm−3), an inductive mode (known as the H mode) with higher densities (1010 cm−3), and a helicon-wave sustained mode (known as the W mode) with very high densities (1011–1012 cm−3). By increasing the injected power, the transitions between these modes can be observed and correspond to sudden increases of the plasma density and of the glow brightness. In this article the performances of the helicon source in terms of density and plasma potential are presented and discussed. Transitions between the different coupling modes are observed and explained thanks to two phenomenological models and a good agreement is found between the experimental values and the model results. It has been shown that the main parameter, which determines the coupling mode, is the plasma density, which depends on the rf power and the gas pressure. So the transition between E and H modes is observed when the density reaches a critical value: for this value the skin depth δ is equal to the radius of the Pyrex tube. A similar result has been shown for the H–W transition: a minimum density, the value of which depends on the magnetic field amplitude, is necessary to observe the first longitudinal helicon-wave mode. This value is also directly linked with the geometry of the reactor and of the antenna.

Characterization of the E to H transition in a pulsed inductively coupled plasma discharge with internal coil geometry: bi-stability and hysteresis

Electrodeless radiofrequency discharges exhibit two modes of operation: a low-density mode in which the power is capacitively coupled to the plasma and which is known as the E-mode, and a higher density mode which is an inductive discharge known as the H-mode. The transition between these modes exhibits hysteresis, i.e. the E to H transition occurs at a different coil current than the reverse H to E transition. Recent theoretical results show that the hysteresis can be qualitatively understood in terms of electron power balance assuming that either the power dissipated or the power absorbed by the plasma electrons has a nonlinear dependence on the electron density. Experiments have been carried out to examine this hypothesis, both by characterizing steady-state E- and H-mode plasmas with a Langmuir probe, and by using a new approach consisting of measuring the internal plasma parameters in a pulsed discharge. In the latter case, the power is time modulated with increasing and decreasing power ramps. This approach allows us to investigate the hysteresis in detail and to study the dynamics of the transition. A number of time-resolved diagnostics including Langmuir probes, current and voltage sensors, optical emission and B-dot probes have been used.

Hysteresis in the E- to H-mode transition in a planar coil, inductively coupled rf argon discharge

Inductively coupled rf discharges typically exhibit two modes of operation, namely, a low-density mode known as the E mode and a higher density mode known as the H mode. The transition between these modes exhibits hysteresis. Experimental observations of these transitions are presented. By means of a mixture of electromagnetic theory and circuit analysis and by invoking the requirement that the power absorbed and lost by the electrons should balance, the possible working points of an inductively coupled rf plasma source are identified in (P, ) space, where P is the power absorbed by the electrons, is the peak rf current in the induction coil and is the electron number density. Once the loci of the operating points have been identified in this manner, it is possible to construct a consistent explanation for all the experimental observations reported in the first part of the paper. In particular, it is possible to present an explanation for the hysteresis-like behaviour manifested by the mode transitions. Basically, the transitions occur when it is no longer possible to balance the power absorbed and lost by electrons.

Characteristics of the high density plasma production by m=0 helicon wave

The characteristics of the high density plasma production by the m = 0 azimuthal mode helicon wave is investigated. By varying the power of the applied radio frequency Prf two discharge modes exist; one is the low density mode (plasma density np<1012 cm−3), at which the discharge is sustained not by a helicon wave but by the antenna induction field at low Prf; the other is the high density mode (np≳1013 cm−3), in which helicon wave is excited at high Prf; in between the two modes, np jumps more than an order of magnitude. The threshold Prf for the density jump is higher for the m = 0 mode than that of the m = +1 and −1 modes excited by the helical antenna.

A study on ion energy distribution functions and plasma potentials in helicon wave plasmas

The characteristics of high-density helicon wave plasma generated in a quartz tube of 10 cm in diameter have been studied. The optimum conditions for efficiently exciting helicon wave plasma have been investigated. It is also observed that plasma fully ionized by helicon waves has ion pumping effect. Whether the discharge is inductive or capacitive, the plasma potential is important, since it determines the energy of the ion incident on the wall. It is investigated theoretically and experimentally that the fluctuation of the plasma potentials influences the ion energy distribution functions monitored by the retarding field energy analyzer. It is also observed that the plasma potential fluctuates with peak-to-peak voltage Vp–p in the low-density mode. The radio-frequency modulation to the plasma potential is weaker in the helicon mode than in the low mode.

Particle trapping, transport, and charge in capacitively and inductively coupled argon plasmas in a Gaseous Electronics Conference Reference Cell

Particle trapping has been observed for both a capacitively coupled and an inductively coupled Gaseous Electronics Conference Reference Cell. Particles in the system were formed by introducing 10% CCl2F2 into an argon discharge for 5–10 min and sputtering the resulting film during argon-only experiments. The particles generated were 200–600 nm in diameter. Particle transport was observed in the postplasma regime for both capacitively and inductively coupled cases. For the capacitively coupled plasma, particles of positive, negative and neutral charge were found during the postplasma and shown to be affected by the presence of a dc field during the postplasma period. The charge on these particles was determined to be between 1.4 and 12.4 electron charges based on a study of particle motion. This compares to a theoretical value of 800–2600 while the plasma is ignited. For the inductively coupled plasma both a low density mode and a high density mode were observed, being distinguished by a sudden increase in the emission from the plasma as rf power to the inductive coil was increased. In the high density mode of the inductively coupled plasma, the particle cloud was observed to reside less than a millimeter from the powered electrode as the high density plasma mode was established. This was distinguished from a trap height of 4–7 mm for the capacitively coupled plasma which corresponds to the sheath thickness.