Narrow Velocity Distribution Research Articles

A Cold Lid on a Warm Ocean: Indian Ocean Surface Rain Layers and Their Feedbacks to the Atmosphere

AbstractOcean surface rain layers (RLs) form when relatively colder, fresher, less dense rain water stably stratifies the upper ocean. RLs cool sea surface temperature (SST) by confining surface evaporative cooling to a thin near‐surface layer, and generate sharp SST gradients between the cool RL and the surrounding ocean. In this study, ocean‐atmosphere coupled simulations of the November 2011 Madden‐Julian Oscillation (MJO) event are conducted with and without RLs to evaluate two pathways for RLs to influence the atmosphere. The first, termed the “SST gradient effect,” arises from the hydrostatic adjustment of the boundary layer to RL‐enhanced SST gradients. The second, termed the “SST effect,” arises from RL‐induced SST reductions impeding the development of deep atmospheric convection. RLs are found to sharpen SST gradients throughout the MJO suppressed and suppressed‐to‐enhanced convection transition phases, but their effect on convection is only detected during the MJO suppressed phase when RL‐induced SST gradients enhance low‐level convergence/divergence and broaden the atmospheric vertical velocity probability distribution below 5 km. The SST effect is more evident than the SST gradient effect during the MJO transition phase, as RLs reduce domain average SST by 0.03 K and narrow vertical velocity distribution, thus delaying onset of deep convection. A delayed SST effect is also identified, wherein frequent RLs during the MJO transition phase isolate accumulated subsurface ocean heat from the atmosphere. The arrival of strong winds at the onset of the MJO active phase erodes RLs and releases subsurface ocean heat to the atmosphere, supporting the development of deep convection.

Enhancing bioflocculation in high-rate activated sludge improves effluent quality yet increases sensitivity to surface overflow rate

High-rate activated sludge (HRAS) relies on good bioflocculation and subsequent solid-liquid separation to maximize the capture of organics. However, full-scale applications often suffer from poor and unpredictable effluent suspended solids (ESS). While the biological aspects of bioflocculation are thoroughly investigated, the effects of fines (settling velocity < 0.6 m3/m2/h), shear and surface overflow rate (SOR) are unclear. This work tackled the impact of fines, shear, and SOR on the ESS in absence of settleable influent solids. This was assessed on a full-scale HRAS step-feed (SF) and pilot-scale HRAS contact-stabilization (CS) configuration using batch settling tests, controlled clarifier experiments, and continuous operation of reactors. Fines contributed up to 25% of the ESS in the full-scale SF configuration. ESS decreased up to 30 mg TSS/L when bioflocculation was enhanced with the CS configuration. The feast-famine regime applied in CS promoted the production of high-quality extracellular polymeric substances (EPS). However, this resulted in a narrow and unfavorable settling velocity distribution, with 50% ± 5% of the sludge mass settling between 0.6 and 1.5 m3/m2/h, thus increasing sensitivity towards SOR changes. A low shear environment (20 s−1) before the clarifier for at least one min was enough to ensure the best possible settling velocity distribution, regardless of prior shear conditions. Overall, this paper provides a more complete view on the drivers of ESS in HRAS systems, creating the foundation for the design of effective HRAS clarifiers. Tangible recommendations are given on how to manage fines and establish the optimal settling velocity of the sludge.

Exploring metal nanoparticles for matter-wave interferometry

High-mass matter-wave interferometry opens interesting perspectives for testing fundamental physics while posing intriguing challenges in experimental technologies. Here we explore the source and detection techniques that will be the basis for the next generation of quantum experiments with objects at least one order of magnitude more massive than the current state of the art. Magnetron sputtering provides a means to produce a continuous metal cluster beam with a large variability in material composition, structure, temperature, velocity, electric, and magnetic properties. The mass-scalable nanoparticles come with high ionization yield and a detected beam brilliance of up to ${10}^{11} {\mathrm{sr}}^{\ensuremath{-}1}$ ${\mathrm{s}}^{\ensuremath{-}1}$ clusters. We find favorably slow and narrow velocity distributions for mass-filtered clusters up to 800 kDa, which can also be readily ionized, manipulated, and detected by 266 nm UV laser light. The source-detector combination that we demonstrate here fulfills the essential requirements for a next generation of nanoparticle interferometry.

Simulation of the Scattering of Continuously Injected Pickup Ions outside the Heliopause

Abstract Hybrid simulations in 2D space and 3D velocity dimensions with continuous injection of pickup ions (PUIs) provide insight into the plasma processes that are responsible for the pitch angle scattering of PUIs outside the heliopause. The present investigation includes for the first time continuous injection of PUIs and shows how the scattering depends on the energy of the PUIs and the strength of the background magnetic field as well as the dependence on the injection rate of the time for the isotropization of the pitch angle distribution. The results demonstrate that, with the gradual injection of PUIs of a narrow ring velocity distribution perpendicular to the background magnetic field, oblique mirror mode waves develop first, followed by the growth of quasiparallel propagating ion cyclotron waves. Subsequently, the PUIs are scattered by the excited waves and gradually approach an isotropic distribution. A time for isotropization is defined to be the time at which T ∣∣/T ⊥, i.e., the ratio of the parallel to perpendicular PUI thermal energy changes from ≈0 to ≈0.15. By varying the PUI injection rate, estimates of the time for the PUI distribution to be isotropized are presented. The isotropization time obtained is shorter, ≈ months, than the time, ≈ years, required by the conventional secondary ENA mechanism to explain the IBEX ENA ribbon.

Transport dynamics in a high-brightness magneto-optical-trap Li ion source

Laser-cooled gases offer an alternative to tip-based methods for generating high-brightness ion beams for focused ion beam applications. These sources produce ions by photoionization of ultracold neutral atoms, where the narrow velocity distribution associated with microkelvin-level temperatures results in a very low emittance, high-brightness ion beam. In a magneto-optical trap-based ion source, the brightness is ultimately limited by the transport of cold neutral atoms, which restricts the current that can be extracted from the ion-generating volume. We explore the dynamics of this transport in a 7Li magneto-optical trap ion source by performing time-dependent measurements of the depletion and refilling of the ionization volume in a pulsed source. An analytic microscopic model for the transport is developed, and this model shows excellent agreement with the measured results.

Three-Dimensional Cooling of an Atom-Beam Source for High-Contrast Atom Interferometry

We present a compact, two-stage atomic beam source that produces a continuous, narrow, collimated and high-flux beam of rubidium atoms with sub-Doppler temperatures in three dimensions, which features very low emission of near-resonance fluorescence along the atomic trajectory. The atom beam source originates in a pushed two-dimensional magneto-optical trap (2D$^+$ MOT) feeding a slightly off-axis three-dimensional moving optical molasses stage that continuously cools and redirects the atom beam. The capture velocity of the moving optical molasses is deliberately chosen to be low, $\sim 3$ m/s, to reduce fluorescence, and the cooling light is detuned by several atomic linewidths from resonance to reduce the absorption cross-section of cooling-induced fluorescence. Near-resonance light from the 2D$^+$ MOT and the push beam does not propagate to the output atomic trajectory due to a 10 degree bend in the atomic trajectory. The atomic beam emitted from the two-stage source has a flux up to $1.6(3)\times 10^9\;\textrm{atoms/s}$, with an optimized temperature of $15.0(2)\;\mu$K. We employ continuous Raman-Ramsey interference measurements at the atom beam output to study the sources of decoherence in the presence of continuous cooling, and demonstrate that the atom beam source effectively preserves high fringe contrast even during cooling. This cold-atom beam source is appropriate for use in atom interferometers and clocks, where continuous operation eliminates dead time, the slow atom beam velocity (6 - 16 m/s) improves sensitivity, the narrow 3D velocity distribution improves fringe contrast, and the low reabsorption of scattered light mitigates decoherence caused by the continuous cooling process.

Constraints on the density distribution of type Ia supernovae ejecta inferred from late-time light-curve flattening

ABSTRACT The finite time, τdep, over which positrons from β+ decays of 56Co deposit energy in type Ia supernovae ejecta lead, in case the positrons are trapped, to a slower decay of the bolometric luminosity compared to an exponential decline. Significant light-curve flattening is obtained when the ejecta density drops below the value for which τdep equals the 56Co lifetime. We provide a simple method to accurately describe this ‘delayed deposition’ effect, which is straightforward to use for analysis of observed light curves. We find that the ejecta heating is dominated by delayed deposition typically from 600 to 1200 d, and only later by longer lived isotopes 57Co and 55Fe decay (assuming solar abundance). For the relatively narrow 56Ni velocity distributions of commonly studied explosion models, the modification of the light curve depends mainly on the 56Ni mass-weighted average density, 〈ρ〉t3. Accurate late-time bolometric light curves, which may be obtained with JWST far-infrared (far-IR) measurements, will thus enable to discriminate between explosion models by determining 〈ρ〉t3 (and the 57Co and 55Fe abundances). The flattening of light curves inferred from recent observations, which is uncertain due to the lack of far-IR data, is readily explained by delayed deposition in models with $\langle \rho \rangle t^{3} \approx 0.2\, \mathrm{M}_{\odot }\, (10^{4}\, \textrm{km}\, \textrm{s}^{-1})^{-3}$, and does not imply supersolar 57Co and 55Fe abundances.

Translational and Rotational Energy Distributions of NO Photodesorbed from Au(100)

We report velocity and internal state distributions of nitric oxide photodesorbed from an Au(100) single crystal using 355 and 266 nm photons. The velocity distributions were measured in all three dimensions independently using our novel 3D-velocity map imaging setup. Combined with the internal energy distributions we reveal two distinct desorption mechanisms for the photodesorption of NO from gold dependent on the photon wavelength. The 355 nm desorption is dominated by a nonthermal mechanism due to excitation of an electron from the gold substrate to the adsorbed NO; this leads to a superthermal and noticeably narrow velocity distribution and a rotational state distribution that positively correlates with the velocity distribution and can be described by a rotational temperature appreciably above the surface temperature. Desorption with 266 nm photons leads to a slower average speed and wider angular distribution and rotational temperatures not too far off the surface temperature. We conclude that in th...

Influence of pulse width on the laser ablation of zinc in nitrogen ambient

Time-resolved spectroscopic measurements of expanding plasma plumes generated by irradiating a solid zinc target with laser pulses of 7 ns and 100 fs durations are carried out in the ambient pressure range of 0.05–200 Torr of nitrogen. At the relatively high input fluence of ~16 J/cm2, fast and slow atomic species are found to appear at different times in the optical time-of-flight (OTOF) spectra, the dynamics of which is primarily determined by the pulse duration of the excitation laser. In fs LPP, the average speed of fast species is unaffected by an increase in ambient pressure, while in ns LPP, the speed is found to reduce with pressure. The slow species shows a sharp peak in the OTOF spectra with a narrow velocity distribution for fs LPP, indicating a large number density and low electron temperature, which is consistent with optical emission spectroscopic (OES) studies. On the other hand, for ns LPP, the OTOF of slow species shows a more broadened profile which can be attributed to strong plume–laser interaction. The dynamics of slow species is heavily influenced by the presence of shock waves, which leads to the occurrence of much slower species at larger pressures.

Survivability of bare, individual Bacillus subtilis spores to high-velocity surface impact: Implications for microbial transfer through space

Laboratory experiments show that endospores of Bacillus subtilis survive impact against a solid surface at velocities as high as 299±28m/s. During impact, spores experience and survive accelerations of at least 1010m/s2. The spores were introduced into a vacuum chamber using an electrospray source and accelerated to a narrow velocity distribution by entrainment in a differentially pumped gas flow. Different velocity ranges were studied by modifying the gas flow parameters. The spores were electrically charged, allowing direct measurement of the velocity of each spore as it passed through an image charge detector prior to surface impact. Spores impacted a glass surface and were collected for subsequent analysis by culturing. Most spores survived impact at all measured velocities. These experiments differ fundamentally from other studies that show either shock or impact survivability of bacteria embedded within or on the surface of a projectile. Bacteria in the present experiments undergo a single interaction with a solid surface at the full impact velocity, in the absence of any other effects such as cushioning due to microbe agglomerations, deceleration due to air or vapor, or transfer of impact shock through solid or liquid media. During these full-velocity impact events, the spores experience extremely high decelerations. This study is the first reported instance of accelerations of this magnitude experienced during a bacteria impact event. These results are discussed in the context of potential transfer of viable microbes in space and other scenarios involving surface impacts at high velocities.

A new Stark decelerator based surface scattering instrument for studying energy transfer at the gas-surface interface.

We report on the design and characterization of a new apparatus for performing quantum-state resolved surface scattering experiments. The apparatus combines optical state-specific molecule preparation with a compact hexapole and a Stark decelerator to prepare carrier gas-free pulses of quantum-state pure CO molecules with velocities controllable between 33 and 1000 m/s with extremely narrow velocity distributions. The ultrahigh vacuum surface scattering chamber includes homebuilt ion and electron detectors, a closed-cycle helium cooled single crystal sample mount capable of tuning surface temperature between 19 and 1337 K, a Kelvin probe for non-destructive work function measurements, a precision leak valve manifold for targeted adsorbate deposition, an inexpensive quadrupole mass spectrometer modified to perform high resolution temperature programmed desorption experiments and facilities to clean and characterize the surface.

Mass-Resolved Velocity Map Imaging of Doubly Charged Photofragments from C60

Abstract Mass-resolved velocity map images have been measured for C60−2n2+ and C60−2n3+ (n ≥ 1) produced by photoionization of C60 with synchrotron radiation. A four-element electrostatic compound lens system was substituted for a conventional three-element system. This produced excellent time resolution, i.e., good mass-to-charge resolution, and negligible overlap between neighboring cluster ions. The translational temperatures for the C60−2n2+ ions were estimated from their velocity map images by assuming the Maxwell–Boltzmann distributions. Moreover, the improved resolution allowed observing a two-dimensional (2D) projection of three-dimensional velocity distribution of each fragment. The 2D projection of C582+ results from convolution of a narrow velocity distribution of the C60 molecular beam and a nascent isotropic profile of photodissociated C582+.

Slowing of femtosecond laser-generated nanoparticles in a background gas

The slowing of Pt nanoparticles in argon background gas was characterized by Rayleigh scattering imaging using a plume of nanoparticles generated by femtosecond laser through thin film ablation of 20 nm-thick Pt films. The ablation was performed at threshold laser energy fluences for complete film removal to provide a well-defined plume consisting almost entirely of nanoparticles traveling with a narrow velocity distribution, providing a unique system to unambiguously characterize the slowing of nanoparticles during interaction with background gases. Nanoparticles of ∼200 nm diameter were found to decelerate in background Ar gas with pressures less than 50 Torr in good agreement with a linear drag model in the Epstein regime. Based on this model, the stopping distance of small nanoparticles in the plume was predicted and tested by particle collection in an off-axis geometry, and size distribution analysis by transmission electron microscopy. These results permit a basis to interpret nanoparticle propagation through background gases in laser ablation plumes that contain mixed components.

Scattering in a quantum dot: the role of resonances

The dynamics of a system in which an electron is incident on a populated quantum dot is studied by resolving the time dependence of the system. Specifically, one electron is initially at rest in the ground state of the dot, whereas the incident electron has a narrow velocity distribution. In addition to the probabilities of reflection and transmission, the probability of excitation is found as a function of energy. Moreover, probabilities of both electrons being ejected or both being captured are obtained. The latter process involves spontaneous emission. It is found that the dynamics is strongly influenced by the presence of doubly excited states; reflection becomes considerably more probable in the energetic vicinity of such states. This, in turn, contributes to an increase in the probability of trapping also the second electron within the dot.

Experimental Study of Steady Quasi-Cone-Jet Electrostatic Sprays of Biobutanol for Engine Applications

AbstractButanol produced from agricultural sources is emerging as a potentially renewable biofuel for use in engine applications. In this work, butanol electrostatic sprays were established within a narrow region of low flow rates. Spray phenomenology was investigated through high-speed visualization for the low flow rate conditions in the vicinity of the lowest voltage for which electrosprays (e-sprays) could be sustained. Spray structure was studied through droplet size and velocity measurements for a combination of conditions, performed by using phase Doppler anemometry. Combined with high-speed spray visualization, these measurements revealed a stable e-spray operation with narrow droplet size and velocity distributions; i.e., spray behavior was close to monodisperse. A similarity analysis was performed to develop an empirical expression correlating appropriately dimensionless average diameter, flow rate, and applied voltage.

Predictions of non-Fickian solute transport in different classes of porous media using direct simulation on pore-scale images

We present predictions of transport through micro-CT images of porous media that include the analysis of correlation structure, velocity, and the dynamics of the evolving plume. We simulate solute transport through millimeter-sized three-dimensional images of a beadpack, a sandstone, and a carbonate, representing porous media with an increasing degree of pore-scale complexity. The Navier-Stokes equations are solved to compute the flow field and a streamline simulation approach is used to move particles by advection, while the random walk method is employed to represent diffusion. We show how the computed propagators (concentration as a function of displacement) for the beadpack, sandstone, and carbonate depend on the width of the velocity distribution. A narrow velocity distribution in the beadpack leads to the least anomalous behavior, where the propagators rapidly become Gaussian in shape; the wider velocity distribution in the sandstone gives rise to a small immobile concentration peak, and a large secondary mobile peak moving at approximately the average flow speed; in the carbonate with the widest velocity distribution, the stagnant concentration peak is persistent, with a slower emergence of a smaller secondary mobile peak, characteristic of highly anomalous behavior. This defines different types of transport in the three media and quantifies the effect of pore structure on transport. The propagators obtained by the model are in excellent agreement with those measured on similar cores in nuclear magnetic resonance experiments by Scheven, Verganelakis, Harris, Johns, and Gladden, Phys. Fluids 17, 117107 (2005).

Qualitative investigation on effects of manifold shape on methanol steam reforming for hydrogen production

Fluid velocity distribution among microchannels plays important role on the reaction performances. In this work, the velocity distribution among microchannels with two different manifold structures is compared by a three-dimensional CFD model under two situations respectively, no reaction and methanol steam reforming occurs. Then the performances of methanol steam reforming in both plates are experimentally investigated, and the effect of manifold shape on the hydrogen production performances is qualitatively analyzed by the combination of simulation results of velocity distribution. It is found that the microchannel plate with right-angle manifold enables narrow velocity distributions under different entrance velocities and reaction temperatures, whether no reaction occurs or methanol steam reforming is progressing, which can be the critical element results in better conversion rate and selectivity of process than that of the microchannel plate with oblique-angle manifold. Optimizing the structural parameters to facilitate a relatively uniform velocity distribution to increase the hydrogen production performances may be a key factor to be considered.

Background Pressure Effects on Ion Velocity Distribution Within a Medium-Power Hall Thruster

5 torr). In addition to varying the background pressure, the radial magnetic field of the thruster was varied (by a factor of 2) between low- and high-strength configurations. The low- strength configuration produced large-magnitude anode current oscillations, whereas the high-strength configuration produced small current oscillations. Ion axial velocity distribution function peaks were used to approximate ion energy and, in turn, axial electric field strength. Acceleration profiles of the tested thruster operatingconditionswerecompared.Highbackgroundpressureoperationwasobservedtoshifttheionacceleration regionupstreaminthedischargechannel. Thewidthofthevelocitydistributionscorrelatedstrongly tothemagnetic field strength. The high magnetic field strength configuration produced narrow velocity distribution functions, whereas the low magnetic field strength configuration led to a broad velocity distribution.

Control of atomic path in projectile cold atom gyroscope

Cold atoms possess low velocity, narrow velocity distribution as well as good matter wave coherence, thus they have been used to achieve cold atom interferometers. The interferometers with Sagnac effect form atom gyroscopes which can measure rotation rate accurately. Here we report our recent progress in building a projectile cold atom gyroscope. We achieve the accurate control of the bidirectional projectile cold atom motion through the use of a homemade direct digital frequency synthesizer.

Nonlinear Stage of Alfvén Ion-Cyclotron Instability

A model describing nonlinear saturated state of Alfvén ion-cyclotron instability is presented. Spirally symmetric exact solutions of Vlasov – Maxwell equations describing nonlinear Alfvén waves in collisionless plasmas are presented. On their basis the equation describing ion distribution function in non-linear equilibrium of a weak-collision plasma with the wave is obtained, taking into account an uniform fast ion injection, ion drag caused by electrons and neglecting ion angle scattering. A solution of this equation and relations of the wave parameters with the injection parameters are obtained analytically in the case of the infinitely narrow velocity distribution of the injected ions